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Article
Volume 4, Number 3
5 March 2003
8003, doi:10.1029/2001GC000223
AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES
Published by AGU and the Geochemical Society
ISSN: 1525-2027
Recycling oceanic crust: Quantitative constraints
Andreas Stracke
National High Magnetic Field Laboratory (NHMFL) and Department of Geological Sciences, Florida State University,
1800 East Paul Dirac Drive, Tallahassee, Florida 32306, USA.
Department of Earth Sciences, Cambridge University, Downing Street, Cambridge CB2 3EQ, UK.
Now at Max-Planck-Institute für Chemie, Abteilung Geochemie, J. J. Becher Weg 27, 55128 Mainz,
Germany.(stracke@mpch-mainz.mpg.de)
Michael Bizimis and Vincent J. M. Salters
National High Magnetic Field Laboratory (NHMFL) and Department of Geological Sciences, Florida State University,
1800 East Paul Dirac Drive, Tallahassee, Florida 32306, USA (bizimis@magnet.fsu.edu; salters@magnet.fsu.edu)
[1] Recycled ancient oceanic crust with variable amounts of aging, or inclusion of sediments of differing
types and origins has often been invoked as a source for present-day ocean island basalts (OIB), but the
current evidence remains largely qualitative. Previous quantitative modeling has shown that much has to be
learned in order to better understand the implications of crustal recycling on mantle heterogeneity. Here,
we present new model calculations incorporating recent constraints on subduction-zone processes and the
composition of subducted sediments. Modeled compositions of the recycled oceanic crust vary widely as a
function of the recycling age and composition of the oceanic crust. HIMU-type sources can only be created
by recycling igneous oceanic crust if it has undergone substantial modification during subduction.
Although the required modifications are qualitatively consistent with dehydration processes in subduction
zones, the many uncertainties prevent a precise estimate of the isotopic composition of ancient recycled
igneous crust. Inclusion of sediments increases the isotopic variability and although the resulting Sr and Nd
isotopic signatures can be similar to enriched mantle (EM) signatures, the Pb isotopic composition of EMtype OIB is difficult to reconcile with the presence of sediment in their sources. The large variability of
modeled compositions of the subducted crust suggests that if mantle heterogeneity is largely formed by
crustal recycling, each OIB is likely to have a unique isotopic composition resulting from specific
combinations of composition, age and subduction modification of the subducted crust. Given the
variability of the recycled components, a small number of relatively well-defined enriched compositions
can only be explained if either the subduction processing of oceanic crust is a far better defined process
than observation would seem to indicate, or, the intramantle disaggregation and mixing of compositionally
diverse recycled materials is surprisingly efficient.
Components: 19,603 words, 8 figures, 5 tables.
Keywords: Recycling; oceanic crust; isotope geochemistry; trace elements; isotopes.
Index Terms: 1025 Geochemistry: Composition of the mantle; 1040 Geochemistry: Isotopic composition/chemistry;
1065 Geochemistry: Trace elements (3670); 3640 Mineralogy and Petrology: Igneous petrology.
Received 30 August 2001; Revised 4 September 2002; Accepted 4 October 2002; Published 5 March 2003.
Stracke, A., M. Bizimis, and V. J. M. Salters, Recycling oceanic crust: Quantitative constraints, Geochem. Geophys. Geosyst.,
4(3), 8003, doi:10.1029/2001GC000223, 2003.
Copyright 2003 by the American Geophysical Union
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1. Introduction
[2] The potential importance of recycled oceanic
crust to contribute to mantle heterogeneity was first
recognized by Armstrong [1968]. Subsequently,
Chase [1981] and Hofmann and White [1982] have
invoked recycled oceanic crust to be a source for
ocean island basalts (OIB; see references given
below).
[3] While recycling of oceanic crust into the
mantle is an integral part of plate tectonics,
exactly how this recycling process manifests itself
in the chemical composition of OIB is still a
debated issue and is key in understanding the
chemical evolution of the Earth’s mantle-crust
system. Over Earth’s history, recycled oceanic
crust amounts to roughly 8–10% of the mass of
the mantle (assuming a production rate of about
3km2/year [Hofmann and White, 1982], an average thickness of the oceanic crust of about 7km
[White et al., 1992]; density of the oceanic crust =
3.3g/cm3, age of the Earth = 4.55*109 years, mass
of the mantle = 4*1024 kg). Depending on the
exact modus of plate tectonics, there might be
considerable uncertainty in this estimate, but it
confirms that recycling of oceanic crust may play
a significant role in assessing the chemical composition of OIB and in understanding mantle-crust
evolution.
[4] White [1985] and Zindler and Hart [1986]
classified OIBs into three isotopically distinct
groups (e.g., HIMU = high m = 238U/204Pb, EMI
and EMII, EM = enriched mantle). Although a
variety of mechanisms have been proposed to
cause these enriched isotopic signatures in OIB,
recycling of oceanic crust with variable amounts
of aging, modification during seafloor alteration
and subduction, or inclusion of sediments of
differing types and origins, has been invoked most
often. The HIMU-type OIB have been explained
by recycling pure igneous oceanic crust [e.g.,
Chauvel et al., 1992, 1997; Halliday et al.,
1988; Hart, 1988; Hauri and Hart, 1993; Hauri
et al., 1996; Hofmann, 1997; Lassiter and Hauri,
1998; Nakamura and Tatsumoto, 1988; Palacz and
Saunders, 1986; Reisberg et al., 1993; Roy-Barman and Allègre, 1995; Salters and White, 1998;
10.1029/2001GC000223
Vidal et al., 1984; Weaver, 1991; Zindler and Hart,
1986]. The EM-type OIB have been explained by
recycling oceanic crust plus minor amounts of
sediment with different composition [e.g., Barling
and Goldstein, 1990; Blichert-Toft et al., 1999;
Chauvel et al., 1992; Eisele et al., 2002; Hart,
1988; Hauri and Hart, 1993; Hauri et al., 1996;
Hemond et al., 1994; Hofmann, 1997; Le Roex et
al., 1990; Rehkämper and Hofmann, 1997; RoyBarman and Allègre, 1995; Weaver, 1991; Weis et
al., 1993; White and Duncan, 1995; Woodhead
and Devey, 1993; Woodhead and McCulloch,
1989; Wright and White, 1987; Zindler and Hart,
1986].
[5] An assumption implied in most of these
studies is that recycling of oceanic crust leads
to the limited number of enriched isotopic components in the mantle (HIMU, EMI and EMII)
and that mixing between these end-members or
components can explain most of the isotopic
variability in OIB [Zindler and Hart, 1986].
However, this is only one possible ‘‘picture’’ of
the isotopic variability in the mantle. Independent
of the way in which the chemical heterogeneities
are introduced into the mantle, two end-member
scenarios are possible to explain the isotopic
variability of OIB sources: (1) There are few
different enriched compositions in the mantle
(HIMU, EMI and EMII) and the isotopic signatures in each OIB are produced by unique mixtures
between these same end-member compositions, or,
(2) each OIB-source has its own unique composition, i.e. there are at least as many end-members as
OIBs.
[6] The model calculations presented here allow
for a wide variety of compositions of the ancient
recycled crust (both in terms of its initial isotopic
and trace element composition), recycling ages,
alteration during subduction and relative quantities
of the recycled materials (basalt and sediment). The
model calculations and input parameters are supplied via Microsoft Excel spread sheets, see auxilliary material available at http://www.g-cubed.org.
The reader is encouraged to use these spreadsheets
and use her/his own input parameters to explore
possible results. Based on our results of the modeling we find that the isotopic composition of
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ancient recycled crust varies considerably as a
function of composition, age and subduction modification. By simple modeling as presented here, it
remains difficult to find suitable combinations of
parameters that yield isotopic compositions of
either HIMU or EM-type basalts.
2. Quantifying the Process of Recycling
Oceanic Crust
[7] In order to calculate the radiogenic isotope
composition of the bulk subducted crust, its age
and initial isotopic composition have to be
known, as well as its parent-daughter (P/D) ratio
for each isotopic system. A detailed look at the
processes involved in recycling oceanic crust,
however, shows that estimating these three
parameters involves many uncertainties and that
it is difficult to uniquely define the final composition and age of the subducted crust. In the
following, the term ‘‘oceanic crust’’ is used for
crust not yet modified by subduction zone processes, whereas the term ‘‘subducted crust’’ is
used for crust that has been modified during
subduction.
[8] Recycling starts with the formation and hydrothermal alteration of the oceanic crust at mid
ocean ridges (MOR), followed by the progressive
deposition of oceanic sediments with increasing
age of the crust. As it enters a subduction zone,
the bulk oceanic crust (igneous crust + sediment)
loses parts of its sedimentary cover which is
scraped off and accumulated in the accretionary
prism. Finally, during subduction, the subducted
crust undergoes a series of phase changes and
different parts of it are dehydrated and/or melted
to different extents [e.g., Bizimis et al., 2000;
Elliott et al., 1997; Tatsumi, 1989; Tatsumi and
Kogiso, 1997; Turner and Hawkesworth, 1997],
before the slab is further physically and mineralogically modified during transport into the deeper
mantle [e.g., Grand et al., 1997; van der Hilst et
al., 1997].
[9] For quantitative modeling, we need to have a
representative estimate of the composition of the
bulk oceanic crust entering the subduction zone and
need to quantify the geochemical processes that
10.1029/2001GC000223
modify the package during subduction. The composition of average fresh igneous crust is well
known [e.g., Hofmann, 1988; Sun and McDonough, 1989] and the composition of the hydrothermally altered igneous crust can be directly
measured at some distance from the ridge [Staudigel et al., 1995, 1996]. Plank and Langmuir [1998]
provide an estimate of the composition and variability of sediments overlying the igneous crust, so
that the input compositions are relatively well
characterized. The extent of alteration during subduction is more difficult to estimate. A number of
recent studies which investigate sub-arc processes
on a quantitative basis will be used in this study
[e.g., Ayers, 1998; Ayers et al., 1997; Brenan et al.,
1995; Johnson and Plank, 2000; Kogiso et al.,
1997; McCulloch and Gamble, 1991; Stalder et
al., 1998].
3. Constraints From Quantitative
Recycling Models
3.1. Recycling Igneous Oceanic Crust
3.1.1. Modeling the Composition of the
Igneous Oceanic Crust
[10] Figure 1 shows the calculated isotopic compositions for ancient fresh and altered basalts
[Hofmann, 1988; Staudigel et al., 1995, 1996;
Sun and McDonough, 1989], and oceanic gabbros
[Hart et al., 1999; Zimmer et al., 1995]. The
details of the calculations are given in the Figure
caption and by using the spread sheet ‘‘basalt +
sediment_recycling’’, the calculations can be
reproduced and different compositions can be
investigated. Also, an inverse approach similar to
that of Hauri and Hart [1993] is undertaken in
order to calculate the trace element composition of
the recycled oceanic crust which is necessary to
yield HIMU isotopic signatures (see Figure 2,
Table 2, and spread sheet ‘‘P/D_HIMUsource’’).
In both cases, the calculations are done for varying recycling ages, i.e. varying initial isotopic
composition of the oceanic crust (the recycling
age is the age at which the recycled crust started
to evolve as a closed system until present-day
melting). The initial isotopic composition of the
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0.5134
MORB
altered MORB, Staudigel et al., 1995
0.5132
0.5Ga
143Nd/144Nd
0.5130
1Ga
1Ga
1Ga
0.5128
HIMU
EMII
2Ga
0.5126
N-MORB
Hofmann, 1988
2Ga
0.5124
EMI
average 735B Gabbro
Hart et al., 1999
0.5122
initial 87Sr/86Sr affected by seawater alteration
no seawater alteration
0.5120
0.702
0.703
0.704
0.705
0.706
a
0.707
0.708
87Sr/86Sr
42
41
HIMU
EMII
208 Pb/ 204 Pb
40
EMI
39
38
MORB
37
1Ga
0.5Ga
2Ga
1Ga
36
b
35
15
16
17
18
19
20
21
22
23
24
206 Pb/ 204 Pb
Figure 1. Shown is the calculated present-day-isotopic composition of ancient recycled oceanic crust as a function
of recycling age (2Ga – 0.5Ga) in a variety of isotope ratio diagrams. Although the Nd and Hf isotope composition of
recycled oceanic crust can be similar to those of HIMU basalts (Figure 1e), none of the compositions used here
(average N-MORB [Hofmann, 1988], oceanic gabbro [Hart et al., 1999], and average altered MORB [Staudigel et
al., 1995, 1996]; see also Table 1) develops 208Pb/204Pb similar to those in OIB (Figure 1b). This indicates that
especially the U-Th-Pb budget of the recycled oceanic crust has to be substantially modified during subduction if the
HIMU source is assumed to originate from recycled oceanic crust. See section 3.1 for detailed discussion, and
appendix B and auxiliary material and spread sheet ‘‘basalt + sed_recycling’’ for details of the calculations. Plotted
data for EMI-type basalts are from the Tristan da Cunha, Pitcairn, and Kerguelen islands and the Walvis ridge, EMII
basalts are from the Society, Samoa, and Marquesas islands and HIMU-type basalts are from St. Helena, and the
Tubaii, Rurutu, Mangaiia, and Rimatara islands, MORB data are from the Atlantic, Pacific and Indian ocean ridges.
For data and references see spread sheet ‘‘basalt + sed_recycling’’.
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0.5134
MORB
0.5132
143 Nd/ 144 Nd
0.5Ga
0.5130
1Ga
HIMU
1Ga
0.5128
2Ga
0.5126
EMII
2Ga
0.5124
EMI
0.5122
c
0.5120
15
16
17
18
19
20
21
22
23
24
206 Pb/ 204 Pb
0.708
EMII
0.707
EMI
87Sr/86Sr
0.706
0.705
0.5Ga
0.704
1Ga
2Ga
0.703
2Ga
2Ga
1Ga
HIMU
1Ga
2Ga
1Ga
d
MORB
0.702
15
16
17
18
19
20
21
22
23
24
206 Pb/ 204 Pb
Figure 1. (continued)
oceanic crust is also a function of the assumed
differentiation age of the mid ocean ridge basalt
(MORB) source (the differentiation age of the
MORB source is the age at which the MORB
source was derived from the Bulk Earth (BE)
reservoir, and is referred to as source age in the
following; source age = 2 Ga in Figure 1). For
details of the modeling and the isotopic evolution
of the MORB source see appendix B and the
spread sheets ‘‘basalt + sediment_recycling’’ and
‘‘P/D_HIMUsource’’.
3.1.2. Isotopic Composition of Ancient
Igneous Oceanic Crust
[11] Due to their high U/Pb ratios, ancient seawater
altered basalts have 206Pb/204Pb ratios that are far
more radiogenic than those of any OIB (recycling
ages >1Ga; see Figures 1b–1d). Owing to their
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0.2836
MORB
176Hf/177Hf
0.2834
0.2832
EMII
0.2830
1Ga
0.2828
1Ga
1Ga
HIMU
EMI
2Ga
0.2826
2Ga
2Ga
e
0.2824
0.2822
0.5120
0.5122 0.5124
0.5126 0.5128
0.5130
0.5132 0.5134
143 Nd/ 144 Nd
Figure 1. (continued)
high Rb/Sr ratios, ancient altered basalts also
develop high 87Sr/86Sr ratios, which are unlike the
low 87Sr/86Sr values of HIMU basalts (Figures 1a,
1d; see also Hart and Staudigel [1989]). Thus, the
Sr and Pb isotope composition of ancient altered
MORB preclude, rather than support the contention
that ancient altered crust yields HIMU compositions [Hart and Staudigel, 1989]. Also, any endmember with such radiogenic Sr and Pb isotope
composition would produce mixing trends oblique
to the OIB arrays in Sr-Nd-Pb isotope diagrams
(Figure 1), which are not readily apparent.
