Earth and Planetary Science Letters 303 (2011) 337–347
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Earth and Planetary Science Letters
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e p s l
Formation of hybrid arc andesites beneath thick continental crust
Susanne M. Straub a,b,⁎, Arturo Gomez-Tuena c, Finlay M. Stuart d, Georg F. Zellmer b,
Ramon Espinasa-Perena e, Yue Cai a,f, Yoshiyuki Iizuka b
a
Lamont Doherty Earth Observatory at the Columbia University, 61 Route 9W, Palisades NY 10964, USA
Institute of Earth Sciences, Academia Sinica, 128 Academia Road, Sec. 2, Nankang, Taipei 11529, Taiwan, ROC
Centro de Geociencias, Universidad Nacional Autónoma de México, Querétaro 76230, Mexico
d
Isotope Geosciences Unit, Scottish Universities Research and Reactor Centre, East Kilbride G75 0QF, UK
e
Instituto de Geofisica, Universidad Nacional Autónoma de México, Ciudad Universitaria, Mexico, D.F. 04510, Mexico
f
Department of Earth and Environmental Sciences, Columbia University, 61 Route 9W, Palisades NY 10964, USA
b
c
a r t i c l e
i n f o
Article history:
Received 7 November 2010
Received in revised form 12 January 2011
Accepted 13 January 2011
Editor: R.W. Carlson
Keywords:
helium isotopes
high-Ni olivine
andesite formation
Mexican Volcanic Belt
a b s t r a c t
Andesite magmatism at convergent margins is essential for the differentiation of silicate Earth, but no
consensus exists as to andesite petrogenesis. Models proposing origin of primary andesite melts from mantle
and/or slab materials remain in deadlock with the seemingly irrefutable petrographic and chemical evidence
for andesite formation through mixing of basaltic mantle melts with silicic components from the overlying
crust. Here we use 3He/4He ratios of high-Ni olivines to demonstrate the mantle origin of basaltic to andesitic
arc magmas in the central Mexican Volcanic Belt (MVB) that is constructed on ~ 50 km thick continental crust.
We propose that the central MVB arc magmas are hybrids of high-Mg# N 70 basaltic and dacitic initial mantle
melts which were produced by melting of a peridotite subarc mantle interspersed with silica-deficient and
silica-excess pyroxenite veins. These veins formed by infiltration of reactive silicic components from the
subducting slab. Partial melts from pyroxenites, and minor component melts from peridotite, mix in variable
proportions to produce high-Mg# basaltic, andesitic and dacitic magmas. Moderate fractional crystallization
and recharge melt mixing in the overlying crust produces then the lower-Mg# magmas erupted. Our model
accounts for the contrast between the arc-typical SiO2 variability at a given Mg# and the strong correlation
between major element oxides SiO2, MgO and FeO which is not reproduced by mantle–crust mixing models.
Our data further indicate that viscous high-silica mantle magmas may preferentially be emplaced as intrusive
silicic plutonic rocks in the crust rather than erupt. Ultimately, our results imply a stronger turnover of slab
and mantle materials in subduction zones with a negligible, or lesser dilution, by materials from the overlying
crust.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Andesite magmas at convergent margins are enriched in silica
compared to magmas erupting at mid-ocean ridges and intra-plate
volcanoes. Determining the cause(s) of silica enrichment is fundamental for models of continental crust formation, arc growth rates
and across-arc mass balances (Plank and Langmuir, 1993; Rudnick,
1995; White et al., 2006). Andesite petrogenesis, however, has long
been controversial, with no consensus even whether andesitic
magmas form in the subarc mantle or in the overlying crust (Rudnick,
1995; Rudnick and Gao, 2002). Many models assume a basaltic flux
from mantle to arc crust, with silicic magmas evolving subsequently in
the upper plate crust through fractional crystallization and crustal
⁎ Corresponding author at: Lamont Doherty Earth Observatory at the Columbia
University, 61 Route 9W, Palisades NY 10964, USA. Tel.: +1 845 365 8464; fax: + 1 845
365 8155.
E-mail address: smstraub@ldeo.columbia.edu (S.M. Straub).
0012-821X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsl.2011.01.013
assimiliation of up half of the mass of the erupted melt (e.g.
Eichelberger, 1978; Leeman, 1983; Hildreth and Moorbath, 1988;
Plank and Langmuir, 1988; Streck et al., 2007; Tatsumi et al., 2008;
Reubi and Blundy, 2009). Other models propose primary andesite
formation beneath the Moho, which may occur by various mechanisms such as hydrous melting of peridotite (Hirose, 1997; Moore and
Carmichael, 1998; Blatter and Carmichael, 1998b; Carmichael, 2002),
slab melting (Defant and Drummond, 1990) or hybridization of slab
and mantle materials by melt rock-reaction processes (Kay, 1978;
Yogodzinksi et al., 1994; Kelemen, 1995; Yogodzinski et al., 1995;
Rapp et al., 1999; Kelemen et al., 2003, 2004; Gomez-Tuena et al.,
2007). These two approaches differ substantially with respect to the
turnover of slab and mantle materials in subduction zones, the rate of
crustal growth, and the overall connectivity between arc magmatism
and the other geochemical cycles of Earth.
A recent study suggests that ‘high-Ni’ olivines, that have been by
now reported from several arcs, e.g. Mexico, Cascades, Setouchi,
Kamchatka and the Aleutians (e.g., GeoROC, 2009) may provide
338
S.M. Straub et al. / Earth and Planetary Science Letters 303 (2011) 337–347
important insights into the origin of arc andesites (Straub et al.,
2008b). High-Ni olivines are characterized by Ni contents of ~ 2200–
5400 ppm at low olivine forsterite contents of Fo80 to Fo89 [where
Fo = molar ratio of Mg/(Mg + Fe2+)]. In Fo–Ni space, they plot well
above the field of olivines from mid-ocean-ridge basalts (MORB)
(Sobolev et al., 2005). Experiment and theory rule out that such highNi olivines crystallize from partial melts of peridotite mantle where
olivine is residual during melting and buffers melt SiO2 (~49–50 wt.