[12] Hauri and Hart [1993] compared calculated P/
D-ratios of the HIMU source with the range of P/D
ratios in modern MORB. They calculated a range of
Rb/Sr, Sm/Nd and Lu/Hf ratios for a range of ages
and present-day isotopic compositions of the HIMU
source, and derived a range of U/Pb, Th/Pb, and Th/
U ratios from the slope of the tie lines between the
MORB and HIMU fields in Pb-Pb isotope diagrams. Hauri and Hart [1993] noted that their
inferred range of P/D ratios of the HIMU source
is similar to the range in fresh basalts and argued
that the alteration signature is removed during
subduction and/or that the altered oceanic crust only
constitutes a small portion of the recycled crust. In
this study, we have calculated P/D ratios of the
HIMU source for recycling/source ages between 3
and 0.5 Ga and a range of isotopic compositions for
the present-day HIMU source (see Tables 1 and 2,
Figure 2, and spread sheet ‘‘P/D_HIMUsource’’).
The calculated range of P/D ratios for the HIMU
source agrees well with the range calculated by
Hauri and Hart [1993], except for Th/U. The range
of Th/U ratios calculated from the Pb-Pb isotope
systematics by Hauri and Hart [1993] extends to
lower values (1.4–3.5) compared to our inversion
calculations (2.9–3.7; Table 1, Figure 2).
[ 13 ]
87
Figure 2 shows the range of calculated
Rb/86Sr, 147Sm/144Nd, 176Lu/177Hf, 238U/204Pb
(m),232Th/238U (k), and 232Th/204Pb (w) ratios for
the HIMU source for recycling/source ages of 3–0.5
Ga (gray field in Figure 2; see also Tables 1, 2, and
spread sheet ‘‘P/D_HIMUsource’’). The broad overlap between the range of 87Rb/86Sr, 147Sm/144Nd,
and 176Lu/177Hf in fresh MORB and the HIMU
source in Figure 2 suggests that, to a first approximation, the Sr, Nd and Hf isotope signatures of
HIMU basalts can be explained by ancient recycled
fresh MORB (see also Figures 1a, 1d, and 1e).
However, the 8 7 Rb/ 8 6 Sr, 1 4 7 Sm/ 1 4 4 Nd and
176
Lu/177Hf ratios required for the HIMU source
depend strongly on the assumed recycling/source
age (i.e. its initial isotopic composition). For any
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Frequency
up to 61
12
60
8
40
4
20
0
0
0.10
0.14
0.18
0.22
0.26
0.30
0
4
8
147Sm/144Nd
12
16
20
24
28 32
36 40
238U/204Pb
40
80
e
b
Frequency
d
altered MORB = 47.8
16
a
60
30
40
20
20
10
0
up to 177
0
0.004 0.010 0.016 0.022 0.028 0.034 0.040 0.046
0
176Lu/177Hf
10 20
30 40 50
60 70 80 90 100
232Th/204Pb
20
c
16
60
12
40
8
20
4
f
altered MORB = 0.36
Frequency
80
0
0
0 0.02 0.06
0.10 0.14 0.18 0.22 0.26 0.30
87Rb/86Sr
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
232Th/238U
Average MORB glasses, this study
N-MORB, Hofmann, 1988
average Gabbro, Hart et al., 1999
altered MORB, Staudigel et al., 1996
bulk crust
range for the HIMU source
recycling/source age = 0.5-3Ga
range for the HIMU source
recycling, source age = 2Ga
Figure 2. Histograms showing the variation and frequency of parent-daughter (P/D) ratios in present-day MORB
glasses (compiled from the LDEO petrological database at http://petdb.ldeo.columbia.edu/petdb; see spread sheet
‘‘P/D_data_MORB’’) compared to the range of P/D ratios calculated for the HIMU source (shaded field).
Calculations assume a range of source ages of 3 – 1Ga and a range of recycling ages of 3 – 0.5Ga (gray field). The
stippled field is the range of P/D ratios for a 2Ga old HIMU source (recycling = source age = 2Ga). Also shown are
the average of the compiled ratios (average MORB, this study), as well as the average N-MORB of Hofmann
[1988], altered MORB [Staudigel et al., 1996], gabbro; [Hart et al., 1999] and our estimate of the bulk igneous crust
(see also Tables 1 and 3).
specific combination of recycling and source ages,
the range in P/D ratios is narrower than the total
range for recycling/source ages between 3 and 0.5
Ga (compare the stippled field for a recycling/source
age of 2 Ga with the gray field for recycling/source
ages between 3 and 0.5 Ga in Figure 2; compare also
Figure 2 with Tables 1 and 2).
[14] In general, most fresh MORBs have 87Rb/86Sr
<0.07, 147Sm/144Nd between 0.19 and 0.22, and
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Table 1. Parent-Daughter (P/D) Ratios in Modern Fresh and Altered MORB and Oceanic Gabbros, as Well as P/D
Ratios Calculated for Recycled Oceanic Crust in Order to Yield HIMU Signaturesa
87
Rb/86Sr
147
Sm/144Nd
176
Lu/177Hf
238
U/204Pb
Total range calculated for recycled oceanic crust (HIMU source, recycling ages 0.5 – 3Ga)
0.012 – 0.121
0.103 – 0.252
0.007 – 0.039
12 – 61
Range in modern MORB glasses (LDEO petrological database)
0.003 – 0.275
0.116 – 0.292
0.009 – 0.044
3 – 35
Average modern MORB glasses (LDEO petrological database)
0.053 ± 40
0.201 ± 30
0.024 ± 5
12 ± 5.6
N-MORB Hofmann [1988]
0.032
0.203
0.028
9.1
N-MORB Sun and McDonough [1989]
0.018
0.218
0.032
9.8
Altered MORB, Staudigel et al. [1996]
0.241
0.228
0.031
47.8
Average gabbro, Gabal Gerf ophiolithe, Zimmer et al. [1995]
0.006
0.169
0.023
5.1
Average gabbro, 735B, Hart et al. [1999]
0.006
0.183
0.027
4.5
Bulk oceanic crust (25% altered MORB + 25% N-MORB + 50% gabbro;
[Hart et al., 1999; Hofmann, 1988; Staudigel et al., 1996])
0.064
0.197
0.028
14.8
232
Th/204Pb
232
Th/238U
45 – 177
2.9 – 3.7
7 – 93
1.5 – 5.3
33 ± 18
2.75 ± 0.5
24.6
2.7
25.8
2.6
17.3
0.36
8.9
1.8
8.2
1.8
19
1.3
a
For the trace element data in modern MORB glasses see spread sheet ‘‘P/D_data_MORB’’ (data are compiled from the Lamont Doherty Earth
Observatory petrological database at http://petdb.ldeo.columbia.edu/petdb).
176
Lu/177Hf between 0.020 and 0.028 (Figures 2a–
2c) so that HIMU isotopic signatures only result
for recycling ages roughly >1.5 Ga for Sr and Nd
isotopes and <1.5 Ga for Hf isotopes (compare
Figure 2 with Tables 1 and 2). If the initial Sr
isotope composition of the igneous crust is modified by seawater alteration, lower 87Rb/86Sr are
required than in case of pristine initial 87Sr/86Sr
ratios (Figure 1a). In contrast to the 87Rb/86Sr,
147
Sm/144Nd, and 176Lu/177Hf ratios, the calculated
Table 2. Calculated Parent-Daughter (P/D) Ratios in Recycled Oceanic Crust in Order to Produce HIMU-Signatures
as a Function of the Recycling and Source Age of the MORB-Sourcea
Recycling age
0.5Ga
1Ga
1.5Ga
2Ga
2.5Ga
3Ga
0.037 – 0.054
0.201 – 0.213
0.032 – 0.036
0.037 – 0.051
0.205 – 0.215
0.034 – 0.037
13.0 – 17.2
3.4 – 3.7
48 – 59
12.1 – 15.4
3.5 – 3.7
45 – 54
Source Age of the MORB Source: 1Ga
87
Rb/86Sr
147
Sm/144Nd
176
Lu/177Hf
0.160 – 0.221
0.005 – 0.025
0.221 – 0.252
0.034 – 0.044
87
Rb/86Sr
147
Sm/144Nd
176
Lu/177Hf
0.010 – 0.094
0.117 – 0.178
up to 0.011
Source Age
0.010 – 0.052
0.178 – 0.209
0.019 – 0.030
of the MORB Source: 2Ga
0.010 – 0.038
0.010 – 0.031
0.199 – 0.219
0.209 – 0.224
0.029 – 0.036
0.034 – 0.039
87
Rb/86Sr
147
Sm/144Nd
176
Lu/177Hf
0.037 – 0.121
0.103 – 0.164
up to 0.007
Source Age
0.037 – 0.079
0.164 – 0.195
0.015 – 0.025
of the MORB Source: 3Ga
0.037 – 0.065
0.037 – 0.058
0.185 – 0.205
0.195 – 0.210
0.024 – 0.031
0.029 – 0.034
238
36.5 – 61.3
2.9 – 3.2
117 – 177
21.8 – 33.7
3.1 – 3.4
74 – 103
U/204Pb
232
Th/238U
232
Th/204Pb
Pb Evolution: Single Stage
16.9 – 24.6
14.5 – 20.0
3.17 – 3.5
3.3 – 3.6
59 – 79
52 – 66
a
The range of the isotopic composition of the HIMU source is: 87Sr/86Sr = 0.7027 – 0.7033; 143Nd/144Nd = 0.5128 – 0.5130; 176Hf/177Hf =
0.2828 – 0.2830; 206Pb/204Pb = 20 – 22; 208Pb/204Pb = 39.5 – 41. The isotopic evolution of the MORB reservoir is described in detail in appendix B,
see spread sheets ‘‘basalt + sed_recycling’’ and ‘‘P/D_HIMUsource’’ and auxiliary material for details of the calculations.
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m (238U/204Pb), k (232Th/238U), and w (232Th/204Pb)
in the HIMU source overlap those of fresh MORB
only to a small degree (Figures 2d–2f; Tables 1
and 2; [Hart and Staudigel, 1989; Hauri and Hart,
1993]). Only the highest m and w values observed
in fresh MORB would be suitable for the HIMU
source, and only for recycling ages >1.5–2 Ga
(compare Table 2 and Figure 2), but most MORB
have m and w that are too low for the HIMU source
(see distribution in Figures 2d and 2e). High m
correspond to high w values in MORB, but neither
m nor w correlate with k. The k of the HIMU source
must be higher than in most modern MORB
(Figure 2f ). Therefore, the HIMU source must be
depleted in Pb relative to Th and U, but also
depleted in U relative to Th or enriched in Th
relative to U compared to modern MORB.
[ 15 ]
176
Although oceanic gabbros have similar
Lu/177Hf ratios than MORB, they have lower
87
Rb/86Sr, m, k, and w and also slightly lower
147
Sm/144Nd ratios, based on the two estimates of
average gabbro compositions given in Table 1
[Hart et al., 1999; Zimmer et al., 1995] (Figures
2a–2f). Due to their lower m and k values (lower
than those of BE), ancient oceanic gabbros develop
even more unradiogenic Pb isotope ratios than the
basalts (206Pb/204Pb < N-MORB, Figures 1b–1d,
Table 1; [Hart et al., 1999]). The gabbros also
develop slightly lower Sr and Nd isotopic composition than the basalts. The 143Nd/144Nd ratios of
the gabbros tend to be too low for HIMU basalts, at
least for recycling ages >1 Ga (Figures 1a, 1c, and
1e, Table 2).
[16] In summary, although some trace element
ratios in altered or fresh oceanic basalts and gabbros are suitable for the HIMU source, the low w in
all these rocks lead to 208Pb/204Pb ratios that are
lower than the 208Pb/204Pb ratios in most OIB, but
especially HIMU-type basalts (see Figures 1b, 2,
Tables 1 and 2). This conclusion is almost independent of the assumed recycling/source age.
Therefore, removal of the alteration signature during subduction [Hauri and Hart, 1993] is insufficient to transform recycled oceanic crust into
appropriate OIB and especially HIMU sources.
The fact that all the components of the oceanic
crust, altered and fresh basalts, and oceanic gab-
10.1029/2001GC000223
bros, have low w also indicates that any mixture
between these components cannot be a suitable
source for OIB without further modification during
subduction.
3.1.3. Effects of Subduction-Zone
Processing on Igneous Oceanic Crust
[17] Oceanic crust is modified during sub-arc processing by preferential extraction of some elements
via fluid-basalt interaction. In the following, it is
investigated whether the resulting subducted crust
can be a suitable source of OIB, and HIMU-type
basalts in particular.
[18] We use four examples for input compositions
of the oceanic crust: the N-MORB of Hofmann
[1988], the average oceanic gabbro of Hart et al.
[1999] and the altered crust of Staudigel et al.
[1995, 1996] (Pb in the altered crust is estimated
by assuming Ce/Pb = 25). In addition, we estimate an average composition of the bulk igneous
crust (basalt and gabbros) by mixing basalt and
gabbros in equal proportion and assuming that
the basaltic portion consists to equal parts of
altered and fresh oceanic crust (bulk crust =
25% fresh N-MORB + 25% altered MORB +
50% gabbro; Tables 1 and 3). It is then calculated
how the P/D ratios of the oceanic crust have to
change during subduction in order to transform
the oceanic crust into appropriate HIMU sources
(assuming an age of 2Ga) and whether the
necessary changes are in agreement with current
constraints on sub-arc alteration mechanisms
[e.g., Ayers, 1998; Ayers et al., 1997; Brenan et
al., 1995; Kogiso et al., 1997; McCulloch and
Gamble, 1991; Stalder et al., 1998]. The P/D
ratios of all four compositions are given in Table
1 and Figure 2. For comparison Figure 2 also
shows the range of P/D ratios for a 2 Ga old
HIMU source (recycling/source age = 2Ga,
stippled field in Figure 2).
3.1.3.1. Rb-Sr System
[19] Comparing the 87Rb/86Sr ratios of all four
examples (N-MORB, gabbro, altered crust, and
bulk crust) with the range of 87Rb/86Sr ratios for a
2 Ga old HIMU source (Figure 2c) shows that
except for the gabbros, the 87Rb/86Sr ratios have
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Table 3. Estimates of Trace Element Mobility During Sub-Arc Magmatism Used in Figures 1, 3, 4, and 8a
Cs
Rb
Ba
Th
U
Nb
Ta
La
Ce
Pb
Nd
Sr
Zr
Hf
Sm
Eu
Ti
Gd
Dy
Y
Er
Yb
Lu
Bulk Igneous
Crust
Mobility
Bulk Subducted
Igneous Crust
GLOSS
Mobility
Fluid-Sediment
Mobility
Melt-Sediment
Sub-Arc Modified
GLOSS (sed-melt)
0.051
2.99
13.87
0.142
0.115
2.03
0.129
3.83
11.95
0.48
9.55
136
82
2.28
3.11
1.13
8212
4.24
5.19
29.1
3.14
3.03
0.45
51%
81%
53%
38%
76%
4%
4%
56%
51%
81%
22%
41%
22%
22%
14%
8%
6%
5%
4%
2%
1%
1%
1%
0.025
0.57
6.59
0.088
0.027
1.95
0.124
1.68
5.89
0.09
7.45
81
64
1.78
2.69
1.04
7735
4.03
5.01
28.5
3.13
2.99
0.45
3.48
57.2
776
6.91
1.68
8.94
0.63
28.8
57.3
19.9
27
327
130
4.06
5.78
1.31
3716
5.26
4.99
29.8
2.92
2.76
0.41
28%
40%
44%
15%
17%
21%
22%
33%
35%
24%
37%
38%
10%
11%
33%
33%
29%
28%
23%
20%
19%
17%
16%
43%
34%
32%
31%
31%
26%
27%
24%
23%
13%
21%
46%
47%
45%
20%
20%
24%
19%
19%
19%
19%
19%
20%
1.97
37.8
528
4.76
1.17
6.65
0.46
21.8
44.4
17.2
21.4
178
69
2.23
4.60
1.04
2826
4.25
4.04
24.2
2.35
2.24
0.33
a
The bulk oceanic crust is derived by mixing 25% altered MORB [Staudigel et al., 1996] + 25%N-MORB [Hofmann, 1988] + 50%gabbro; [Hart
et al., 1999]. GLOSS is the ‘‘global subducted sediment’’ as given by Plank and Langmuir [1998]. Mobility estimates for sub-arc basalt alteration
are taken from Kogiso et al. [1997] but are modified to acquire HIMU isotopic signatures for a recycling age of 2Ga. Mobility estimates for fluidsediment exchange at 700C and melt-sediment exchange at 900C are derived according to the parameters given by Johnson and Plank [2000]
(their Table 1 and 8, formula for mobility as given in their paragraph 38).
to decrease. In case of the fresh basalts (NMORB), however, it is clear from their overall
compositional variability (Figure 2c and Tables 1
and 2) that the required amount of Rb loss
decreases with decreasing 87Rb/86Sr of the basalt.