%), MgO (~ 10–15 wt.%), Mg# (~71–73) [where Mg# = molar ratio of
Mg/(Mg + Fe2+) in bulk rock] and Ni of equilibrium melts (numbers
exemplify moderate extents of melting b10–15% of fertile mantle)
(Langmuir et al., 1992; Sobolev et al., 2005; Straub et al., 2008b). The
melt Ni is limited, because the amount of Ni in mantle olivines is
limited to ~ 2000–3000 ppm (Blatter and Carmichael, 1998a; GeoROC,
2009). The experimentally well-constrained mineral/melt partition
coefficient KdNi of 10–11 (Hart and Davis, 1978; Beattie et al., 1991;
Wang and Gaetani, 2008) then predicts that melt Ni will not exceed
200–300 ppm in a melt of MgO with 10–12 wt.% (Langmuir et al.,
1992; Straub et al., 2008b). Consequently, the maximum Ni content of
early crystallizing magmatic olivine of ≥Fo89.5, controlled by the same
KdNi, cannot exceed 2000–3000 ppm Ni either. As olivine crystallization rapidly depletes melt Ni, any derivative melt (=melt derived by
fractional crystallization) can only crystallize low-Fo olivines with less
Ni (Straub et al., 2008b).
High-Ni olivines observed in intraplate basalts (Sobolev et al.,
2005) have been suggested to indicate the presence of secondary
veins of olivine-free ‘reaction pyroxenites’ in a peridotite mantle
source. Such veins may form following the infiltration of the silicic
melts from subducted eclogite into the surrounding peridotite mantle
that would locally transform mantle olivines into reaction pyroxenes
at sub-solidus conditions (Sobolev et al., 2005). Because reaction
pyroxenes inherit ca. 80–86% of the Ni content of the original olivine
(as being diluted by the addition of silica, Sobolev et al., 2005), but
have a three times lower KdNi than olivine (Beattie et al., 1991), their
partial melts are Ni-rich and precipitate high-Ni olivines at shallow
pressures, give or take additional dilution by partial melts from
peridotite (Sobolev et al., 2005, 2007). Clearly, the Sobolev et al.
(2005) model has appeal for subduction zones where highly reactive
siliceous fluids and partial melts from the slab continuously infiltrate
the overlying peridotite mantle wedge (Johnson et al., 1999; Rapp
et al., 1999; Kessel et al., 2004). In a recent study, Straub et al. (2008b)
related high-Ni olivines in andesites of central Mexican Volcanic Belt
(MVB) to hybridized pyroxenite/peridotite mantle sources. However,
high-Ni olivines may also crystallize in magmas produced through
mixing of basaltic mantle melts with larger portions of silicic crustal
melts. In order to test conclusively for the mantle origin of the olivinebearing central MVB magmas, we analyzed He isotope ratios of highNi olivines. Here, we present the results, together with a first
systematic study of olivine zoning patterns.
2. Geological setting and samples
Quaternary volcanism of the central MVB is related to subduction
of the Cocos plate at the Middle American Trench (Fig. 1). Despite the
ca. 50 km thick Paleozoic to Precambrian continental arc crustal
basement (Perez-Campos et al., 2008), basaltic and basaltic andesite
magmas are common next to the ubiquitous andesitic and dacitic
series along the broad arc front (Siebe et al., 2004a; Schaaf et al., 2005;
Gómez-Tuena et al., 2007). Many basaltic and andesitic magmas of the
composite volcano Popocatepetl and the adjacent monogenetic Sierra
Chichinautzin Volcanic Field (Fig. 1) contain olivines as single or
principal phenocrysts. We selected 25 olivine-rich volcanic rocks with
SiO2 = 50.0–61.2 wt.%, MgO = 3.2–9.7 wt.% and Mg# = 50–72 from
monogenetic volcanoes Guespalapa (7 samples), Chichinautzin (5),
Texcal Flow (4), Suchiooc (4), Cuatepel (2), and one sample each from
Tuxtepec, Yecahuazac and Popocateptl volcanoes (Table 1). The
sample set includes high-Nb arc basalts and basaltic andesites (18–
16 ppm Nb, Nb/La ~ 0.7–1.2) next to the volumetrically dominant lowNb (4–14 ppm Nb; Nb/La b 0.1–0.8) calc-alkaline basalts to andesites
with the typical high ratios of fluid mobile large-ion lithophile
element to high-field strengths elements (Table 1). We report bulk
rock analyses for selected major and trace elements, measured by DCP
and ICP-MS methods, respectively, major element oxides and Ni
abundances in olivines measured by electron microprobe, 3He/4He
isotope analyses on olivine separates as well as Nd isotopic
compositions of bulk rock measured by thermal ionization mass
spectrometry. Analytical details are given in the Appendix. The new
data are presented in Tables 1 and 2, and Appendix Table 3.
3. Results
In order to streamline data presentation and discussion, samples
were grouped into basaltic (olivine-normative) and andesitic (quartznormative) compositions on the basis of their CIPW norm (Fig. 2A;
Table 1). Basaltic samples (n= 12) have 50.0 to 54.6 wt.% SiO2, 6.7–9.7
MgO, and 90–220 ppm Ni at high Mg# = 62–72. Andesitic samples
Fig. 1. Central Mexican Volcanic Belt with Quaternary composite volcanoes Nevado de Toluca and Popocatepetl flanking the Holocene Sierra Chichinautzin Volcanic Field. Slab
contours in inset after Pardo and Suarez (1995). Samples are of (b 5 ka) Holocene age (Siebe et al., 2004b), except for the basaltic andesite from Popocatepetl that has a roughly
estimated age of several 100 ka (Schaaf et al., 2005).
S.M. Straub et al. / Earth and Planetary Science Letters 303 (2011) 337–347
(n= 13) are enriched in SiO2 = 53.7–61.2 wt.% at moderately elevated
MgO (3.2–7.9 wt.%) and Ni (8–165 ppm) abundances, but overlap
widely with the basalts in Mg# = 50–70 values. Olivines from all
samples have the high Ni abundances typical for the central MVB
(Straub et al., 2008b) with a maximum of 5300 ppm Ni at Fo80–90 (Fig. 3,
Appendix Table 1). This is also true for olivines of some of the most mafic
basalts with up to 9.7 wt.% MgO for which data were obtained for the
first time. Including the data from Straub et al. (2008b) (not shown in
Fig. 3), the Ni-rich olivines exhibit a similar distribution in Fo–Ni space,
with comparable maximum Ni for basalt and andesite series. Bulk rock
Mg# ranges from compositions that are in equilibrium with mantle
mineral assemblages (Mg# = 70–72) to the low Mg# values typical of
continental crust (Mg# = 60–50) (Rudnick, 1995). Despite this range,
the average Fo of the olivine correlates with bulk rock Mg# with a slope
that is consistent with the well-constrained, constant mineral/melt
= 0.3 ± 0.03 (Roeder and Emslie, 1970)
exchange coefficient KDoliv/melt
Fe/Mg
(Fig. 2B). As discussed by Straub et al. (2008b), the olivines must then
crystallize in, or close to, equilibrium with their basaltic and andesitic
host magmas, and their composition must pertain to melt origin and
differentiation.