Some fresh basalts with the lowest 87Rb/86Sr
observed even require a slight increase in 87Rb/
86
Sr (Figure 2c). The required decrease in 87Rb/
86
Sr for the altered crust has to be an order of
magnitude higher than for the fresh basalts (Figure
2c; Table 1). The necessary Rb loss has to be even
more severe in case the initial Sr isotopic composition is raised by seawater alteration (see Figure
1a). As a consequence, the Rb gained during
alteration (no Sr is gained [Staudigel et al.,
1995, 1996]) has to be removed almost quantitatively from the altered oceanic crust during subduction.
[20] Note that the discussion above is valid only
for a recycling and source age of 2Ga. For a given
source age, calculated 87Rb/86Sr ratios of the
HIMU source increase with decreasing recycling
age (decreasing initial isotopic composition). For
a given recycling age, 87Rb/86Sr ratios of the
HIMU source increase with increasing source
age (decreasing initial isotopic composition; see
Table 2). Therefore, required changes in 87Rb/86Sr
ratios of the oceanic crust are systematically
different as a function of recycling/source age.
However, relatively old source ages (>1.7 Ga) are
required based on the Sr isotope evolution of the
Bulk Earth (BE) and the present-day isotopic
composition of MORB, because the initial
87
Sr/86Sr ratio of the BE reservoir becomes greater
than that of present-day MORB roughly between
1.7 and 2.2 Ga ago (for present-day MORB
values of 0.7020 and 0.7027 and present-day BE
of 0.7050).
[21] Rb partitions preferentially into subduction
zone fluids compared to Sr [e.g., Ayers, 1998;
Ayers et al., 1997; Brenan et al., 1995; Keppler,
1996; Kogiso et al., 1997; McCulloch and Gamble,
1991]. Estimates of bulk fluid-slab partition coefficient ratios (DRb/DSr), a measure of the relative
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mobility of Rb and Sr, differ significantly: DRb/DSr
= 0.2 [Ayers, 1998], DRb/DSr = 0.5 [McCulloch and
Gamble, 1991]). With the given partition coefficients for Dslab/fluid [Ayers, 1998; Brenan et al.,
1995; McCulloch and Gamble, 1991; Stalder et al.,
1998] and assuming the fraction of the fluid interacting with the oceanic crust is 5%, the calculated
Rb loss is between 26 and 49% and the Sr loss is
between 3 and 30%. The Rb and Sr loss increases
proportionally with the amount of fluid. Kogiso et
al. [1997] estimate a Rb and Sr loss of 63% and
41%, respectively. Thus, sub-arc alteration lowers
the 87Rb/86Sr ratios of the oceanic crust, but due to
the range of Dslab/fluid and the dependence on the
amount of fluid interacting with the oceanic crust,
the exact magnitude of this effect is uncertain.
[22] Note that for all the studies considered, the
amount of Rb loss relative to Sr loss is insufficient
to account for the required Rb loss (relative to Sr)
of the altered crust, indicating that altered crust
alone cannot be an appropriate HIMU source, even
after modification during subduction. This conclusion is independent of the assumed recycling and
source age. For the bulk igneous crust, the Rb
budget is dominated by the altered crust, so that the
necessary amount of Rb loss depends critically on
the proportion of the altered crust (see Figure 2c).
For the bulk crust composition considered here,
approximately three times as much Rb compared to
Sr has to be removed in order to get 87Rb/86Sr
ratios similar to those of the HIMU source (source
and recycling age = 2Ga), which is within the
range of the experimental estimates of Rb and Sr
mobility.
3.1.3.2. Sm-Nd and Lu-Hf Systems
[23] Overall, for the four examples considered,
147
Sm/ 144 Nd ratios are close to the required
147
Sm/144Nd ratios of a 2 Ga old HIMU source
(Figure 2a, and compare Tables 1 and 2). The
147
Sm/144Nd ratios of the basalt, gabbro, and the
bulk crust have to increase, while the 147Sm/144Nd
ratios of the altered oceanic crust have to decrease
slightly (Figure 2a). The 176Lu/177Hf ratios of all
four compositions (fresh or altered basalts, gabbros, and bulk crust) have to increase during
subduction (Figure 2b, Tables 1 and 2).
10.1029/2001GC000223
[24] As for Rb and Sr, however, the necessary
changes in 147Sm/144Nd and 176Lu/177Hf ratios
are dependent on the assumed recycling/source
age. 147Sm/144Nd and 176Lu/177Hf ratios have to
decrease with decreasing recycling ages (for any
given source age). For any given recycling age,
older source ages require lower 147Sm/144Nd and
176
Lu/177Hf ratios in the subducted crust to be
suitable HIMU sources (Figure 1e, Tables 1, and
2; see also Salters and White [1998]). Since the
147
Sm/144Nd and 176Lu/177Hf ratios of the oceanic
crust are not expected to change significantly
during hydrothermal alteration, the range of
147
Sm/144Nd and 176Lu/177Hf ratios of the fresh
oceanic crust and gabbros (Tables 1 and 2, Figures
2a, 2b) should be representative of the oceanic
crust in general. Based on the most abundant
147
Sm/144Nd and 176Lu/177Hf ratios in MORB
(0.19 147Sm/144Nd 0.22; 0.020 176Lu/177Hf
0.028; see distribution in Figures 2a and 2b), the
147
Sm/144Nd ratios of the oceanic crust are within
the range of appropriate values for the HIMU
source only for recycling ages 1.5 Ga, while
176
Lu/177Hf ratios are within the range of HIMU
values only for recycling ages 1.5 Ga (compare
Tables 1 and 2).
[25] Unfortunately, few direct studies of the behavior of Lu and Hf during subduction processes are
available, so that their behavior is inferred from
that of chemically similar elements (other heavy
rare earth elements (HREE, e.g., Yb) or Y and Zr,
respectively). The behavior of the high field
strength elements (HFSE) Zr and Hf depends
strongly on the amount of rutile in the subducted
slab [Ayers, 1998; Brenan et al., 1994, 1995;
McCulloch and Gamble, 1991; Stalder et al.,
1998]. HFSE are very efficiently retained by rutile,
so that even small amounts of rutile (<1%) can be
sufficient to fractionate the REE from the HFSE
[e.g., Brenan et al., 1995; Stalder et al., 1998].
Available experimental D’s indicate that Zr (Hf ) is
less efficiently retained in the slab than the HREE
(Lu, inferred from Y and Yb), but not necessarily
the MREE (e.g., Sm), despite the presence of
residual rutile: DZr/Yb = 0.044, DZr/Sm = 1 [Stalder
et al., 1998]; DZr/Y = 0.041, DZr/Sm = 0.2 [Ayers,
1998] (Stalder et al. [1998] assume 1.5% residual
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rutile, Ayers [1998] assumes 1% residual rutile). In
case of the study of Stalder et al. [1998], Zr (Hf ) is
less efficiently retained in the oceanic crust than
Yb (Lu), thus increasing the Lu/Hf in the oceanic
crust, but leaving the Sm/Hf ratio unchanged.
Using the Ds of Ayers [1998] both Y (Lu) and
Sm do not fractionate from Zr (Hf ), despite DZr/Y
and DZr/Sm 6¼ 0, because the absolute values of
DSm, DY, and DZr are so high that, effectively, Sm,
Y and Zr (Sm, Lu and Hf ) do not fractionate during
sub-arc alteration for a large range of fluid/slab
ratios. Contrary to that, empirical slab/fluid partition coefficients [McCulloch and Gamble, 1991]
indicate that Zr (Hf ) is more effectively retained in
the slab than Sm and Yb (Lu) so the Sm/Zr and Yb/
Zr (Sm/Hf, Lu/Hf ) ratios in the oceanic crust
decrease during subduction.
[26] Experimental studies of subduction zone processes indicate that 147Sm/144Nd ratios in the
oceanic crust increase during subduction [Ayers,
1998; Kogiso et al., 1997; Stalder et al., 1998], in
agreement with empirically determined Dslab/fluid
[McCulloch and Gamble, 1991], but the magnitude
of this increase is difficult to estimate. Dslab/fluid are
rarely given for both Sm and Nd, but experimental
and empirical studies indicate a higher mobility of
the LREE compared to the MREE and HREE
[Ayers, 1998; Kogiso et al., 1997; McCulloch and
Gamble, 1991; Stalder et al., 1998]. The relative
differences between D slab/fluid for LREE and
MREE are large: DLa/DSm 0.035 [Stalder et
al., 1998] and 0.01 [Ayers, 1998]. Absolute values
of D’s are also variable and can change from values
<1 to values >1 for La [Ayers, 1998; Stalder et al.,
1998] and can be two orders of magnitude different
for Sm. Kogiso et al. [1997] estimate that about
14% Sm and 31% Nd (56% La) are extracted from
the oceanic crust.
[27] Based on the above discussion, 147Sm/144Nd
of the oceanic crust increase during sub-arc alteration, although there is significant uncertainty
regarding the magnitude of that increase. Assuming that 147Sm/144Nd ratios between 0.19 and 0.22
are typical of the oceanic crust (see distribution in
Figure 2a) and that 147Sm/144Nd further increase
during subduction requires recycling ages to be
>1.5 Ga (see Figure 2a and compare Tables 1 and
10.1029/2001GC000223
2). The presence of residual rutile is critical to LuHf fractionation during sub-arc alteration. Although HFSE (e.g., Hf ) are compatible in rutile,
experimental slab-fluid partitioning studies suggest
that Hf can be more or equally effectively extracted
from the oceanic crust than either Lu or Sm, even
in case of residual rutile (see above). Thus,
176
Lu/177Hf (and Sm/Hf ) ratios in the subducted
slab are expected to increase in the oceanic crust
during subduction. Most 176Lu/177Hf in the fresh
oceanic crust are between 0.020 and 0.028 (Figure
2b), and are therefore too low to be suitable HIMU
sources for recycling ages >1.5 Ga (see Figure 2b
and compare Tables 1 and 2). Therefore, because
Sm-Nd systematics suggest recycling ages >1.5Ga,
subducted crust is likely to be an appropriate
HIMU source only for the case where subduction
alteration leads to increasing 176Lu/177Hf in the
oceanic crust. Decreasing the 176Lu/177Hf ratios in
the oceanic crust during subduction as suggested
by empirical slab-fluid partition coefficient estimates [McCulloch and Gamble, 1991], on the other
hand, would lead to very low 176Lu/177Hf ratios in
the subducted crust which are only suitable for the
HIMU source in case of recycling ages 1 Ga
(Table 2). These young recycling ages, however,
are difficult to reconcile with the recycling ages
>1.5 Ga inferred from the Sm-Nd systematics (see
above).
3.1.3.3. U-Th-Pb System
[28] For the N-MORB, gabbro, and the bulk crust
m, w, and k are too low compared to a 2 Ga old
HIMU source (Figures 2d–2f ). For the altered
crust, w and k are too low, while m is too high
(Figures 2d–2f ). Thus for all four examples, w and
k have to increase to become suitable HIMU
sources, and except for the altered crust, m also
has to increase. In contrast to other isotopic systems, this observation is almost independent of the
assumed recycling age, as only some MORB with
the highest w and k overlap those of the HIMU
source (see distribution in Figures 2e and 2f; Table
2). Note that different source ages have little
influence on the initial isotopic composition of
the oceanic crust, because single or two stage
source evolution models are very similar (see
appendix B.2).
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[29] Experimental slab-fluid partition coefficient
ratios are between 51 and 81 for DU/DPb, and
between 41 and 49 for DTh/DPb [Ayers, 1998;
Brenan et al., 1994, 1995]. Kogiso et al. [1997]
estimate that about 85% Pb, 29% U, and 38% Th
are extracted from the oceanic crust. All experimental studies agree that Pb behaves more incompatibly than U and Th, thus increasing w and m in
the oceanic crust, although the magnitude of this
increase is highly variable [Ayers, 1998; Brenan et
al., 1994, 1995; Kogiso et al., 1997].
[30] Partition coefficients estimated based on the
composition of arc basalts suggest that U behaves
more incompatible than Th and Pb during subduction (DU/DTh = 0.5, DU/DPb = 0.5; [McCulloch and
Gamble, 1991]). Independent evidence that U
behaves more incompatible than Th during subarc alteration is provided by the 238U excesses
observed in arc lavas. The 238U excesses in arclavas are commonly attributed to metasomatism by
a fluid derived from the subducted crust which is
enriched in U relative to Th, i.e. Th is more
efficiently retained in the subducted slab than U
[e.g., Elliott et al., 1997; Gill and Williams, 1990,
1997; Turner et al., 1996]. Experimentally estimated slab-fluid partition coefficient ratios DU/DTh
depend on the amount of garnet in the subducted
slab (eclogite) and the oxygen fugacity during slabfluid interaction [Brenan et al., 1994, 1995]. DU/
DTh decreases with decreasing abundance of garnet
in the subducted crust and increasing oxygen
fugacity [Brenan et al., 1994, 1995]. DU/DTh is
<1, i.e. U behaves more incompatible than Th, at
high oxygen fugacities and modal proportions of
garnet in the oceanic crust (=eclogite) <60%
[Brenan et al., 1994, 1995]). The given DU/DTh
of Ayers [1998] depends strongly on the amount of
rutile (DUrutile = 89, DThrutile = 0.1) and can change
from values >1 for modal abundances of rutile
>0.5% to values <1 for modal abundances of rutile
<0.5%. In the study of Kogiso et al. [1997], 29% U
and 38% Th are extracted from the oceanic crust,
suggesting that U is less incompatible than Th.
Thus, although there is some disagreement on the
relative partitioning of Th and U during sub-arc
processes, the 238U excesses [Elliott et al., 1997;
Gill and Williams, 1990, 1997; Turner et al., 1996]
10.1029/2001GC000223
observed in island arc lavas are considered to be
strong evidence that U is more efficiently extracted
from the oceanic crust during subduction than Th.
[31] The k values in fresh oceanic crust (e.g., NMORB, gabbro and bulk crust) are lower than
required for the HIMU source, almost independent
of the assumed age of the HIMU source (Figure
2f ). That U behaves less incompatible during subarc alteration than Th is therefore also required for
recycled oceanic crust to become suitable HIMU
sources. The m and w in most basalts (Figures 2d
and 2e) are also lower than required for the HIMU
source, and the inferred increase of m and w during
sub-arc alteration can transform subducted oceanic
crust into suitable HIMU sources if the magnitude
of increase of m and w (and k) is appropriate for the
assumed recycling/source age. The large variability
in existing estimates, however, prevents a more
precise estimate of possible compositions and
recycling ages. In the altered oceanic crust m is
too high and w is too low to be an appropriate
HIMU source. Assuming that little Th is extracted
from the oceanic crust during subduction, w can
only increase by preferential extraction of Pb
compared to Th. This in turn leads to increasing
m, and U has to be extracted almost quantitatively
from the altered oceanic crust to yield appropriate m
and w values. At the same time U extraction is
limited by the k values, so that it is difficult to find
U, Th and Pb mobility parameters that are in
agreement with experimental studies (see above)
and at the same time transform altered oceanic
crust into suitable HIMU sources.
3.1.4. The Trace Element Composition
of Recycled Igneous Oceanic Crust
[32] Since the preferential removal of some elements during subduction (especially Pb, but also
alkali elements and U) leads to distinct trace
element signatures, another possible test for the
plausibility of recycled oceanic crust as HIMU
source comes from its trace element patterns. Ce/
Pb and Nb/U ratios in HIMU basalts reported by
Chauvel et al. [1992, 1997] are higher than those in
typical OIB [Newsom et al., 1986] and are at least
qualitatively consistent with the required Pb and U
loss during subduction.
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1000
calculated OIB melt
PUM normalized
100
10
calculated OIB source
1
average Tubaii basalt
average St. Helena basalt
average Tristan da Cunha basalt
average Society basalt
0.10
Cs Rb Ba Th U Nb Ta La Ce Pb Nd Sr Zr Hf Sm Eu Ti Gd Dy Y Er Yb Lu
Figure 3. Shown is the trace element pattern of a calculated OIB source and its melt. The composition of the
subducted crust is equal to those of the subduction-modified bulk oceanic crust (see section 3.1.2 and Tables 1 and 3),
the degree of melting is 1% and assumes fractional melting of an eclogitic crust in the garnet-stability field (see
spread sheet ‘‘TE_OIBmelts’’, and appendix B and auxiliary material for details of the calculations). The trace
element pattern of the calculated OIB melt agrees fairly well with those of average HIMU basalts from Tubaii
(compiled from the GEOROC database; http://georoc.mpch-mainz.gwdg.de) and St. Helena [Chaffey et al., 1989]
(see also see spread sheet ‘‘TE_OIBmelts’’). Also shown are average EMI basalts from Tristan da Cunha (compiled
from the GEOROC database; http://georoc.mpch-mainz.gwdg.de) and EMII basalts from Society [White and
Duncan, 1995]. Normalizing values are the primitive mantle values (PUM) of McDonough and Sun [1995].
[33] As one example for a possible HIMU source
derived from recycled oceanic crust, we have taken
our estimate of the bulk oceanic crust composition
(Tables 1 and 3) and assume a source and recycling
age of 2Ga. The estimates of element mobility
during sub-arc alteration are based on the mobility
estimates of Kogiso et al. [1997], but are modified
in order to achieve average present-day HIMU
isotopic signatures (Table 3). The required changes
in P/D ratios are large: 238U/204Pb, 232Th/204Pb,
232
Th/238U, 147Sm/144Nd, and 176Lu/177Hf have to
increase by 26%, 228%, 160%, 11%, and 28%,
respectively, and 87Rb/86Sr has to decrease by 68%
(Table 3).
[34] Melting of the subducted crust directly as an
eclogitic assemblage produces trace element patterns similar to those of HIMU basalts for a degree
of melting of about 1%, although with more
pronounced Nb and Ta anomalies. In case the
oceanic crust is assumed to be admixed to depleted
or slightly enriched mantle (assuming less than
25% basalt in the mixture), the trace element
patterns are also broadly similar to those of modern
HIMU basalts (Figure 3), but even lower degrees
of melting (<0.5%), depending on the amount of
basalt in the mixture, are required (see also spread
sheet ‘‘TE_OIBmelts.xls’’). Note that in the latter
case, mixing of the subducted crust with ambient
mantle will produce isotopic mixing trends
between HIMU and the composition of the ambient
mantle. For both cases, the trace element compositions of OIB require very low degrees of melting,
even if produced entirely from enriched sources
such as the subducted oceanic crust, which is in
contrast to the larger degrees of melting inferred
from major element systematics (several percent).
3.1.5. Recycling Igneous Oceanic Crust:
Summary and Conclusions
[35] It is virtually impossible to find a composition
of the present-day oceanic crust that yields a
HIMU-like isotopic composition without further
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modification during sub-arc processing. This conclusion mainly stems from the Th/Pb systematics in
the oceanic crust, which indicate that, almost
independent of the assumed recycling age, the
Th/Pb in most ridge basalts are too low to be
comparable to those in the HIMU source. Thus,
subduction plays a paramount role in understanding crust-mantle recycling and the composition of
ancient recycled oceanic crust.
[36] The Rb/Sr ratios decrease while the Sm/Nd
(and likely the Lu/Hf ) ratios of the oceanic crust
increase during subduction. All experimental studies agree that Pb is preferentially removed compared to U and Th, thus increasing the U/Pb and
Th/Pb ratio. The U-Th fractionation is dependent
on factors such as the abundance of garnet and
rutile in the subducted slab, and the oxygen fugacity, but U-Th disequilibrium studies in island arc
volcanics indicate that U is lost preferentially compared to Th during subduction (Th/U increases),
which is required for recycled basalts to become
suitable HIMU sources.
[37] Although there has been substantial progress
in quantifying element transfer during sub-arc
processing of the oceanic crust, and experimental
studies agree in most cases on the relative fractionation of elements, the magnitude of fractionation of parent-daughter elements pairs remains
uncertain. Furthermore, there are few constraints
on likely recycling ages and depending on the
relative proportion of altered and fresh basalt and
gabbro in the bulk subducted oceanic crust, the
composition of the oceanic crust can be quite
variable. Thus, it remains hard to make a general
estimate of the present-day isotopic composition
of ancient subduction-modified oceanic crust, as
for all three relevant parameters (age, composition before and after subduction-processing) it
remains plausible to assume a large range of
values.
[38] Some conclusions, however, are possible
based on the analyses in section 3.1: (1) Hydrothermally altered basalt alone is unlikely to be a
source of HIMU basalts, because Rb (U) loss
during sub-arc processing is insufficient to transform hydrothermally altered crust into suitable
10.1029/2001GC000223
HIMU sources. (2) If the HIMU source mainly
consists of subducted basalts, then very low
degrees of melting (approximately <1%) are
required based on the trace element systematics
of HIMU-like basalts. (3) Subducted oceanic crust
is only a likely HIMU source in case Lu/Hf ratios
of the oceanic crust increase during subduction. (4)
Ancient subducted crust can only be a HIMU
source if U and Pb are removed preferentially to
Th from the subducted slab. (5) There are likely to
be large variations in the isotopic composition of
ancient oceanic crust as a function of recycling
age, composition and subduction modification
(Figure 1). Thus, ancient subducted crust is likely
to have a range of isotopic compositions. A narrow
range of isotopic compositions (e.g., similar to
HIMU: St. Helena and Austral plume) can only
result if similar material is processed in a similar
way at about the same time. Otherwise, oceanic
crust with different initial composition and age
would have to be processed differently as a function of time and composition, but always in such a
way that the end-product has similar isotopic
signatures.
3.2. Recycling of Oceanic Crust Plus
Sediment
3.2.1. Modeling the Isotopic Composition
of Recycled Crust Plus Sediment
[39] In order to model the effects of sediment
subduction, 2 and 10% of subduction-modified
sediment with different compositions (we use the
28 bulk sediment compositions of Plank and Langmuir [Plank and Langmuir, 1998]) are mixed with
subducted oceanic crust). We use the subducted
bulk igneous crust derived in the previous section (a
mix of altered and fresh basalts and gabbro), which
has HIMU isotopic signatures (Table 3). Recycling
and source ages of 2 Ga are assumed for both the
basalt and the sediment (a single stage evolution for
the sediment reservoir is assumed in order to model
the initial isotopic composition of the sediment; see
appendix and spread sheet ‘‘basalt + sediment_recycling’’) (auxiliary material). To estimate the composition of subducted sediments, mobility estimates
for sediment-melting at 900C are used (Table 3),
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which are calculated according to the parameters
given by Johnson and Plank (their Tables 1 and 8,
formula for mobility as given in paragraph 38),
since there is some geochemical evidence that
sediments melt during subduction [e.g., Elliott et
al., 1997; Johnson and Plank, 2000].
[40] Sediment melt-interaction during subduction
changes the Sm/Nd and Th/U ratios by less than
1% [Johnson and Plank, 2000]; U/Pb and Th/Pb
decrease by about 20% relative to unmodified
sediments, whereas the Rb/Sr and Lu/Hf ratios
increase by about 21% and 47%, respectively
(Table 2). Sediment-fluid exchange [Johnson and
Plank, 2000] results in an increase of Th/U, U/Pb,
Th/Pb and Sm/Nd by about 3%, 9%, 11%, and 6%,
whereas Rb/Sr and Lu/Hf decrease by 3% and 6%
compared to unmodified sediments, respectively
(Table 2). Thus, the changes caused by sedimentmelt or sediment-fluid interaction lead to relatively
minor differences in the isotopic composition compared to unmodified sediments (at least for recycling ages of 2Ga, see below). In contrast, required
changes to transform the bulk igneous crust into a
suitable HIMU source are substantially larger (see
above).
[41] Rather than mixing basalts and sediment, most
previous models [e.g., Chauvel et al., 1992; Hart
and Staudigel, 1989; Rehkämper and Hofmann,
1997] calculate the isotopic composition of the
sediment end-member only, implying that the
composition of the bulk recycled crust (sediment
and basalt) is dominated by the sediment. While
this is true for the most incompatible elements,
which have significantly higher concentrations in
sediments compared to basalts (Cs, Rb, K, U, Th,
Pb,. . .), especially the middle and heavy REE (SmLu) and some HFSE (e.g., Zr, Hf ) have similar
concentrations in sediments and basalts (Figure 5).
As a consequence, mixtures of basalt and sediment
can differ significantly in composition and develop
different isotopic compositions compared to the
pure sediment (see spread sheet ‘‘basalt+sediment_recycling’’ which gives both the basalt and sediment end-member and a range of mixtures between
the basalt and sediment). Results of the modeling
of basalt and sediment are shown in Figure 4 (see
figure caption, appendix B and auxiliary material,
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and spread sheet ‘‘basalt + sed_recyling’’ for
details of the modeling).
3.2.2. Effect of Sediment Addition
[42] Compared to recycled igneous crust alone
(section 3.1), the effect of sediment addition is
largest for Pb and Sr isotopes, and smallest for Nd
and Hf isotopes. Note that the difference in concentration between average MORB [Hofmann,
1988] and average subducted sediment (GLOSS
[Plank and Langmuir, 1998]) decreases in the order
Pb > Sr > Nd > Hf: Pbsed/Pbbasalt 40; Srsed/Srbasalt
2.9; Ndsed/Ndbasalt 2.4; Hfsed/Hfbasalt 1.4
(Figure 5). Compared to MORB, sediments have
similar Lu/Hf and Th/Pb ratios; Sm/Nd and U/Pb
ratios are lower, and Rb/Sr and Th/U ratios are
higher than in the basalts (Figure 6). Sediment-melt
interaction during subduction leaves the Th/U and
Sm/Nd ratios relatively unchanged, but increases
the Rb/Sr and Lu/Hf ratios and decreases the U/Pb
and Th/Pb ratios. As a consequence, the differences
in P/D ratios and resulting isotopic differences in
ancient recycled sediments and basalts are enhanced
by sub-arc alteration.
[43] Because of the low U/Pb ratio and the high U
and Pb concentrations in sediments compared to
basalt, the Pb isotopic composition of the bulk
subducted crust decreases dramatically as a function of the amount and composition of the added
sediment. As for MORB, U/Pb ratios in the sediments correlate positively with the Th/Pb ratios, but
neither U/Pb nor Th/Pb correlate with the Th/U
ratios. Since U/Pb and Th/Pb ratios are decreased
by similar amounts during sediment-melt interaction, low 206Pb/204Pb ratios in most ancient recycled sediments are associated with low 208Pb/204Pb
ratios. The low values of Th/Pb in the sediments
(values comparable to MORB) and the high Pb
contents therefore lead to 206,208Pb/204Pb ratios in
subducted sediments that are, in general, lower than
those in OIB, even if the sediment is mixed with
basalts with HIMU-like compositions as for the
examples in Figure 4 and even for small amounts
of sediment (<5 –10% sediment). Note that the
estimates of Pb mobility during subduction of
Johnson and Plank [2000] are subject to technical
difficulties, so that the changes in U/Pb and Th/Pb
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0.5134
MORB
a
0.5132
143Nd/144Nd
0.5130
EMII
0.5128
HIMU
0.5126
0.5124
GLOSS
EMI
0.5122
average SAS
0.5120
0.702
0.704
0.706
0.708
0.710
0.712
0.714
87Sr/86Sr
42
208 Pb/204 Pb
41
b
HIMU
EMII
40
10% sediment
REE rich sediments
SiO2 rich sediments
CaO rich sediments
other sediments
EMI
39
38
2% sediment
REE rich sediments
SiO2 rich sediments
CaO rich sediments
other sediments
37
MORB
36
subduction-modified MORB
35
15
16
17
18
19
20
21
22
23
24
206 Pb/204 Pb
Figure 4. Various isotope ratio diagrams show the effects of sediment recycling on the present-day Sr, Nd, Hf, and
Pb isotopic composition of ancient bulk subducted crust. 2 and 10% of subduction-modified sediment [Johnson and
Plank, 2000; Plank and Langmuir, 1998] are mixed with the bulk igneous crust (see section 3.1.2 and Tables 1 and
3). Calculations assume a recycling age of 2Ga. Also shown are the mixing lines between the bulk igneous crust and
average slowly accumulating sediment (SAS = ‘‘pelagic’’ sediment, see spread sheet ‘‘basalt + sediment_recycling’’)
and global subducted sediment (GLOSS [Plank and Langmuir, 1998]), respectively. Tick marks are for 2 and 10%
sediment. For further details of the calculation see section 3.2.1., spread sheet ‘‘basalt + sediment_recycling’’, and
appendix B and auxiliary material. See section 3.2 for a detailed discussion. Plotted MORB and OIB data and
references as in Figure 1.
ratios during subduction might be different [see
Johnson and Plank, 2000 for details]. However,
206
Pb/204Pb-208Pb/204Pb systematics in OIB would
require the subducted sediments to have higher U/
Pb and Th/Pb but lower Th/U ratios. This is only
possible if the Pb mobility is increased and at the
same time the U but not the Th mobility decreased,
which is considered unlikely.
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0.5134
143Nd/144Nd
c
MORB
0.5132
0.5130
0.5128
HIMU
0.5126
EMII
0.5124
EMI
0.5122
0.5120
15
16
17
18
19
20
21
22
23
24
206Pb/204Pb
0.714
d
0.712
87Sr/86Sr
0.710
0.708
EMII
EMI
0.706
HIMU
0.704
MORB
0.702
15
16
17
18
19
20
21
22
23
24
206 Pb/ 204 Pb
Figure 4. (continued)
[44] The higher Rb/Sr and lower Sm/Nd ratios in
sediments compared to basalts lead to very high
87
Sr/86Sr and low 143Nd/144Nd ratios in ancient
subducted sediments. In addition, the Sr (and to
a lesser degree the Nd) concentrations in sediments are substantially higher than those in
basalts (see above), so that the combined Sr
and Nd isotope systematics in the ancient
recycled crust are very sensitive to the amount
of sediment. Note that calculated 87Sr/86Sr in the
bulk subducted crust are only within the range of
OIB values for small amounts of sediment (<5%;
Figures 4a and 4d).