Whole rock 143Nd/144Nd ranges from 0.51273 to 0.51299 and is
comparable to the range previously reported for Sierra Chichinautzin
and Popocateptl magmas (LaGatta, 2003; Siebe et al., 2004a; Schaaf
et al., 2005). Results of 3He/4He analyses are listed in Table 2, and
presented in Fig. 4 and Appendix Fig. 1. Irrespective of the bulk magma
composition, olivines of all samples have 3He/4He between 6.8 and
8.0 Ra, with an average 3He/4He of 7.3 ± 0.3 Ra (where Ra is the
atmospheric 3He/4He; 1.39× 10−6) (Fig. 4). The average 3He/4He of
olivines from basaltic magmas (7.5 ± 0.3 Ra) is indistinguishable from
andesitic magmas (7.1 ± 0.3 Ra), with no correlation between 3He/4He
and bulk rock 143Nd/144Nd, SiO2, MgO and Ni, or the average Fo and Ni of
olivines (Appendix Fig. 1). Mixing lines in the 143Nd/144Nd vs. 3He/4He
space (Fig. 4), calculated for a range of feasible crustal He abundances,
indicate that N94% of the He is mantle-derived, and hence mantle melts
must control the composition of the olivines.
4. Discussion
4.1. Ruling out silica enrichment by melt differentiation in the overlying
crust
At crustal levels, melt silica may increase by crystal fractionation,
and/or assimilation of siliceous crustal material (e.g. Leeman, 1983;
Hildreth and Moorbath, 1988; Annen et al., 2006; Reubi and Blundy,
2009). In the central MVB magmas, however, the composition of the
olivines rules out either mechanism. Fractional crystallization cannot
work because olivines of the individual magmas mostly define
separate groups in Fo–Ni space, with different Fo at similar Ni. In a
series of derivate melts, however, where melt Mg# and Ni are
controlled by olivine, trends of fractional crystallization follow single
trajectories of decreasing Ni with decreasing Fo (individual blue lines
and the
in Fig. 3C,D). This reflects the joint control of the Kdoliv/melt
Ni
on melt and olivine (Straub et al., 2008b). Thus, the olivine
KDoliv/melt
Fe/Mg
compositions unambiguously preclude that variations within and
between basaltic and andesitic series were caused by fractional
crystallization. This is also true for magma series from single
monogenetic volcanoes that differ only by a few weight percent in
MgO and SiO2 (e.g. Chichinautzin, Texcal Flow, Table 1). The olivine
data thus indicate that basaltic and andesitic magmas have different
origins, which is consistent with the different ratios of Sr–Nd–Pb
isotopes among most of the magmas investigated, including those
from single volcanic centers (Siebe et al., 2004a; Schaaf et al., 2005;
Straub et al., 2008a; Straub et al., 2009) (Table 1).
The high 3He/4He of the olivines immediately excludes lower crust
contamination as cause of silica enrichment. This is because olivines
crystallize at upper crust levels mostly at pressures of up to 400 MPa,
339
and rarely of up to 600 MPa, corresponding to ~ 20 km crustal depth
(Cervantes and Wallace, 2003; Roberge et al., 2009). Since the
measured He isotope ratios reflect the composition of melt and gas
inclusions trapped in the olivines, it is possible that the bulk rock
matrix was contaminated by upper crustal material (e.g. Stuart et al.,
2000; Martelli et al., 2008). However, this is unlikely for the central
MVB. With a bulk melt SiO2 as high as 61.2 wt.%, uptake of crustal
silica would have to be on the order of several tens of percent of the
melt mass erupted. This disagrees with the whole rock Sr–Nd–Pb
isotope ratios that have a limited range consistently more primitive
than the non-magmatic upper crust of the central MVB (MartinezSerrano et al., 2004; Siebe et al., 2004a; Schaaf et al., 2005). The
moderate enrichment relative to a MORB-source mantle (Schaaf et al.,
2005; Straub et al., 2008b) does not require crustal contamination and
is either attributable to an inherently enriched mantle, or to the
addition of slab components (Siebe et al., 2004a; Schaaf et al., 2005;
Cai et al., 2008; Straub et al., 2008a, 2009).
In summary, the combined olivine and bulk rock data rule out
crustal processes as cause for the melt silica increase. We conclude
that the central MVB basalts and andesites originate from below the
Moho with their trace element and isotope chemistry reflecting a
subarc mantle modified by slab components, in agreement with
previous models of sub-Moho formation of arc andesites proposed for
the MVB (Moore and Carmichael, 1998; Blatter and Carmichael,
1998b; Carmichael, 2002) and arcs elsewhere (e.g. Aleutians, Kelemen,
1995; Yogodzinski et al., 1995; Kelemen et al., 2004).
4.2. Commonalities between basalts and andesites
Our data reveal substantial commonalities between central MVB
basalts and andesites which argue against a fundamentally different
genetic mechanism for the two series. In both series, the mantle
signature is strong, with high melt Mg# ~70, MgO and Ni, and 3Herich high-Ni olivines. The ubiquitous high-Ni olivines argue for genetic
processes inherent to melt formation rather than to other variables.
For example, oxygen fugacities in central MVB magmas range
between +0.5 and +1.5 relative to the fayalite–magnetite–quartz
buffer, which does not significantly change the melt ferric/ferrous
ratio (Straub et al., 2008b). The increase of melt oxygen fugacity
(Kelley and Cottrell, 2009; Brounce et al., 2010) would increase the
melt Mg# and olivine Fo, which is opposite to the trend observed. The
regular sub-parallel patterns of olivine in the Fo–Ni space (Fig. 3A,B)
argue against their origin by Fe diffusion in the mantle in a direction
perpendicular to these trends (Tatsumi et al., 2002). A silicate melt
control of the Ni is emphasized by the excellent correlation of Ni with
MgO in normally and inversely zoned olivines that disagrees with a Ni
control by occasionally co-existing sulfide melts (Straub et al., 2008b)
(Appendix Table 1 and Figs. 2 and 3).
The evidence for the presence of slab components in subarc mantle
of the central MVB (e.g. Schaaf et al., 2005; Straub et al., 2008a, 2009)
implies addition of slab silica, either as slab ‘fluids’ or slab ‘melts’
(Rapp et al., 1999; Kessel et al., 2005). A significant slab flux does not
contradict the high 3He/4He of the olivines, as any crustal He on the
slab must have long been driven off during the flat, prolonged slab
subduction beneath the Mexican forearc (Fig. 1) while other
elements, including H2O, were retained to greater depth (e.g.
Gomez-Tuena et al., 2007). Notably, olivine melt inclusions show
that basaltic and andesitic series have similar maximum H2O
abundances (Cervantes and Wallace, 2003; Roberge et al., 2009).
Hence, melt water cannot drive andesite formation relative to ‘drier’
basaltic melts, either by degassing during shallow differentiation, or
hydrous mantle melting (Carmichael, 2002). Basaltic and andesitic
series have also similar ratios of trace elements (Kay, 1978; Defant
and Drummond, 1990), including low Sr/Y (b33) and a limited range
of Gd/Yb (2.2± 0.1) (Wallace and Carmichael, 1999; Siebe et al., 2004a;
Schaaf et al., 2005) slightly higher than MORB (Gd/Yb = 1.5 + 0.2,
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S.M. Straub et al. / Earth and Planetary Science Letters 303 (2011) 337–347
Table 1
Bulk rock data of Sierra Chichinautzin and V. Popocateptl samples analyzed for olivine compositions.
Location
Sample
Longitude W
Latitude N
Laboratorya
Groupb
V.Popocateptl
V.Cuatepel
V.Cuatepel
V.Guespalapa
V.Guespalapa
V.Guespalapa
V.Guespalapa
V.Guespalapa
Texcal Flow
V.Chichinautzin
V.Chichinautzin
V.Chichinautzin
V.Chichinautzin
V.Tuxtepec
V.Yecahuazac
V.Guespalapa
Texcal Flow
Texcal Flow
Texcal Flow
Texcal Flow
V.Suchiooc
V.Suchiooc
V.Suchiooc
V.Suchiooc
V.Chichinautzin
SPO34
SPO60
SPO56
CH-07-12
CH-07-14
CH-07-9
CH-07-6
CH-08-3
CH-09-2
S1
S8
S4
S9
S15f
CH-08-15
MCH-06-9
CH-05-16
MCH-06-12
CH-08-17
MCH-06-11
CH-08-8
CH-07-1
CH-08-11
ASC45B
MCH-06-5
98°39′18″
98°51′28″
98°51′33″
99°10′10″
99°10′02″
99°10′20″
99°10′38″
99°11′56″
99°11′52″
99°08′17″
99°09′03″
99°08′34″
99°09′14″
99°16′28″
99°05′41″
99°11′02″
99°12′21″
99°06′24″
99°08′32″
99°10′36″
99°06′18″
99°06′02″
99°03′58″
99°04′04″
99°04′40″
19°03′24″
19°05′08″
19°05′25″
19°05′29″
19°0525″
19°05′26″
19°05′26″
19°04′34″
19°04′49″
19°05′18″
19°05′52″
19°05′21″
19°05′37″
19°07′31″
19°04′26″
19°05′25″
19°04′23″
18°58′55″
18°53′41″
19°00′33″
19°04′03″
19°03′33″
18°57′52″
18°57′26″
19°06′02″
LaGatta (2003)
Harvard
LaGatta (2003)
WSU/CGEO
WSU/CGEO
WSU/CGEO
WSU/CGEO
WSU/CGEO
WSU/CGEO
Harvard
Harvard
Harvard
Harvard
Harvard
WSU/CGEO
WSU/CGEO
WSU/CGEO
WSU/CGEO
WSU/CGEO
WSU/CGEO
WSU/CGEO
WSU/CGEO
WSU/CGEO
LaGatta (2003)
WSU/CGEO
And
And
And
And
And
And
And
And
And
And
And
And
And
Bas
Bas
Bas
Bas
Bas
Bas
Bas
Bas
Bas
Bas
Bas
Bas
SiO2
Fe2O3
MgO
Mg#
58.90
6.25
5.40
66.3
54.99
54.50
55.81
56.58
61.18
55.71
56.45
54.39
54.81
53.71
50.16
53.13
54.58
52.62
49.98
50.52
51.67
52.97
54.17
51.60
7.77
7.63
7.34
7.15
5.84
7.28
7.46
8.66
8.25
9.31
8.76
7.92
8.10
8.87
10.20
10.13
9.73
8.20
8.17
9.21
7.92
7.79
6.92
6.20
5.35
5.79
3.24
4.95
3.89
5.41
9.73
8.05
6.71
8.13
7.96
8.20
6.98
8.83
8.75
7.60
69.9
69.9
68.2
66.4
67.6
64.4
49.7
56.5
51.8
57.0
71.6
69.8
65.3
67.6
64.0
64.8
62.0
71.0
70.9
65.3
53.49
9.60
6.75
61.6
Qtzc
Olivc
4.92
8.58
3.24
0.61
1.02
2.08
3.31
10.24
2.26
6.35
3.93
3.97
2.66
17.48
6.59
1.44
8.77
16.44
16.47
10.52
6.13
4.23
14.16
16.92
0.08
Ni
Sr
109
434
164
165
125
110
137
100
8
47
23
77
220
168
115
179
119
145
91
169
197
123
362
349
373
401
456
488
518
486
510
517
938
766
514
548
520
488
485
527
507
548
133
535
a
Indicates laboratory were major and trace elements were obtained (Harvard = Langmuir Laboratory; WSU = Washington State University, CGEO = Centro de Geociencias). Data
for samples SPO34, SPO56 and ASC45B are from LaGatta (2003). Major element oxides in wt.%, trace element abundances in ppm. Total Fe given as Fe2O3.
b
Andesitic (quartz-normative) and basaltic (olivine-normative).
c
CIPW norm: Qtz = quartz-normative, Oliv = olivine normative. Calculation with program created by K. Hollocher at the Union College of Schenectady (http://www.union.edu/
PUBLIC/GEODEPT/COURSES/petrology/norms.htm).
d
Nd isotope analyses at Institute of Earth Sciences, Taipei, Taiwan.
e
Nd isotope analyses from Lamont Doherty Earth Observatory, Palisades, NY, USA.
f
Nd isotope ratio of Sample S15 is seen as equivalent to LaGatta (2003) sample pw115 (taken at the same quarry) and which has identical major and trace element chemistry.