[45] Calculated 143Nd/144Nd ratios in the bulk
subducted crust fall well within the range of those
in OIB, and in particular, EMI and EMII type
basalts. However, combined Nd-Sr and Nd-Pb
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0.2836
MORB
0.2834
177Hf/176Hf
0.2832
EMII
0.2830
HIMU
0.2828
0.2826
EMI
e
0.2824
0.2822
0.5120 0.5122 0.5124 0.5126 0.5128 0.5130 0.5132 0.5134
143 Nd/ 144 Nd
Figure 4. (continued)
isotope systematics (Figures 4a, 4c, and 4e) are
rarely appropriate for OIB, which is an effect of
the low Pb and high Sr isotope values of the
subducted sediment. 176Hf/177Hf ratios in the bulk
subducted crust for all but the special cases of
sediments with high Lu/Hf ratios deviate little
from those in the pure oceanic crust (see below,
Figure 4e) and are by itself not a very sensitive
tracer of subducted sediments. Combined Nd and
Hf isotope systematics show that in case less than
about 10–15% subducted sediments are mixed
with a basalt that will develop HIMU signatures,
resulting Nd and Hf isotope ratios tend to plot
within or slightly below the ‘‘mantle array’’ in a
Nd-Hf isotope diagram (Figure 4e). Nd and Hf
isotope compositions that plot above the ‘‘mantle
array’’ only result in case large amounts of sediment (>10–15%) and/or sediments with high Lu/
Hf ratios are mixed with the oceanic crust (see
below, Figure 4e).
3.2.3. Differences in Isotopic Composition
Due to Different Sediments?
[ 46 ] The compositions of EMI and EMII-type
basalts have often been explained by recycling of
sediments. Both EMI and EMII-type basalts are
characterized by high 87Sr/86Sr and low 143Nd/144Nd
and 176 Hf/ 177 Hf ratios; the 143 Nd/ 144 Nd and
176
Hf/177Hf ratios are systematically lower in EMI
compared to EMII-type basalts. The most significant
isotopic difference between EMI and EMII-type
basalts are their Pb isotope systematics: EMI-type
basalts have variable 206Pb/204Pb, and high and
almost constant 208 Pb/ 204 Pb ratios; EMII-type
basalts have variable 208Pb/204Pb for relatively constant 206Pb/204Pb ratios with intermediate values
(see Figures 1 and 4). The time integrated Th/U
ratios in both EMI and EMII-type basalts (as
expressed by (208Pb/206Pb)* = (208Pb/204Pbmeasured
29.476)/(206Pb/204Pbmeasured 9.307); [Galer
and O’Nions, 1985]) are variable, but are higher in
EMI than in EMII.
[47] These isotopic differences between EMI and
EMII-type basalts have often been attributed to
different sediment compositions; ‘‘pelagic’’ sediments have been favored for the EMI source and
‘‘terrigenous’’ sediments for the EMII source (see
references given in the introduction). While the
classification into pelagic and terrigeneous sediments is a useful genetic classification, sediments
in either category display a large range of chemical
compositions relative to the total compositional
variability observed in marine sediments, so that
this classification is less useful for geochemical
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subducted sediments
PUM-normalized
100
10
1
N-MORB, Hofmann, 1988
GLOSS
REE-rich sediments
CaO-rich sediments
SiO2-rich sediments
0.1
Cs Rb Ba Th U Nb Ta La Ce Pb Nd Sr Zr Hf SmEu Ti Gd Dy Y Er Yb Lu
Figure 5. Plot of the trace element patterns of bulk subducted sediments [Plank and Langmuir, 1998]. Most trace
elements in bulk subducted sediments show about a ten-fold variation (gray area), with decreasing variability with
increasing compatibility (compatibility increases from left to right). Subducted sediments dominated by slowly
accumulating sediments (e.g., red clays with a large proportion of metalliferous components or phosphate) are REE
enriched. Sediments with a large proportion of biogenic siliceous or carbonate components are characterized by
depletions in HSFE (e.g., Hf, Zr, Nb, Ta, Th, Ti), and carbonate dominated sediments show characteristic Sr and Ba
enrichments and Rb and Cs depletions. Shown for comparison is the average N-MORB of Hofmann [1988].
Normalizing values are the primitive mantle values of McDonough and Sun [1995].
purposes [see Plank and Langmuir, 1998]. In
general, the geochemical composition of marine
sediments is determined by three factors: the abundance of biogenic and detrital phases, the nature of
the source of the detrital components, and the
sedimentation rate (see Plank and Langmuir
[1998] for a detailed discussion). Systematic differences in the isotopic composition of the bulk
recycled crust as a function of sediment type, as
suggested to explain the differences between the
EMI and EMII sources, are only expected in case
the P/D ratios in sediments (e.g., Rb/Sr, Sm/Nd, U/
Pb. . .) vary systematically as a function of sediment type.
[48] Subducted sediments dominated by slowly
accumulating sediment (e.g., red clays with a large
proportion of metalliferous components or phosphate; referred to as ‘‘SAS’’ = Slowly Accumulating Sediments in the following) scavenge REE
from seawater and are enriched in REE compared
to most sediments [e.g., Ben Othman et al., 1989;
Plank and Langmuir, 1998] (Figure 5). They are,
therefore, characterized by distinct trace element
ratios between REE and other trace elements, such
as high La/Nb, or (Lu, Sm)/Hf ratios (Figures 5
and 7). SAS also tend to have some of the lowest
U/Pb and Th/Pb ratios for the bulk sediment
compositions considered here. Carbonate dominated biogenic sediments show characteristic Sr
and Ba enrichments and Rb and Cs depletions
leading to high Sr/Nd and Ba/Th ratios, and the
lowest Rb/Sr ratios of all sediments (Figures 5 and
7). Silica dominated biogenic sediments are
depleted in Sr (Figure 5). Both sediments dominated by biogenic siliceous or carbonate components are also depleted in HFSE relative to most
sediments due to the low concentrations of HFSE
in seawater and the incompatibility of HFSE in the
mineralogic structure of marine organisms. Thus,
both SAS and most, but not all, biogenic sediments
have distinctively higher Lu/Hf ratios compared to
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a
160
30
20
10
120
80
40
0
0
0.10
0.14
0.18
0.22
0.26
Frequency sediment/MORB
MORB
Frequency MORB
MORB
d
30
sediment
20
10
0
0
0.30
4
8 12 16 20 24 28 32 36 40 44 48 52
1 4 7 S m / 1 4 4 Nd
2 3 8 U / 2 0 4 Pb
40
b
30
MORB
20
0.1-0.47
10
200
20
100
0
0.04
sediment
10
0
0.02
0.06
0.08
0
0.1
0
10 20
30 40 50
1 7 6 L u / 1 7 7 Hf
60 70 80 90 100
2 3 2 T h / 2 0 4 Pb
20
20
120
MORB
c
Frequency sediment
range in MORB
16
Frequency sediment
e
30
sediment
0
MORB
Frequency sediment/MORB
300
Frequency MORB
Frequency sediment
40
12
sediment
8
4
0
f
16
12
sediment
8
Frequency MORB
Frequency sediment
40
200
40
sediment
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0.6
1.0
1.4
1.8
2.2
2.6
3.0
3.4
87Rb/86Sr
40
4
0
0
0 0.2
80
0
2
4
6
8
10
12
14
16
232Th/238U
Figure 6. Histograms showing the variation and frequency of parent daughter (P/D) ratios in present-day MORB
glasses as compiled from the LDEO petrological database http://petdb.ldeo.columbia.edu/petdb) compared to the
range of P/D ratios in modern subducted sediments [Ben Othman et al., 1989; Plank and Langmuir, 1998]. Sm/Nd
and U/Pb ratios in modern sediments are lower, whereas Rb/Sr ratios are substantially higher than those in MORB.
Lu/Hf and Th/Pb ratios in MORB and sediments show a similar range and Th/U ratios in the sediment tend to be
higher and are more variable than those in MORB.
other sediments (Figures 5–7), and, upon aging,
will develop high 176Hf/177Hf ratios. The typically
high Lu/Hf ratios of SAS and most biogenic sediments, however, are the only distinct difference in
P/D ratios as a function of sediment type (Figure
7), and most of the bulk sediment compositions
considered here [Plank and Langmuir, 1998] show
no systematic variation of P/D ratios with sediment
type (Figures 6 and 7).
[49] SAS have often been classified as ‘‘pelagic’’
sediments in previous studies [e.g., Ben Othman et
al., 1989; Chauvel et al., 1992; Rehkämper and
Hofmann, 1997; Weaver, 1991]. While the Sr-Nd
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0.125
0.125
0.100
0.100
0.075
0.075
0.050
0.050
0.025
0.025
0.000
0.000
0.1
0
0.11 0.12 0.13 0.14 0.15 0.16 0.17
0.125
0.125
0.100
0.100
0.075
0.075
0.050
0.050
0.025
0.025
0.000
0
4
8
10
20
30
40
50
232Th/204Pb
147Sm/144Nd
176Lu/177Hf
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16
0.000
0.01
0.1
1
10
87Rb/86Sr
238U/204Pb
Plank and Langmuir, 1998
REE rich sediments
SiO2 rich sediments
CaO rich sediments
other sediments
Ben Othman et al., 1989
Figure 7. Plot of parent-daughter (P/D) ratios in modern subducted sediments [Ben Othman et al., 1989; Plank and
Langmuir, 1998]. Some of the bulk subducted sediments [Plank and Langmuir, 1998] have high Lu/Hf ratios due
either to REE enrichment in slowly accumulating sediments or HFSE depletion in CaO and SiO2 rich sediments, but
have otherwise similar P/D ratios.
isotope evolution of ‘‘average’’ SAS roughly follows the trend of EMI basalts (Figure 4a), they
develop the most radiogenic Hf isotope ratios of all
sediments (Figure 4e). The Hf-Nd isotope characteristics of SAS are only similar to those of EMI
and EMII-type basalts for small amounts of sediment (<10%). SAS also have the most unradiogenic Pb isotope ratios of all sediments, for both
206
Pb/204Pb and 208Pb/204Pb (Figures 4b, 4c, and
4d). The most characteristic feature of EMI-type
basalts, the high and constant 208Pb/204Pb ratios for
given 206Pb/204Pb, are therefore not a typical feature of ancient ‘‘pelagic’’ sediments (see Figures 4b
and 5–7; see also [Ben Othman et al., 1989; Plank
and Langmuir, 1998]). Moreover, of all sediments,
SAS are not only the presently least abundant but
are generally also of very small volume [Plank and
Langmuir, 1998], so that recycling large amounts
of such sediments over geologic time seems
unlikely.
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10
calculated OIB source:
igneous crust + 5%sediment
PUM
normalized
100
1
calculated igneous crust
+ 2%GLOSS
calculated igneous crust only
average Tristan da Cunha basalt
average Society basalt
0.1
Cs Rb Ba Th U Nb Ta La Ce Pb Nd Sr Zr Hf Sm Eu Ti Gd Dy Y Er Yb Lu
Figure 8. Shown are the trace element patterns of calculated OIB sources, which are derived from melting of
recycled oceanic crust plus various amounts of average subduction-modified sediment (0, 2, and 5%; sediment =
GLOSS [Johnson and Plank, 2000; Plank and Langmuir, 1998]). Also shown is the melt of the OIB source
consisting of the bulk igneous crust (see section 3.1.2 and Tables 1 and 3) and 5% of sediment. Calculations assume a
degree of melting of 4%, fractional melting of an eclogitic assemblage in the garnet-stability field. See Table 3 for
basalt and sediment compositions and for mobility estimates. See spread sheet ‘‘TE_OIBmelts’’, and appendix B and
auxiliary material for details of the calculations. With the exception of Pb, the trace element pattern of the calculated
melt agrees fairly well with those of average EMI basalts from Tristan da Cunha (compiled from the GEOROC
database; http://georoc.mpch-mainz.gwdg.de) and EMII basalts from Society [White and Duncan, 1995] (see also
spread sheet ‘‘TE_OIBmelts’’). Normalizing values are the primitive mantle values of McDonough and Sun [1995].
[50] As an analog for ‘‘terrigeneous’’ sediments
[Ben Othman et al., 1989], we take the average
composition of global subducted sediments
(GLOSS [Plank and Langmuir, 1998]). GLOSS
and the other non-SAS sediments form a trend that
is roughly parallel to those of EMII-type basalts in
a Nd-Sr isotope diagram (Figure 4a), but for the Sr
isotope ratios to be similar to those of EMII-type
basalts, the amount of sediment in the bulk subducted crust needs to be smaller than about 10%.
GLOSS does not develop the radiogenic Hf isotope
composition of SAS. The Nd-Hf isotope compositions of GLOSS and the other non-SAS sediments
form a trend away from HIMU toward the ‘‘mantle
array’’. Sediment proportions >10% are needed to
develop compositions with higher 176Hf/177Hf
ratios for given 143Nd/144Nd ratios than those of
the ‘‘mantle array’’. The 206Pb/204Pb and 208Pb/
204
Pb ratios in non-SAS type sediments vary
widely, but are generally lower than in most OIB.
Sediments with a small range in U/Pb but variable
Th/Pb ratios would be required to reproduce the
variable 208Pb/204Pb for almost constant 206Pb/
204
Pb ratios that are typical of EMII-type basalts,
but no type of sediment with these characteristics
can presently be identified.
3.2.4. Trace Element Composition of
Subducted Oceanic Crust Plus Sediment
[51] The distinct trace element concentrations of
subducted sediments relative to those of fresh or
subduction-modified basalts have a considerable
leverage on the trace element composition of the
bulk subducted crust (see spread sheet ‘‘TE_OIBmelts’’). Figure 8 shows the effect of addition of 2,
5, and 10% of sediment (GLOSS [Plank and
Langmuir, 1998]) to the igneous oceanic crust
(the bulk igneous crust as calculated in section
3.1, see Table 3). The effect of sediment addition is
largest for Pb; only about 3% of sediment erase the
pronounced negative Pb anomaly of the subducted
igneous crust. Cs, Rb, Ba, Th, U, and La, and Ce
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are also strongly affected by the addition of small
amounts of sediment (<10%), whereas Nb, Ta, the
MREE and HREE, and Sr, Zr, and Hf are hardly
affected. Therefore, sediment addition to the igneous crust makes the trace element pattern steeper,
causes positive Pb anomalies, and for >5% sediment, Nb and Ta become depleted relative to
neighboring elements.
[52] Melting of the subducted crust with minor
amounts of sediment (<5%) directly as an eclogitic
assemblage produces trace element patterns similar
to those of EM-type basalts for degrees of melting
<5%, although with positive Pb anomalies and
more depleted HREE. In case the bulk subducted
crust is assumed to be admixed to depleted or
slightly enriched mantle, the trace element patterns
can also be similar to those of modern EM-type
basalts. In this case, different combinations of the
amount of sediment in the bulk subducted crust and
the amount of bulk subducted crust in the mantle
source (<25%) are possible, but lower degrees of
melting (<1%) are required compared to melting of
the subducted crust as an eclogitic assemblage,
depending on the amount of bulk subducted crust
in the mixture (see also spread sheet ‘‘basalt +
sediment_recycling’’). As in case of directly melting the bulk subducted crust, however, positive Pb
anomalies and depletions in Nb and Ta result. Note
that mixing of the bulk subducted crust with
ambient mantle will produce isotopic mixing trends
between and the composition of the bulk subducted
crust and those of the ambient mantle.
[53] The trace element patterns of average HIMU
and EM-type basalts are very similar (Figure 3, see
spread sheet ‘‘TE_OIBmelts’’). Sediment addition
to the igneous subdcuted crust produces steeper
trace element patterns than for the igneous crust
alone. Therefore, progressively larger degrees of
melting are required with increasing amount of
sediment to account for the similar slopes of the
trace element patterns of EM-and HIMU-type
basalts, in case the isotopic difference between
HIMU and EM-type basalts is assumed to be the
addition of sediment. However, even for only a few
percent of sediment in the bulk subducted crust
(>3%), positive Pb anomalies result, which is
10.1029/2001GC000223
contrary to the negative Pb anomalies observed in
EM-type basalts.