Niu and Batiza, 1997), except two basalts (S15 and CH08-15) with Gd/
Yb of 4.3 and 3.2, respectively (Table 1). Whatever the causes for the
low Sr/Y (presumably partial Sr loss beneath the forearc, GomezTuena et al., 2007) and Gd/Yb ratios (such as re-equilibration of
HREE-depleted slab components with the surrounding mantle prior
to melting, Kelemen et al., 2004; Gomez-Tuena et al., 2007, 2008),
there is no evidence that andesites had a stronger ‘slab melt
signature’, and hence a genesis substantially different from that of
the basalts.
4.3. Andesite formation from hybridized peridotite and pyroxenite
mantle sources
Straub et al. (2008b) proposed that primary andesite magmas were
partial melts from ‘reaction pyroxenites’ in the subarc mantle. While
building on these results, our extended data set allows for further
refinement and expansion. In short, we propose that arc andesite
formation starts with the infiltration of silicic slab components in the
subarc peridotite mantle and creation of discrete segregations of silicadeficient and silica-excess reaction pyroxenites in a peridotite matrix
(Fig. 5). Upon melting, these reaction pyroxenites produce initial highMg# basaltic and dacitic melts that mix in variable proportions, and
with subordinate amounts of basaltic partial peridotite melts, to
produce a broad spectrum of high-Mg# N70 basaltic, andesitic and
dacitic melts during ascent through mantle and lower crust. In the upper
crust, further differentiation by moderate fractional crystallization and
recharge melt mixing generates the broader spectrum of high-to low
Mg# basaltic, andesitic and dacitic magmas erupted (Fig. 5).
4.3.1. Subarc mantle lithologies and the composition of initial mantle
melts
The composition and lithology of the subarc mantle plays a key
role in the revised model. Important constraints on the mantle can be
inferred from the spatial and temporal distribution of the Holocene
Sierra Chichinautzin monogenetic volcanoes. While forming a
compositional continuum from basalt to dacite, individual volcanoes
are fairly homogenous with only a few wt% difference in major
element oxides, but have distinct patterns of incompatible trace
elements and Sr–Nd–Pb isotope ratios (Wallace and Carmichael, 1999
LaGatta, 2003; Siebe et al., 2004a, 2005; Schaaf et al., 2005). Four of
the seven Holocene monogenetic volcanoes investigated (Chichinautzin, Guespalapa, Suchiooc, Texcal Flow) erupt within only a few
kilometers and a few thousand years (Siebe et al., 2004b), yet their
isotopically distinct magmas range from low-Nb calc-alkaline magmas (e.g. ~ 5 ppm Nb at Guespalapa) to high-Nb arc basalts with up to
~35 ppm Nb (Chichinautzin). Despite their proximity, these magmas
must originate from separate, compositionally distinct domains in the
subarc mantle. The ubiquitous high-Ni olivines indicate that these
‘domains’ were separate segregations of reaction pyroxenite formed
by infiltration of silicic slab components along different ascent
pathways (Straub et al., 2008b).
Melt formation beneath the central MVB occurs below the 50 km
thick crust, and likely in the hot center of the mantle wedge in the
presence of garnet. At depths N2 GPa (~66 km), and in the presence of
garnet, both silica-deficient or quartz-normative pyroxenites may
exist that produce distinct types of partial melts (Kogiso et al., 2004).
Silica-deficient pyroxenites produce Ne-normative basaltic melts that
resemble partial melts of peridotite with high melt MgO (~12–14 wt.
%) and low melt SiO2 (~48–50 wt.%) (Kogiso et al., 2004). In contrast,
quartz-normative pyroxenites are equilibrated with dacitic melts
of SiO2 ~65 wt.% and MgO ~3 wt.% (Rapp et al., 1999, 2010). Which
pyroxenite type forms in the reaction process depends on how much
slab silica is added to a finite parcel of peridotite. The destruction
of olivine in a peridotite parcel is complete before its bulk composition becomes quartz-normative, but excess-silica pyroxenite may
eventually form through continued addition of slab silica, owing to a
S.M. Straub et al. / Earth and Planetary Science Letters 303 (2011) 337–347
341
Table 1
Bulk rock data of Sierra Chichinautzin and V. Popocateptl samples analyzed for olivine compositions.
La
Gd
Y
Yb
Sr/Y
Gd/Yb
143Nd/144Ndd
6.41
15.63
3.96
20.52
1.92
21.15
2.06
0.512836
5
3.61
3.72
4.08
4.40
7.26
7.61
21.75
29.71
30.19
35.74
3.82
8.44
13.61
26.13
19.00
22.85
23.67
10.32
12.92
24.37
9.08
9.29
9.55
10.29
19.19
13.49
30.71
38.87
39.92
44.52
43.99
31.16
16.55
26.67
16.43
20.43
21.68
15.91
16.34
21.08
3.63
3.58
3.59
3.63
3.86
4.19
7.16
8.49
8.72
9.45
9.19
6.03
4.75
6.35
5.67
5.82
6.30
4.48
4.55
5.51
19.65
19.17
18.69
18.44
19.18
21.24
36.64
41.57
43.09
45.15
28.56
22.97
24.07
32.06
31.75
30.75
33.90
22.83
22.88
28.39
1.84
1.73
1.64
1.64
1.74
1.89
3.31
3.67
3.87
3.96
2.15
1.88
2.17
2.86
2.88
2.73
3.06
2.03
2.08
2.51
18.42
18.23
19.97
21.73
23.77
22.97
14.13
11.69
11.83
11.46
32.85
33.37
21.38
17.10
16.38
15.87
14.31
23.07
22.15
19.29
1.97
2.07
2.19
2.22
2.22
2.21
2.17
2.31
2.26
2.38
4.28
3.21
2.18
2.22
1.97
2.13
2.06
2.20
2.19
2.19
0.512948
0.512916
0.512922
0.512925
0.512823
0.512936
0.512801
0.512743
0.512746
0.512733
9
7
7
6
4
5
9
6
8
6
0.512934
0.512928
0.512824
0.512987
7
6
10
6
0.512866
0.512958
0.512942
0.512942
8
5
6
6
33.05
37.41
8.16
39.77
3.45
13.46
2.36
Nb
repetitive slab flux in once established conduits (e.g. Hall and Kincaid,
2001).