3.2.5. Recycling of Oceanic Crust Plus
Sediment: Summary and Conclusions
[54] The initial compositional difference between
sediments and the igneous oceanic crust (Figure 6)
is enhanced by subduction modification. The subduction of minor amounts of sediment (<10%) in
addition to the igneous oceanic crust leads to lower
Nd and Pb and higher Sr isotope ratios, whereas Hf
isotope ratios are, in general, relatively little changed
compared to recycled basalt alone (Figure 4). Of all
the isotopic systems investigated, the combined Sr
and Nd isotope systematics of the bulk subducted
crust (igneous crust + sediments) are most compatible with the isotopic composition of EM-type
basalts. The Nd and Sr isotope composition of
average SAS (often labeled ‘‘pelagic’’ sediment)
seems to follow the trend of EMI-type basalts,
whereas other sediment compositions (including
GLOSS [Plank and Langmuir, 1998], often labeled
‘‘terrigeneous’’sediments) are more compatible with
the trend of EMII-type basalts. The trends of EM-type
basalts in Sr-Pb and Nd-Pb isotope space could also
be interpreted as being part of a mixing hyperbola
between a HIMU source (consisting of recycled
basalt) and sediment. The Pb isotope values, however, are too unradiogenic to be compatible with this
interpretation. As for subduction of the igneous
oceanic crust alone, the Pb isotope systematics are
therefore most difficult to reconcile with the notion of
ancient recycled crust in OIB sources. Even if mixed
with a HIMU source as for the examples in Figure 4,
the high Pb contents and low U-Th/Pb ratios in
sediments lead to very unradiogenic Pb isotope
ratios, even for small amounts of sediment (<5%).
Both sediment-melt and sediment-fluid alteration
appear to affect the Th/Pb and U/Pb to a similar
degree, so that the U/Pb and Th/Pb characteristics
of the sediments are preserved during subduction
processing. Therefore, the required P/D characteristics of EMI and EMII-type basalts (variable U/Pb
but relatively constant Th/Pb for EMI and variable
Th/Pb but relatively constant U/Pb for EMII-type
basalts) would have to be an inherent characteristic of
specific types of sediment, but no such group of
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sediments can presently be identified. As a consequence, the characteristic Pb isotope features of EMI
and EMII basalts are difficult to reconcile with
ancient recycled sediment in their source.
[55] The trace element pattern of the bulk subducted crust (igneous crust + sediment) are sensitive to the amount of added sediment. The addition
of sediment to the igneous crust results in low Nb
and Ta and high Pb concentrations relative to
neighboring elements, and, in general, in a progressively steeper trace element pattern with
increasing amount of sediment (Figure 8). Increasing amounts of sediment in the source therefore
require an increasing degree of melting (F) to
maintain a constant slope of the trace element
pattern of the associated melt. EM and HIMU-type
basalts have trace element patterns with very similar
slopes (Figure 3; see also spread sheet ‘‘TE_OIBmelts’’). Thus, if the difference between HIMU and
EM-type basalts is assumed to be a larger amount of
sediment in the EM sources, higher F are required
for EM-type basalts to compensate for the steeper
trace element pattern of their sources. Because most
OIB underwent substantial amounts of fractional
crystallization, it is difficult, however, to find independent support for this notion in the major element
chemistry of EM and HIMU-type basalts.
[56] As for the subducted igneous crust alone, the
ancient subducted crust including sediment is
expected to show large variability as a function
of composition, age, and differences in subduction
modification. Thus, as in case of the HIMU signature, creating similar isotopic signatures by recycling oceanic crust plus sediment requires that very
specific sediment compositions have to be recycled
at similar times in order to yield similar isotopic
signatures upon aging.
4. Conclusions
[57] Creating HIMU sources by recycling subduction-modified oceanic crust requires substantial
modification of the trace element budget of the
oceanic crust during subduction. The required
modifications are qualitatively consistent with
dehydration processes in subduction zones, but a
better quantitative understanding of subduction
10.1029/2001GC000223
zone processes is needed for a more precise estimate of the final composition of the subducted
igneous crust. Although the addition of sediment
tends to produce signatures which broadly point
toward EM-type OIB for the Sr and Nd isotope
system, suggesting that the sources of EM-type
basalts may contain a component with crustal
affinities, this notion is difficult to reconcile with
the Pb isotope systematics.
[58] No matter if sediment is involved or not,
recycling of oceanic crust results in a large isotopic
variability as a function of composition, recycling
age and possible differences in subduction alteration. As a consequence, if the isotopic signatures
of OIB are attributed to recycling of oceanic crust
(with or without sediment), each OIB source is
likely to represent a unique combination of composition, age and subduction modification of the
ancient recycled crust. Even for this scenario,
however, the grouping of OIB into several ‘‘families’’ or groups with similar isotopic composition
[e.g., White, 1985; Zindler and Hart, 1986] suggests that crustal recycling cannot be an entirely
random process, but that there must either be some
broadly reproducible combination of composition,
age and alteration, or that there is some as of yet
unknown physical and/or chemical process that
aids in reducing the apparent variability of the
subducted crust.
[59] Attributing mantle heterogeneity to a few endmembers (e.g., HIMU, EMI and EMII), which
ultimately originate from crustal recycling, on the
other hand, requires a very specific combination of
age and composition for each end-member. Therefore, one single event for each proposed endmember, rather than continuous subduction and
recycling of oceanic crust into the Earth’s mantle
would have to be invoked to attribute mantle
heterogeneity to a few end-members in the context
of crustal recycling. These constraints, however,
can be somewhat relaxed when taking into account
that the isotopic composition of the proposed endmembers might range to more extreme values than
those observed in OIB. In this case, a larger
number of different recycling ages and/or compositions may be allowed. Alternatively, the alteration, sedimentation, and subduction processing of
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oceanic crust could be a by far better defined
process than observation would seem to indicate,
resulting in surprising uniformity among discrete
parcels of recycled material, or, the intramantle
disaggregation and mixing of compositionally
diverse recycled materials has an averaging affect
and leads to the small number of relatively welldefined enriched compositions evidenced in
MORB and OIB isotopic trajectories. In this case,
physical and chemical processes have to be identified which are responsible for creating the different end-member compositions.
[60] It should be pointed out that based on the
assumption that crustal recycling is the major
processes to create mantle heterogeneity, each
view, those of only a few or those of many different
isotopic components in the mantle, appears to
require that the mantle operates in a very different
manner. Assuming only a few end-members
requires that the original variability of the oceanic
crust is averaged whereas the creation of many
different components requires the ability to preserve the heterogeneity of the subducted crust on
some level over millions to billions of years.
Either view has to be compatible with geophysical constraints on the preservation of chemical
heterogeneities in the mantle and only a combination of geochemical and geophysical constraints can be successful in developing a more
focused picture of mantle heterogeneity and mantle dynamics.
Appendix A: Os Isotopes as a Tracer
of Crustal Recycling
[61] A detailed quantitative analytical treatment of
crustal recycling for Os isotopes is not presented.
This is because owing to the relatively limited
amount of available data and the challenging
analytical task, the Os isotope composition (and
evolution) of the mid ocean ridge basalt (MORB)
and Bulk Earth (BE) reservoir is not as well constrained as for other isotopic systems (Sr, Nd, and
Pb isotopes), and a quantitative treatment is even
more problematic (see below; see Hauri and Hart
[1993] and Roy-Barman and Allègre [1995] for
quantitative modeling).
10.1029/2001GC000223
[62] 187Os/188Os ratios in OIB higher than those in
BE have often been regarded as a unique tracer for
recycled material in the Earth’s mantle because to
date, Re/Os ratios higher than those in BE have
only been observed in crustal rocks (oceanic and
continental [e.g., Hauri and Hart, 1993; Hauri et
al., 1996; Lassiter and Hauri, 1998; Reisberg et
al., 1993; Roy-Barman and Allègre, 1995; Widom
et al., 1999; Widom and Shirey, 1996]). However,
recent data for continental and oceanic rocks
[Peucker-Ehrenbrink, 2001] show that it is difficult to distinguish between ancient oceanic crust
and continentally derived material based on Os
isotope ratios, and while Os isotope ratios may be
a good indicator of recycled material in general,
they do not appear to be discriminative between
different types of recycled materials (oceanic crust
and sediment). Also, extremely high Re/Os ratios
( 200) recently reported in oceanic gabbros lead
to very radiogenic 187Os/188Os ratios (187Os/188Os
= 1) over geologically short time (50m.y.) [Hart et
al., 1999] suggesting that such a component
cannot be present in pristine form in any OIB
source.
[63] Furthermore, recent Os isotope studies in OIB
have cast some doubt on the uniqueness of attributing radiogenic 187Os/188Os ratios to recycled
crust. It has been suggested that the radiogenic
187
Os/188Os ratios in OIB can also be a signature of
core-mantle interaction [e.g., Bennett et al., 1996;
Brandon et al., 1998, 1999; Walker et al., 1995;
Widom and Shirey, 1996]. An ‘‘enriched plume
component’’ with radiogenic Os isotope values
(187Os/188Os = 0.130 0.135) common to many
OIB and equivalent to the lower mantle has also
been invoked [Shirey and Walker, 1998; Widom
and Shirey, 1996]. As an alternative to both the
crustal recycling and core-mantle interaction models, melting of variable amounts and/or compositions of residual sulfide have recently been
suggested as an explanation for the Os isotope
and Re and platinum group element (PGE) systematics in Hawaiian basalts [Bennett et al., 2000].
[64] Therefore, although Os isotope systematics are
a potentially powerful tracer of ancient recycled
crust, alternative explanations have to be considered and it even appears possible that radiogenic
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187
Os/188Os values in OIB may not be a unique
feature of recycled crust.
Table A.1. MORB Source Evolution: A Comparison
of Different Approachesa
Age
(Ga)
Appendix B: Quantitative Modeling
of the Composition of Ancient Recycled
Oceanic Crust
[65] In order to determine the present-day isotopic
composition of ancient recycled crust, the age and
the initial isotopic composition, as well as the
parent-daughter ratio (P/D) for each isotopic system investigated have to be known. The following
is a brief description of how each of these variables
is determined using the most recent constraints on
the processes involved and a comparison with the
approach taken in previous studies. The provided
spread sheets give the reader access to all the
calculations and input parameters. The reader is
encouraged to change the modeling parameters
according their own preference and to investigate
the influence on the final composition of the
recycled crust. For each spread sheet, a ‘‘user
guide’’ is provided in auxiliary material which
explains its general outline. Remaining questions
about the modeling and the use of the spread sheets
should be addressed to the first author.
B.1. Age of the Recycled Crust
[66] The age of the recycled crust plays a crucial
role, as it is one of the main factors that determine
its initial isotopic composition. Unfortunately, due
to the lack of more detailed constraints on mantle
dynamics, the age of the recycled oceanic crust is
essentially a free parameter. However, as shown by
Hauri and Hart [1993], the Pb isotopic composition
of MORB and HIMU basalts indicate that recycling
ages are likely to be between 0.77 and 2.1Ga.
B.2. Initial Isotopic Composition of the
Recycled Crust
B.2.1. Oceanic Crust
[67] For each isotopic system, the initial isotopic
composition of the recycled basalts is assumed to
be equal to those of the MORB source at the time
of recycling. The Pb isotope evolution of the
This
study
Hart and Chauvel Rehkämper Hauri and
Staudigel et al. and Hofmann
Hart
87
0
1
2
3
4
4.5
0.7027
0.7026
0.7024
0.7022
0.7016
0.7010
0.7004
0.6997
Sr/86Sr
0.7026
0.7018
0.7010
0.7002
0.6994
0.6990
0.7022
0.7023
0.7024
0.5132
0.5117
0.5102
0.5087
0.5072
0.5067
0.5133
0.5117
0.5100
143
0
0.5132
1
0.5116
2
0.5100
3
4
4.55
0.5133
0.5118
0.5104
0.5089
0.5074
Nd/144Nd
0.5132
0.5117
0.5102
0.5087
0.5072
0.5067
0.7025
0.7017
0.7010
0.7002
0.6994
0.6990
176
0
1
2
0.28335
0.28243
0.28148
0
1
2
3
4
4.55
17.72
16.34
14.73
12.86
10.66
9.31
17.30
16.00
14.48
12.71
10.64
0
1
2
3
4
4.55
37.34
35.76
34.10
32.36
30.52
29.48
37.19
35.63
33.99
32.26
30.45
Hf/177Hf
0.28355
0.28253
0.28148
206
Pb/204Pb
17.61
16.25
14.67
12.81
10.65
9.31
Pb/204Pb
37.25
35.68
34.04
32.32
30.51
29.48
17.72
16.34
14.73
12.86
10.66
9.31
?
?
?
?
?
?
37.76
36.09
34.34
32.51
30.58
29.48
?
?
?
?
?
?
208
a
Numbers in italics indicate the source age of the MORB source,
which is the age where the MORB source was derived from the Bulk
Earth (BE) reservoir, bold numbers give the present-day isotopic
composition of the MORB source. For details see appendix B.
References are as follows: Hart and Staudigel [1989], Chauvel et al.
[1992], Rehkämper and Hofmann [1997], and Hauri and Hart [1993].
MORB source is modeled by a single stage evolution starting from Bulk Earth (BE) at 4.55 Ga ago
with m = 238U/204Pb = 8.2 and 232Th/238U = 3.8,
similar to the approach taken in previous studies
[e.g., Chauvel et al., 1992; Rehkämper and Hofmann, 1997; Roy-Barman and Allègre, 1995]
(Table A.1). Hart and Staudigel [1989] model the
Pb isotopic composition of the recycled oceanic
crust by a two stage evolution model with a first
stage equivalent to BE evolution between 4.55 and
4.0 Ga and a second stage evolution from BE at 4.0
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to present-day MORB ( 206 Pb/ 204 Pb = 17.3,
208
Pb/204Pb = 37.3l; Table A.1 [Allègre et al.,
1983a, 1983b]). Hart and Staudigel [1989] point
out that no substantial difference in the Pb isotopic
evolution of the MORB mantle results if a single
stage evolution is preferred and Table A.1 shows
that the two approaches differ only slightly. In the
spread sheets, the Pb evolution is readily changed
to a two stage evolution similar to that used by
Hart and Staudigel [1989] and allowing for a
variable age for the beginning of the second stage.
However, it should be kept in mind that all these
possible Pb evolution models are likely to be an
oversimplification, because recent models predict
an open system evolution of the mantle with
respect to its U (Th) and Pb concentration, and
consequently its Pb isotope composition [Elliott et
al., 1999; Galer and O’Nions, 1985; White, 1993].
[68] The initial Sr, Nd, and Hf isotope composition
of ancient oceanic crust is modeled by a two stage
evolution of the MORB reservoir, with a first stage
equivalent to BE evolution between 4.55 and XGa
and a second stage evolution from BE at XGa to
present-day MORB (87Sr/86Sr = 0.7027, 143Nd/
144
Nd = 0.5132, and 176Hf/177Hf = 0.28335 (BE
values today: 87Sr/86Sr = 0.705, 143Nd/144Nd =
0.512638, 176Hf/177Hf = 0.2828; see spread sheet
‘‘basalt + sed_recycling’’ and Table A.1). The
beginning of the second stage can be chosen
according to personal preference, but has been
chosen to be 2 Ga for the examples shown in the
Figures 1 and 4. The implications of different
differentiation ages will be discussed and are readily investigated by choosing different source ages
of the MORB source in the spread sheet ‘‘basalt +
sed_recycling’’. Hauri and Hart [1993] have taken
a similar approach but use a slightly different
composition of the present-day BE and MORB
reservoir (BE values today: 87Sr/86Sr = 0.7047,
143
Nd/144Nd = 0.512638; 176Hf/177Hf = 0.28288;
MORB values today: 8 7 Sr/ 8 6 Sr = 0.7022,
143
Nd/144Nd = 0.5133, 176Hf/177Hf = 0.28355;
see spread sheet ‘‘basalt-recycling’’ and Table
A.1). Hart and Staudigel [1989] chose a source
age of 4 Ga similar to their Pb isotope evolution
(MORB reservoir today: 87 Sr/ 86 Sr = 0.7022,
143
Nd/144Nd = 0.5133, Table A.1 [Allègre et al.,
10.1029/2001GC000223
1983a, 1983b]). Rehkämper and Hofmann [1997],
on the other hand, use a single stage evolution
starting from BE at 4.55 Ga ago and assume
present-day MORB values of 87Sr/86Sr = 0.7025
and 143Nd/144Nd = 0.5132, similar to the approach
taken by Chauvel et al. [1992] (Table A.1).
[69] As shown in Table A.1, despite the different
approaches, the initial Pb and Nd isotopic composition of the MORB reservoir at any given age differ
only slightly. The initial Sr isotope composition, on
the other hand, differs significantly as a function of
the assumed source age of the MORB-source: the
older the source age, the lower the 87Sr/86Sr of the
MORB source at any given time. Furthermore,
depending on the value chosen for the present-day
MORB and BE reservoir, the initial 87Sr/86Sr ratio
of the BE reservoir becomes greater than that of
present-day MORB roughly between 1.7 and 2.2
Ga ago (for present-day MORB values of 0.7020
and 0.7027 and present-day BE of 0.7050), requiring relatively old source ages of the MORB source.
This should be kept in mind when adjusting the
source ages of the MORB source in the spread sheet
‘‘basalt + sed_recycling’’.
[70] Furthermore, the Sr isotope composition of the
MORB reservoir is affected by hydrothermal alteration processes at MOR [Staudigel et al., 1981b,
1995]. Based on the composition of fresh and
hydrothermally altered crust [Kawahata et al.,
1987, 1995; Staudigel et al., 1981a, 1996], it has
been estimated that about 20–30% of the Sr in the
altered basalts is replaced by seawater Sr [Hart and
Staudigel, 1989; Rehkämper and Hofmann, 1997].
The reader has the option whether or not to allow
for modification of the Sr isotope composition of
ancient MORB, and can apply variable exchange
rates and choose the isotopic composition of
ancient seawater according to their own preference
(see spread sheet ‘‘basalt + sed_recycling’’). For
the examples shown in Figure 1, a 25% exchange
rate as suggested by Rehkämper and Hofmann
[1997] is adopted, and the Sr isotopic composition
of ancient seawater is approximated using the
seawater evolution curve by Shields and Veizer
[2002] (see spread sheet ‘‘basalt + sed_recycling’’;
see also [Rehkämper and Hofmann, 1997]). Chauvel et al. [1992] have taken a different approach by
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simply increasing the initial 87 Sr/86Sr of the
recycled basalt by 0.5%, based on the 87Sr/86Sr
ratios in fresh and altered basalts given by Kawahata et al. [1987].
Table A.2. Sediment Source Evolution: A Comparison
of Different Approachesa
Age
(Ga)
This
study
87
B.2.2. Subducted Sediment
[ 71 ] The initial isotopic composition of the
recycled sediment is assumed to be equal to those
of the sediment source (i.e. the continental crust
(CC)) at the time of recycling. The reader can
choose between three different types of evolution
models for the sediment source in case of Sr, Nd,
and Hf isotopes (see spread sheet ‘‘basalt + sed_recycling’’). (1) A simple BE evolution model. The
BE evolution should provide a close estimate of the
initial isotopic composition for old recycling ages
(about 2Ga), but should be less appropriate for
young recycling ages (2Ga). In any case, the BE
evolution provides a maximum estimate of the
initial Nd and Hf and a minimum estimate of the
initial Sr isotopic composition. (2) A single stage
evolution starting from BE at Xga ago to average
present-day values of pelagic sediments (87Sr/86Sr
0.7200, 143Nd/144Nd 0.5122, 176Hf/177Hf 0.2824; the reader can adjust these values according to their own preference; see spread sheet
‘‘basalt + sed_recycling’’). Hart and Staudigel
[1989] use a similar model where the sediment
source is derived from BE 4 Ga ago and develops
with average crustal composition to present-day
values of average sediment (87Sr/86Sr 0.719,
143
Nd/144Nd 0.5103, 206Pb/204Pb 18.9, and
208
Pb/204Pb 38.9; Table A.2). This approach is
also similar to that used by Rehkämper and Hofmann [1997] and Chauvel et al. [1992] with the
difference that both studies use a single stage
evolution from BE at 4.55 Ga ago to present-day
average sediment values (87Sr/86Sr 0.7114 and
143
Nd/144Nd 0.5122; Chauvel et al. [1992] uses
a similar approach but different values for modern
sediments; Table A.2). (3) It can also be assumed
that the sediment source is derived from BE at Xga
ago and subsequently evolves with an average
crustal composition [Rudnick and Fountain,
1995] until the time of recycling, in which case
the initial isotopic composition of the sediment
depends on its crustal residence time.
0
1
2
3
4
4.55
Hart and
Staudigel
Chauvel
et al.
Sr/86Sr (terrigenous/pelagic sediment)
0.71901 0.76838/0.73381
0.71140
0.71429
0.70874
0.70951 0.71076
0.70605
0.70465
0.70331
0.69970
0.70053
0.69900
143
0
1
2
3
4
4.55
Rehkämper and
Hofmann
Nd/144Nd
0.51034
0.50962
0.50889
0.50816
0.50740
0.51222
0.51102
0.50981
0.50860
0.50737
0.50670
206
0
1
2
3
4
4.55
18.70
17.07
15.16
12.93
10.54
9.35
18.81
17.22
15.36
13.18
10.64
Pb/204Pb
18.70
17.07
15.16
12.93
10.54
9.35
208
0
1
2
3
4
4.55
a
38.63
36.76
34.80
32.74
30.63
29.52
38.84
36.89
34.85
32.71
30.45
Pb/204Pb
38.63
36.76
34.80
32.74
30.63
29.52
18.70
17.07
15.16
12.93
10.54
9.35
38.63
36.76
34.80
32.74
30.63
29.52
See notes in Table A.1.
[72] There are significant differences in the initial
Sr, Nd and Hf isotopic composition of the sediment
source depending on the preferred evolution
model. The effects of different initial isotopic
composition of the sediment reservoir can readily
be investigated by choosing between the three
different evolution models in the spread sheet
‘‘basalt + sed_recycling’’. Compared to BE values,
both the single stage evolution and evolution in the
CC lead to lower initial Nd and Hf and to higher Sr
isotope ratios for similar source ages. These different initial isotope ratios also lead to lower presentday Nd and Hf and higher Sr isotope ratios in the
bulk subducted crust. The slope of the mixing line
between pure oceanic crust and sediment, on the
other hand does not change as it depends only on
the compositional difference between oceanic crust
and sediment. Therefore, the main effect is to
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develop more enriched (crustal-like) isotopic compositions for a given recycling age. This effect is
most pronounced for Sr isotopes but is less significant for Nd and Hf isotopes (see spread sheet
‘‘basalt + sed_recycling’’).
[73] Contrary to the Sr, Nd, and Hf isotopes, the Pb
isotopic evolution of the CC is well recorded in
galenas and has been reproduced by the Stacey and
Kramers [1975] model. Therefore, the Stacey and
Kramers [1975] model is used here to estimate the
Pb isotopic evolution of the sediment reservoir,
analogous to Chauvel et al. [1992] and Rehkämper
and Hofmann [1997] (Table A.2). Again, two-stage
models similar to those chosen for Sr, Nd and Hf
isotopes are readily implemented in the spread
sheet ‘‘basalt + sed_recycling’’.
B.3. Trace Element Composition of the
Recycled Crust
B.3.1. Oceanic Crust
[74] In principle, any plausible basalt composition
can be used for the modeling. A variety of different
estimates of average MORB [Hofmann, 1988; Sun
and McDonough, 1989], present and ancient gabbros [Hart et al., 1999; Zimmer et al., 1995] and
altered oceanic crust [Staudigel et al., 1995, 1996]
is used in the examples presented in Figure 1 and
listed in the spread sheet ‘‘basalt + sed_recycling’’.
Compared to average MORB [Hofmann, 1988; Sun
and McDonough, 1989], hydrothermally altered
oceanic crust is characterized by an increase in
alkali element (Cs, Rb, K) and U concentrations,
whereas the rare earth elements (REE) and high
field strength elements (HFSE) remain relatively
unchanged [Staudigel et al., 1995, 1996] (see Table
1). Previous estimates of the altered oceanic crust
[Staudigel et al., 1981a, 1981b, 1983] used by Hart
and Staudigel [1989] have slightly lower U/Pb, Th/
Pb and Rb/Sr ratios but similar Sm/Nd and Lu/Hf
ratios. The gabbros [Hart et al., 1999; Zimmer et al.,
1995] have broadly similar composition to the
average MORBs [Hofmann, 1988; Sun and McDonough, 1989] but have lower U/Pb and Th/Pb ratios.
[75] Chauvel et al. [1992] and Rehkämper and
Hofmann [1997] and Hart and Staudigel [1989]
10.1029/2001GC000223
all use a single composition of altered oceanic
crust. Hart and Staudigel [1989] use an estimate
based on the studies of Staudigel et al. [e.g.,
Staudigel et al., 1981a, 1981b, 1983], whereas
Chauvel et al. [1992] and Rehkämper and Hofmann estimated the composition of the altered
oceanic crust by modifying average MORB [Hofmann, 1988]. Chauvel et al. [1992] decrease the Pb
and increase the U concentration by 56% and 12%,
respectively; other elements remain unchanged,
which is at odds with the increase in alkali elements (Cs, Rb, K) observed in the altered oceanic
crust [Staudigel et al., 1995; 1996]. Rehkämper
and Hofmann [1997] increase the U and Rb concentration by 15% and 40%, respectively, and
decrease the Pb concentration by 35%. Compared
to the altered oceanic crust [Staudigel et al., 1995;
1996] the estimates of Chauvel et al. [1992] and
Rehkämper and Hofmann result in significantly
lower Rb/Sr, U/Pb, and Th/Pb ratios but have
broadly similar Sm/Nd and Lu/Hf ratios.
B.3.2. Recycled Sediments
[76] Plank and Langmuir’s [1998] estimates of the
bulk composition of sediment approaching subduction zones are used in this study in conjunction
with Johnson’s and Plank’s [2000] estimates of
sub-arc modification.
[77] Hart and Staudigel [1989] used the compositions of recent pelagic sediments given by White et
al. [1985], whereas Chauvel et al. [1992] and
Rehkämper and Hofmann [1997] used averages
of the terrigenous and pelagic sediments given by
Othman et al. [1989] (which are also listed in the
spread sheet ‘‘basalt + sed_recycling’’). Because
the actual composition of subducted sediments
represents a composite of sediments of different
lithologies, the estimates of Plank and Langmuir
[1998] can be significantly different from those of
a single lithology.
Acknowledgments
[78] The editors B. White and H. Staudigel and two anonymous reviewers are thanked for thoughtful comments which
helped to improve the presentation of the manuscript and the
documentation of the modeling. A.S. was in part supported by
a HSP-III doctoral fellowship by the German Academic
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Exchange Service (DAAD). This work was partly supported
by NSF award EAR-0124965 to V.S.
References
Allègre, C. J., S. R. Hart, and J. F. Minster, Chemical structure
and evolution of the mantle and the continents determined by
inversion of Nd and Sr isotopic data I, Theoretical models,
Earth Planet. Sci. Lett., 66, 177 – 190, 1983a.
Allègre, C. J., S. R. Hart, and J. F. Minster, Chemical structure
and evolution of the mantle and the continents determined by
inversion of Nd and Sr isotopic data, II, Numerical experiments and discussion, Earth Planet. Sci. Lett., 66, 191 – 213,
1983b.
Armstrong, R. L., A model for the evolution of strontium and
lead isotopes in a dynamic Earth, Rev. Geophys., 6, 175 –
199, 1968.
Ayers, J. C., Trace element modeling of aqueous fluid-peridotite interaction in the mantle wedge of subduction zones,
Contrib. Mineral. Petrol., 132, 390 – 404, 1998.
Ayers, J. C., S. K. Dittmer, and G. D. Layne, Partitioning of
elements between peridotite and H2O at 2.0 – 3.0 GPa and
900 – 1100C, and application to models of subduction zone
processes, Earth Planet. Sci. Lett., 150, 381 – 398, 1997.
Barling, J., and S. L. Goldstein, Extreme isotopic variations in
Heard Island lavas and the nature of mantle reservoirs, Nature, 348, 59 – 62, 1990.
Beattie, P., The generation of uranium series disequilibria by
partial melting of spinel peridotite: Constraints from partitioning studies, Earth Planet. Sci. Lett., 117, 379 – 391, 1993.
Ben Othman, D., W. M. White, and J. Patchett, The geochemistry of marine sediments, island arc magma genesis, and
crust – mantle recycling, Earth Planet. Sci. Lett., 94, 1 – 21,
1989.
Bennett, V. C., T. M. Esat, and M. D. Norman, Two mantleplume components in Hawaiian picrites inferred from correlated Os-Pb isotopes, Nature, 381, 221 – 224, 1996.
Bennett, V. C., M. D. Norman, and M. O. Garcia, Rhenium
and platinum group element abundances correlated with
mantle source components in Hawaiian picrites: Sulphides
in the plume, Earth Planet. Sci. Lett., 183, 513 – 526,
2000.
Bizimis, M., V. J. M. Salters, and E. Bonatti, Trace and REE
content of clinopyroxenes from supra-subduction zone peridotites: Implications for melting and enrichment processes in
island arcs, Chem. Geol., 165, 67 – 85, 2000.
Blichert-Toft, J., F. A. Frey, and F. Albarede, Hf isotope evidence for pelagic sediments in the source of Hawaiian basalts, Science, 285, 879 – 882, 1999.
Brandon, A. D., M. D. Norman, R. J. Walker, and J. W. Morgan, 186Os-187Os systematics of Hawaiian picrites, Earth
Planet. Sci. Lett., 174, 25 – 42, 1999.
Brandon, A. D., R. J. Walker, J. W. Morgan, M. D. Norman,
and H. Prichard, Coupled 186Os and 187Os evidence for coremantle interaction, Science, 280, 1570 – 1573, 1998.
Brenan, J. M., H. F. Shaw, D. L. Phinney, and F. J. Ryerson,
Rutile-aqueous fluid partitioning of Nb, Ta, Hf, Zr, U and
10.1029/2001GC000223
Th: Implications for high-field strength element depletions in
island-arc basalts, Earth Planet. Sci. Lett., 128, 327 – 339,
1994.
Brenan, J. M., H. F. Shaw, F. J. Ryerson, and D. L. Phinney,
Mineral-aqueous fluid partitioning of trace-elements at
900C and 2.0 Gpa: Constraints on the trace-element chemistry of mantle and deep-crustal fluids, Geochim. Cosmochim. Acta, 59, 3331 – 3350, 1995.
Chaffey, D. J., R. A. Cliff, and B. M. Wilson, Characterization
of the St. Helena magma source, in Magmatism in the Ocean
Basins, edited by A. D. Saunders and M. J. Norry, Geol. Soc.
Spec. Publ., 42, 257 – 276, 1989.
Chase, C. G., Oceanic island lead: Two-stage histories and
mantle evolution, Earth Planet. Sci. Lett., 52, 277 – 284,
1981.
Chauvel, C., A. W. Hofmann, and P. Vidal, HIMU-EM: The
French-Polynesian Connection, Earth Planet. Sci. Lett., 110,
99 – 119, 1992.
Chauvel, C., W. McDonough, G. Guille, R. Maury, and
R. Duncan, Contrasting old and young volcanism in
Rurutu Island, Austral chain, Chem. Geol., 139, 125 – 143,
1997.
Eisele, J., M. Sharma, J. G. Galer, J. Blichert-Toft, C. W.
Devey, and A. W. Hofmann, The role of sediment recycling
in EMI inferred from Os, Pb, Hf, Nd, Sr isotope and trace
element systematics in the Pitcairn hotspot, Earth Planet.
Sci. Lett., 196, 197 – 212, 2002.
Elliott, T., T. Plank, A. Zindler, W. White, and B. Bourdon,
Element transport from slab to volcanic front at the Mariana
arc, J. Geophys. Res., 102, 14,991 – 15,019, 1997.