In a simple model, the ubiquity of high-Ni olivines in the central
MVB magmas signifies that they formed primarily through mixing of
initial basaltic and dacitic melts produced by partial melting of silicadeficient and silica-excess pyroxenites, with or without some dilution
by peridotite partial melts. Notably, this model accounts for key
characteristics observed in the central MVB magmas. These are (i) the
variable SiO2 at given Mg# in bulk rocks at strong linear correlations of
SiO2, MgO and FeO; (ii) the combination of high-Ni olivines, moderate
melt Ni abundances and near-primary character of the erupted
magmas, and (iii) the pervasive evidence of melt mixing preserved in
forsteritic olivines in equilibrium with mantle melts. This will be
discussed in the following sections.
4.3.2. Formation of high-Mg# basaltic to dacitic initial melts
Initial basaltic and dacitic melts from reaction pyroxenite, or any of
their hybrids, always have high melt Mg#. This is because neither FeO
nor MgO are transported in slab components in significant amounts
(Johnson and Plank, 1999; Rapp et al., 1999; Kessel et al., 2005) and
hence reaction pyroxenites inherit the FeO, MgO, and the Mg#, of the
original peridotite mantle. Consequently, partial pyroxenite melts
must have similar high melt Mg# as peridotite melts, as the KDopx/melt
Fe/Mg
(~0.255) is only slightly lower than the KDoliv/melt
(~ 0.295) (Straub
Fe/Mg
et al., 2008b). Fig. 6 shows the range of initial hybrid melts with their
variable SiO2 ~ 50–65 wt.% at a high Mg# ~ 72. Because this range is
produced by binary melt mixing, our model predicts linear correlations between major element oxides SiO2, MgO and FeO. Because
these high-Mg# melts have comparatively low MgO ~ 3–10 wt.% and
FeO ~2.5–8 wt.%, only a few percent (maximum of 15%) loss of olivine,
and/or pyroxenes, suffice to produce melt Mg#'s as low as 50 (Fig. 6).
Such minor loss leaves the initial melt FeO and SiO2 nearly unchanged,
and melt MgO is only moderately lowered with decreasing Mg#
2 STEM
143Nd/144Nde
2 STEM
0.512736
18
0.512825
0.512982
0.512910
14
14
18
0.512757
10
(Fig. 6). Thus, the binary mixing of the initial mantle melts should be
prominent in the erupted melts. Indeed, the samples with the high
3
He/4He olivines all show strong linear correlations of the major
elements SiO2, FeO and MgO (Fig. 7), with or without a correction for
fractionation, that run between experimentally generated basaltic and
dacitic mantle melts from pyroxenite and peridotite lithologies
(Hirose and Kushiro, 1993; Kushiro, 1996; Hirschmann et al., 2003).
The array extends to include the dacitic magmas of the central MVB
which are often aphyric and free of olivine, but share origin with their
coeval mafic magmas on the basis of their trace elements and Sr–Nd–
Pb isotope ratios. Thus, our genetic model accounts readily for the
contrast between linear correlations between SiO2, MgO and FeO that
are typical for arc magmas worldwide (Fig. 3 in, Reubi and Blundy,
2009), and the SiO2 variability at a given Mg# (Fig. 2A). Notably, this
contrast is not expected for andesites formed through mixing of
basaltic mantle melts with crustal components (Eichelberger, 1978;
Reubi and Blundy, 2009) which produces hyperbolic mixing curves in
the SiO2 vs Mg# space, next to requiring large amounts of crustal silica
(in the order of several tens of percent) that are inconsistent with the
3
He/4He ratios observed.
4.3.3. The formation of high-Ni olivines in low Ni magmas
Any basaltic or dacitic melt from reaction pyroxenites, or any hybrid
of such melts, will precipitate high-Ni olivines at shallow pressures,
is three times lower than the Kdoliv/melt
for any
because the Kdopx/melt
Ni
Ni
melt MgO (Beattie et al., 1991). However, because the KdNi increases
exponentially in melts with MgO b10 wt.% (Hart and Davis, 1978;
Beattie et al., 1991; Wang and Gaetani, 2008), the Ni abundances of the
initial basaltic and dacitic pyroxenite melts are different. Only basaltic
pyroxenite melts will have high melt Ni of 400–500 ppm, which reflects
the low Kdopx
Ni ~3–4 of reaction orthopyroxene (Straub et al., 2008b). In
contrast, dacitic pyroxenite melts have much lower Ni of 130–170 ppm,
because the Kdopx
Ni ~10–13 is much higher in dacitic melts. Consequently,
342
S.M. Straub et al. / Earth and Planetary Science Letters 303 (2011) 337–347
high-Ni olivines do not necessarily signify Ni-rich initial melts. Because
andesitic hybrids of the initial basaltic and dacitic melts will have
intermediate Ni abundances between 130–500 ppm, only a few percent
of olivine and/or pyroxene fractionation are required to reach the low Ni
(10–220 ppm) of the erupted magmas. Thus, the central MVB magmas
represent near-primary mantle magmas despite comparatively moderate melt Ni abundances.
4.3.4. The record of mantle melt hybridization in early crystallizing
olivines
Closer examination of the olivine zoning patterns shows that
forsteritic olivines preserve traces of the inferred mixing of initial
pyroxenite melts. In Fo–Ni space (Fig. 3), early crystallizing high-Ni
olivines in equilibrium with pyroxenite partial melts must plot within the
ochre pyroxenite field, while low-Ni olivine from initial peridotite melts
must plot in the green peridotite field. The large black arrow between
pyroxenite and peridotite fields marks the melts produced by mixing of
initial peridotite and pyroxenite melt. This mixing array, however, is
poorly recorded by the olivines because they mostly crystallize when the
hybridization of initial melts has been far progressed.
There are various traces of the initial melt mixing. First, while no
single sample that can be considered to represent a peridotite partial
melt (or its derivative), not all olivine arrays backtrack to the pyroxenite
field. The lower-Ni olivines of several samples (e.g. sample MCH06-12
from Texcal Flow) backtrack to the area between peridotite and
pyroxenite fields, indicating that they precipitate from a hybrid highMg# melt composed of pyroxenite and peridotite component melts.
Peridotite component melts either increase melt MgO or decrease melt
Ni, either of which reduces the Ni content of early-crystallizing olivine.
Second, zoning patterns of olivines in all samples investigated record a
melt hybrid origin. Invariably, olivine cores, forsteritic or not, define the
existence of multiple component melts that cannot be related by
fractional crystallization, and prove the co-existence of different melt
batches in a hand specimen. Invariably, the olivine rims record melt
hybridization by their complex normal and inverse zoning pattern. In
the Appendix, three selected examples illustrate the hybrid nature of the
central MVB magmas. Notably, while olivine reliably reproduces the
melt Mg# (Roeder and Emslie, 1970), it does neither constrain the SiO2,
MgO or FeO abundances of the equilibrium melts, nor the melt Ni given
the compositional dependency of the KdNi. Partial melts from
pyroxenite, producing high-Ni olivines, may thus have either basaltic
or dacitic composition, or hybrid andesitic compositions. In contrast,
low-Ni olivines always represent basaltic melts from peridotite, give or
take a few percent SiO2 increase due to hydrous melting (Baker et al.,
1994). While only undegassed melt inclusions may safely record the
silica content of initial melts, some clues are provided by the bulk rock
composition that must be close to the weighted average of all initial
component melts. In principle, the higher the bulk rock silica, the higher
must be the proportion of the initial dacitic melts. The high-Ni olivines of
Tuxtepec basalt S15 (SiO2 = 50.2 wt.%, MgO= 9.7 wt.%) do not require
dacitic melts, but only basaltic melts from silica-deficient pyroxenites,
and possibly a minor melt component from peridotite. In contrast, the
higher SiO2 abundances of Suchiooc basalt CH08-11 (SiO2 = 51.6 wt.%,
MgO= 7.6 wt.%) and Popocatepetl andesite SPO34 (SiO2 = 56.7 wt.%,
MgO= 6.9 wt.%) suggest a larger proportion of initial dacitic melts from
silica-excess pyroxenite that mingle with basaltic melts in order to
produce the intermediate hybrid magmas erupted. The broader range of
initial melts is reflected in the broad range of high-Ni and low-Ni
olivines, all of which require different initial melts (Appendix Fig. 2B,C).
4.4. Crustal differentiation
4.4.1. Upper crustal processing of central MVB magmas
Crustal-level magma differentiation obliterates the strong signals
of the mantle melts and is essential to produce low Mg# b 70 magmas
crystallizing olivine with bFo88. Lowering the melt Mg# to b 70 must
Table 2
Air-normalized 3He/4He ratios of olivines from Sierra Chichinautzin and Popocatepetl volcanics.
Sample
SPO34
SPO60
SPO56
S15
CH-08-15
CH-07-12
CH-07-14
CH-07-9
CH-07-6
MCH-06-9
CH-08-3
CH-09-2
CH-05-16
MCH-06-12
MCH-06-12
CH-08-17
MCH-06-11
CH-08-8
CH-07-1
ASC45B
CH-08-11
S1
S8
S4
S4
MCH-06-5
S9
a
b
c
d
e
Sample Hea
CH09-5
CH09-4
CH09-6
CH09-9
CH09-10
CH-08-6
CH09-14
CH08-5
S10
He 10−9
(cc STP/g)d
Groupb
Sample Agec
years
Olivine size fraction
micrometer
Weight
g
3
He/4He
(R/Ra) ± 1s
4
Andesitic
Andesitic
Andesitic
Basaltic
Basaltic
Andesitic
Andesitic
Andesitic
Andesitic
Andesitic
Andesitic
Andesitic
Basaltic
Basaltic
Basaltic
Basaltic
Basaltic
Basaltic
Basaltic
Basaltic
Basaltic
Andesitic
Andesitic
Andesitic
Andesitic
Basaltic
Andesitic
Pleistocenee
Holocene
Holocene
Holocene
Holocene
2800
2800
2800
2800
2800
2800
2800
700
700
250–500
250–500
250–500
250–500
250–500
250–500
250–500
250–500
250–500
500–1000
250–500
250–500
250–500
500–1000
250–500
500–1000
500–1000
500–1000
500–1000
500–1000
500–1000
250–500
500–1000
500–1000
250–500
500–1000
500–1000
0.91
1.04
0.99
1.06
0.98
1.03
0.73
1.05
1.01
1.07
1.04
1.06
1.00
1.22
1.10
1.22
1.19
1.15
2.22
0.96
1.30
0.67
1.17
0.62
1.01
1.24
1.11
7.08 ± 0.39
7.01 ± 0.19
7.04 ± 0.79
7.00 ± 0.51
7.08 ± 0.19
6.90 ± 0.11
6.96 ± 0.15
6.81 ± 0.13
7.59 ± 0.08
7.39 ± 0.15
7.39 ± 0.36
7.53 ± 0.06
7.76 ± 0.07
8.00 ± 0.17
1.5
9.9
1
1.9
6.1
19.9
32.7
15.9
205.4
28.1
1.1
124.5
76
8.9
7.15 ± 0.43
7.90 ± 0.19
7.73 ± 0.13
7.79 ± 0.19
7.54 ± 1.30
7.33 ± 0.13
6.98 ± 0.33
7.23 ± 0.10
6.96 ± 0.08
1
7
6.9
6
0.4
23.9
2.5
19.5
48.4
7.66 ± 0.06
7.45 ± 0.18
48.4
14.2
700
700
Holocene
Holocene
Holocene
Holocene
1600
1600
1600
1600
1600
Samples used to obtain olivines for He work, which are a second hand specimen from the same location as sample for bulk rock analyses.
Andesitic (quartz-normative), basaltic (olivine-normative).
Sample ages are based on Siebe et al. (2004b), Bulletin of Volcanology; sample of Holocene age likely b 5000 years.
Uncertainty in He concentrations is approximately ± 20%, governed by incomplete crushing of samples.
Estimated b 1 Ma, presumably a few 100,000 years.