Elliott, T., A. Zindler, and B. Bourdon, Exploring the Kappa
Conundrum: The role of recycling in the lead isotope evolution of the mantle, Earth Planet. Sci. Lett., 169, 129 – 145,
1999.
Galer, S. J. G., and R. K. O’Nions, Residence time of thorium,
uranium and lead in the mantle with implications for mantle
convection, Nature, 316, 778 – 782, 1985.
Gill, J. B., and R. W. Williams, Th isotope and U-series studies
of subduction-related volcanic rocks, Geochim. Cosmochim.
Acta, 54, 1427 – 1442, 1990.
Grand, S. P., and S. Widiyantoro, Global seismic tomography:
a snapshot of convection in the Earth, Geol. Soc. Am. Today,
7, 1 – 7, 1997.
Halliday, A. N., A. P. Dickin, and A. E. Fallick, Mantle dynamics: A Nd, Sr, Pb and O isotopic study of the Cameroon
Line volcanic chain, J. Petrol., 29, 181 – 211, 1988.
Halliday, A. N., D. C. Lee, S. Tommasini, G. R. Davies, C. R.
Paslick, J. G. Fitton, and D. E. James, Incompatible traceelements in OIB and MORB and source enrichment in the
sub-oceanic mantle, Earth Planet. Sci. Lett., 133, 379 – 395,
1995.
Hart, S. R., Heterogeneous mantle domains: Signatures, genesis and mixing chronologies, Earth Planet. Sci. Lett., 90,
273 – 296, 1988.
Hart, S. R., J. Blusztajn, H. J. Dick, P. S. Meyer, and K.
Muehlenbachs, The fingerprint of seawater circulation in a
500-meter section of ocean crust gabbros, Geochim. Cosmochim. Acta, 63, 4059 – 4080, 1999.
31 of 33
Geochemistry
Geophysics
Geosystems
3
G
stracke et al.: recycling oceanic crust
Hart, S. R., and T. Dunn, Experimental Cpx melt partitioning
of 24 trace-elements, Contrib. Mineral. Petrol., 113, 1 – 8,
1993.
Hart, S. R., and H. Staudigel, Isotopic characterization and
identification of recycled components, in Crust/Mantle Recycling at Convergence Zones, edited by S. R. Hart and
L. Gulen, pp. 15 – 28, Kluwer Acad., Norwell, Mass., 1989.
Hauri, E., T. P. Wagner, and T. L. Grove, Experimental and
natural partitioning of Th, U, Pb and other trace elements
between garnet, clinopyroxene and basaltic melts, Chem.
Geol., 117, 149 – 166, 1994.
Hauri, E. H., and S. R. Hart, Re-Os Isotope systematics of
HIMU and EMII oceanic island basalts from the South-Pacific Ocean, Earth Planet. Sci. Lett., 114, 353 – 371, 1993.
Hauri, E. H., J. C. Lassiter, and D. J. DePaolo, Osmium isotope systematics of drilled lavas from Mauna Loa, Hawaii,
J. Geophys. Res., 101, 11,793 – 11,806, 1996.
Hemond, C., C. W. Devey, and C. Chauvel, Source compositions and melting processes in the society and austral plumes
(South-Pacific Ocean): Element and isotope (Sr, Nd, Pb, Th)
geochemistry, Chem. Geol., 115, 7 – 45, 1994.
Hofmann, A. W., Chemical differentiation of the Earth: The
relationship between mantle, continental-crust, and oceaniccrust, Earth Planet. Sci. Lett., 90, 297 – 314, 1988.
Hofmann, A. W., Mantle geochemistry: The message from
oceanic volcanism, Nature, 385, 219 – 229, 1997.
Hofmann, A. W., and W. M. White, Mantle plumes from ancient oceanic-crust, Earth Planet. Sci. Lett., 57, 421 – 436,
1982.
Johnson, K. T. M., Experimental determination of partition
coefficients for rare earth and high-field-strength elements
between clinopyroxene, garnet, and basaltic melt at high
pressures, Contrib. Mineral. Petrol., 133, 60 – 68, 1998.
Johnson, M. C., and T. Plank, Dehydration and melting experiments constrain the fate of subducted sediments, Geochem. Geophys. Geosys., 1, Paper number 1999GC000014,
2000.
Kawahata, H., M. Kusakabe, and Y. Kikuchi, Strontium, oxygen, and hydrogen isotope geochemistry of hydrothermally
altered and weathered rocks in DSDP hole 504B, Costa Rica
rift, Earth Planet. Sci. Lett., 85, 343 – 355, 1987.
Kennedy, A. K., G. E. Lofgren, and G. J. Wasserburg, An
experimental study of trace element partitioning between
olivine, orthopyroxene and melt in chondrules: equilibrium
values and kinetic effects, Earth Planet. Sci. Lett., 115, 177 –
195, 1993.
Keppler, H., Constraints from partitioning experiments on the
composition of subduction-zone fluids, Nature, 380, 237 –
240, 1996.
Kogiso, T., Y. Tatsumi, and S. Nakano, Trace element transport during dehydration processes in the subducted oceanic
crust, 1, Experiments and implications for the origin of
ocean island basalts, Earth Planet. Sci. Lett., 148, 193 –
205, 1997.
Lassiter, J., and E. Hauri, Osmium-isotope variations in Hawaiin lavas: evidence for recycled oceanic lithosphere in the
Hawaiian plume, Earth Panet. Sci. Lett., 164, 483 – 496,
1998.
10.1029/2001GC000223
Le Roex, A. P., R. A. Cliff, and B. J. I. Adair, Tristan-DaCunha, South-Atlantic: Geochemistry and petrogenesis of a
basanite phonolite lava series, J. Petrol., 31, 779 – 812,
1990.
McCulloch, M. T., and J. A. Gamble, Geochemical and geodynamical constraints on subduction zone magmatism, Earth
Planet. Sci. Lett., 102, 358 – 374, 1991.
McDonough, W. F., and S.-S. Sun, The composition of the
Earth, Chem. Geol., 120, 223 – 253, 1995.
Nakamura, Y., and M. Tatsumoto, Pb, Nd, and Sr isotopic
evidence for a multicomponent source for rocks of CookAustral Islands and heterogeneities of mantle plumes, Geochim. Cosmochim. Acta, 52, 2909 – 2924, 1988.
Newsom, H. E., W. M. White, K. P. Jochum, and A. W.
Hofmann, Siderophile and chalcophile element abundances in oceanic basalts, pb-isotope evolution and growth
of the Earths core, Earth Planet. Sci. Lett., 80, 299 – 313,
1986.
Palacz, Z. A., and A. D. Saunders, Coupled trace element and
isotope enrichment in the Cook-Austral-Samoa islands,
southwest Pacific, Earth Planet. Sci. Lett., 79, 270 – 280,
1986.
Peucker-Ehrenbrink, B., Re-Os systematics and PGE-budgets
of contintental and oceanic crust, paper presented at GERM
workshop, Nat. Sci. Foundation, Arlington, Va., 2001.
Plank, T., and C. H. Langmuir, The chemical composition of
subducting sediment and its consequences for the crust and
mantle, Chem. Geol., 145, 325 – 394, 1998.
Rehkämper, M., and A. W. Hofmann, Recycled ocean crust
and sediment in Indian Ocean MORB, Earth Planet. Sci.
Lett., 147, 93 – 106, 1997.
Reisberg, L., A. Zindler, F. Marcantonio, W. White, D. Wyman, and B. Weaver, Os Isotope Systematics in Ocean Island
Basalts, Earth Planet. Sci. Lett., 120, 149 – 167, 1993.
Roy-Barman, M., and C. J. Allègre, 187Os/186Os in oceanic
island basalts: Tracing oceanic crust recycling in the mantle,
Earth Planet. Sci. Lett., 129, 145 – 161, 1995.
Rudnick, R. L., and D. M. Fountain, Nature and composition
of the continental crust: A lower crustal perspective, Rev.
Geophys., 33, 267 – 309, 1995.
Salters, V. J. M., and W. M. White, Hf isotope constraints on
mantle evolution, Chem. Geol., 145, 447 – 460, 1998.
Shaw, D. M., Trace element fractionation during anatexis,
Geochim. Cosmochim. Acta, 34, 237 – 243, 1970.
Shields, G., and J. Veizer, Precambrian marine carbonate isotope database: Version 1.1, Earth Planet. Sci. Lett., 3, 12,
2002.
Shirey, S. B., and R. J. Walker, The Re-Os system in cosmochemistry and high-temperature geochemistry, Ann. Rev.
Earth Planet. Sci., 26, 423 – 500, 1998.
Stacey, J. S., and J. D. Kramers, Approximation of terrestrial
lead isotope evolution by a two-stage model, Earth Planet.
Sci. Lett., 26, 207 – 221, 1975.
Stalder, R., S. F. Foley, G. P. Brey, and I. Horn, Mineral-aqueous fluid partitioning of trace elements at 900 – 1200C and
3.0 – 5.7GPa: New experimental data for garnet, clinopyroxene, and rutile, and implications for mantle metasomatism,
Geochim. Cosmochim. Acta, 62, 1781 – 1801, 1998.
32 of 33
Geochemistry
Geophysics
Geosystems
3
G
stracke et al.: recycling oceanic crust
Staudigel, H., G. R. Davies, S. R. Hart, K. M. Marchant, and
B. M. Smith, Large scale isotopic Sr, Nd and O isotopic
anatomy of altered oceanic crust: DSDP/ODP sites 417/
418, Earth Planet. Sci. Lett., 130, 169 – 185, 1995.
Staudigel, H., and S. R. Hart, Alteration of basaltic glass:
Mechanisms and significance for the oceanic crust-seawater
budget, Geochim. Cosmochim. Acta, 47, 337 – 350, 1983.
Staudigel, H., S. R. Hart, and S. H. Richardson, Alteration of
the oceanic crust: processes and timing, Earth Planet. Sci.
Lett., 52, 311 – 327, 1981a.
Staudigel, H., K. Muelhenbachs, S. H. Richardson, and S. R.
Hart, Agents of low temperature ocean crust alteration, Contrib. Mineral. Petrol., 77, 150 – 157, 1981b.
Staudigel, H., T. Plank, W. M. White, and H. U. Schmincke,
Geochemical fluxes during seafloor alteration of the basaltic
upper crust: DSDP sites 417 and 418, in Subduction: Top to
Bottom, Geophys. Monogr. Ser., vol. 96, edited by G. E.
Bebout, D. W. Scholl, S. H. Kirby, and J. P. Platt, pp. 19 –
38, AGU, Washington, D.C., 1996.
Sun, S.-S., and W. F. McDonough, Chemical and isotopic
systematics of oceanic basalts: implications for mantle composition and processes, in Magmatism in the Ocean Basins,
edited by A. D. Saunders and M. J. Norry, Geol. Soc. Spec.
Publ., 42, 313 – 345, 1989.
Tatsumi, Y., Migration of fluid phases and genesis of basalt
magmas in subduction zones, J. Geophys. Res., 94, 4697 –
4707, 1989.
Tatsumi, Y., and Y. Kogiso, Trace element transport during dehydration processes in the subducted oceanic
crust, 2, Origin of chemical and physical characteristics in
arc magmatism, Earth Planet. Sci. Lett., 148, 207 – 221,
1997.
Tatsumoto, M., R. J. Knight, and C. J. Allègre, Time differences in the formation of meteorites as determined from the
ratio of lead-207 to lead-206, Science, 180, 1278 – 1283,
1973.
Turner, S., and C. Hawkesworth, Constraints on flux rates and
mantle dynamics beneath island arcs from Tonga-Kermadec
lava geochemistry, Nature, 389, 568 – 573, 1997.
Turner, S., C. Hawkesworth, N. Rogers, J. Bartlett, T. Worthington, J. Hergt, J. Pearce, and I. Smith, U-238-Th-230 disequilibria, magma petrogenesis, and flux rates beneath the depleted
Tonga-Kermadec island arc, Geochim. Cosmochim. Acta, 61,
4855 – 4884, 1997.
Turner, S., C. Hawkesworth, P. van Calsteren, E. Heath,
R. Macdonald, and S. Black, U-series isotopes and destructive plate margin magma genesis in the Lesser Antilles,
Earth Planet. Sci. Lett., 142, 191 – 207, 1996.
van der Hilst, R., S. Widiyantoro, and E. R. Engdahl, Evidence
for deep mantle circulation from global tomography, Nature,
386, 578 – 584, 1997.
Veizer, J., and W. Compston, 87Sr/86Sr in Precambrian carbonates as an index of crustal evolution, Geochim. Cosmochim.
Acta, 40, 905 – 915, 1976.
10.1029/2001GC000223
Vidal, P., C. Chaucel, and R. Brousse, Large mantle heterogeneity beneath French Polenesia, Nature, 307, 536 – 538,
1984.
Walker, R. J., J. W. Morgan, and M. Horan, Osmium 187
enrichments in some plumes: a consequence of core-mantle
interaction?, Science, 269, 819 – 822, 1995.
Weaver, B. L., The origin of ocean island basalt end-member
compositions: Trace element and isotopic constraints, Earth
Planet. Sci. Lett., 104, 381 – 397, 1991.
Weis, D., F. A. Frey, H. Leyrit, and I. Gautier, Kerguelen
Archipelago: Geochemical and isotopic study of the Southast
Province lavas, Earth Planet. Sci. Lett., 118, 101 – 119, 1993.
White, R. S., D. McKenzie, and A. K. O’Nions, Oceanic crustal thickness from seismic measurements and rare earth element inversions, J. Geophys. Res., 97, 19,683 – 19,715,
1992.
White, W., and R. Duncan, Geochemistry and geochronology
of the Society Islands: New evidence for deep mantle recycling, Isotope Studies of Crust Mantle Evolution, edited by
S. R. Hart and A. Basu, AGU, Washington, D.C., 1995.
White, W. M., Sources of oceanic basalts: Radiogenic isotopic
evidence, Geology, 13, 115 – 118, 1985.
White, W. M., U-238/Pb-204 in MORB and open system evolution of the depleted mantle, Earth Planet. Sci. Lett., 115,
211 – 226, 1993.
White, W. M., B. Dupré, and P. Vidal, Isotope and trace element
geochemistry of sediments from the Barbados Ridge-Demerara Plain region, Atlantic Ocean, Geochim. Cosmochim. Acta,
49, 1875 – 1886, 1985.
Widom, E., K. A. Hoernle, S. B. Shirey, and H. U. Schmincke,
Os isotope systematics in the Canary Islands and Madeira:
Lithospheric contamination and mantle plume signatures,
J. Petrol., 40, 279 – 296, 1999.
Widom, E., and S. B. Shirey, Os isotope systematics in the
Azores: Implications for mantle plume sources, Earth Planet. Sci. Lett., 142, 451 – 465, 1996.
Woodhead, J. D., and C. W. Devey, Geochemistry of the pitcairn seamounts, 1, Source character and temporal trends,
Earth Planet. Sci. Lett., 116, 81 – 99, 1993.
Woodhead, J. D., and M. T. McCulloch, Ancient seafloor signals in pitcairn-island lavas and evidence for large-amplitude, small length-scale mantle heterogeneities, Earth
Planet. Sci. Lett., 94, 257 – 273, 1989.
Wright, E., and W. M. White, The Origin of Samoa: New
evidence from Sr, Nd, and Pb isotopes, Earth Planet. Sci.
Lett., 81, 151 – 162, 1987.
Zimmer, M., A. Kroener, K. P. Jochum, T. Reischmann, and
W. Todt, The Gabal Gerf complex: A precambrian N-MORB
ophiolite in the Nubian shield, NE Africa, Chem. Geol., 123,
29 – 51, 1995.
Zindler, A., and S. Hart, Chemical Geodynamics, Ann. Rev.
Earth Planet. Sci., 14, 493 – 571, 1986.
Zindler, A., and E. Jagoutz, Mantle Cryptology, Geochim.
Cosmochim. Acta, 52, 319 – 333, 1988.
33 of 33