S.M. Straub et al. / Earth and Planetary Science Letters 303 (2011) 337–347
343
fortuitous that two simultaneous melt batches from the same, or a
similar, pyroxenite mantle source, evolved separately by either
pyroxene or olivine fractionation to similar melt Mg# values in
order to crystallize olivine with vastly different Ni abundances just
prior to mixing. An alternative, but far more complex and speculative
scenario involves melt-rock reaction processes within the crust
between olivine-rich stagnating crustal magma bodies and later
ascending silicic melts. Possibly, silicic olivine-bearing, viscous
derivate magmas may stall in upper crustal conduits as semi-solidified
‘plugs’ at elevated temperatures. If such ‘plugs’ were remobilized by
later hot, recharging melts, melt-rock reactions may result that are
comparable to those in mantle peridotite infiltrated by silicic melts. If
silica addition transformed olivine phenocrysts to pyroxene, which
melt again owing to the heat of the recharging magmas, low-Mg#
melts rich in Ni may be produced that are capable of crystallizing
high-Ni olivines at low ~Fo85 values. In all, while the details of
complex crustal processing may be difficult to reconstruct, our data
indicate their existence as well as that silicic melts may be
preferentially affected relative to the less viscous basaltic magmas.
Fig. 2. A: Bulk rock Mg# vs SiO2 wt.% divided into basaltic (olivine-normative) and
andesitic (quartz-normative) compositions (see also Table 1). Other central MVB magmas
are from Wallace and Carmichael (1999), LaGatta (2003), Schaaf et al. (2005), and
unpublished data by Straub and Gomez-Tuena; mid-ocean ridge basalts (MORB) from
GeoRoc(2009). B: Bulk rock Mg# vs Mg# calculated from averaged olivine composition per
= 0.32 and a ferric/ferrous ratio of 0.18± 0.04 (Straub et al.,
sample, using a KDoliv/melt
Fe/Mg
= 0.29–0.33. Total error
2008b). Gray band indicates equilibrium for acceptable KDoliv/melt
Fe/Mg
bars are 2 standard deviations of average Mg# derived from olivine (core and rims) per
sample. Average of upper (UC), middle (MC) and lower (LC) continental crust after
Rudnick and Gao (2002).
occur by fractional crystallization of olivine and/or pyroxenes. All the
same, in Fo–Ni space, trends below Fo88 are dominated by crustal melt
mixing (red arrows in Fig. 3C,D) rather than fractional crystallization
(blue model curves). Crustal melt mixing trends, fully recorded by
olivine chemistry, are typical for late-stage ‘recharge melt mixing’ in
upper crustal conduits, where batches of more evolved derivate melts
mingle with recharging, less evolved melts just prior to eruption (e.g.
Kent et al., 2010). In most samples, olivines with bFo88 can be
modeled through a combination of olivine fractionation and recharge
melt mixing (Fig. 3C,D). The notable exceptions are Ni-rich olivines in
two most silica-rich samples (sample SPO60 and CH08-3 with 58.9
and 61.2 wt.% SiO2, respectively) that have as much as ~ 4000–
4900 ppm Ni at a low ~Fo85 (lilac colors in Fig. 3D, for more details see
Appendix Fig. 3). In the Guespalapa sample CH08-3, only a single
fosteritic olivine is in equilibrium with a pyroxenite initial melt. This
olivine presumably represents a parental melt, but only low Ni
olivines with ~2000 ppm Ni at ~Fo85 can be derived from it (Appendix
Fig. 3B). Two possibilities exist to create the high-Ni olivines at this
low ~Fo85. First, they may crystallize from Ni-rich derivate melt that
formed by pyroxene fractionation of the pyroxenite partial melt
(Appendix Fig. 3B). While straightforward, it seems uncommonly
4.4.2. Lower crustal hot zone and consequence for crustal formation
Crustal processing is key to andesite genesis in the MASH (melting,
assimilation, storage, homogenization, Hildreth and Moorbath, 1988)
and ‘lower crustal hot zone’ models (Annen et al., 2006). These
models propose that the flux to the Moho is basaltic and that silicic
melts were produced exclusively in the crust by fractional crystallization, re-melting and crustal contamination, driven by the heat
released from the mafic magmas, and complemented by the periodic
delamination of mafic residues (e.g., Tatsumi et al., 2008). Our model
conclusively excludes crustal contamination, a major component of
the MASH model, neither does it require the existence of a ‘lower
crustal hot zone’ that may also be questioned on the basis of magma
transfer rates from mantle to upper crust (Zellmer, 2008). Because our
data preclude silica increase by crustal contamination and fractional
crystallization, re-melting of solidified bodies of basaltic mantle melts
stalled in the lower crust, or possibly at the crust–mantle boundary
(underplating), were the only possibility of crustal silica increase. The
challenge is then to produce silicic high-Mg# melts capable of
crystallizing high-Ni olivine with this mechanism. First, this requires
the basaltic mantle melts to be already Ni-rich, such as a partial melt
from silica-deficient pyroxenite. Second, in order to maintain the
high-Mg#, the source lithology must be high-Mg# cumulates or
intrusiva, which are refractory to melting. Third, silicic melts
produced at low degrees of melting had low melt Mg#'s.
We neither have the evidence to support such a mechanism, nor to
conclusively reject it. However, if silicic melts were produced this
way, large amounts of cumulates are required in order to balance their
observed volumes. In contrast, our genetic model with its silicic initial
melts produces minor amounts of residual cumulates. How much
cumulate forms depends on the average composition of the mantle
melts that pass the Moho. If the erupted magmas approximately
reflect this spectrum, residual cumulates should not exceed 10–15% of
total melt mass, and less, if the flux was more silicic. These numbers
are considerably lower than any previous estimates (e.g., White et al.,
2006). Notably, in line with a recent study of the Cascades andesites
(Kent et al., 2010), our model allows for the possibility that much of
the silicic mantle melts may be trapped within the crust as calcalkaline plutons rather than erupt. Central MVB volcanics are on
average andesitic at SiO2 ~ 57 ± 4 wt.% and high Mg#'s ~ 65 ± 6
[n = 375 samples (Wallace and Carmichael, 1999; Schaaf et al.,
2005)] and thus significantly more mafic than the average of silicic
upper and middle continental crust (Rudnick and Gao, 2002)
(Fig. 2A). Within and around the central MVB, voluminous complexes
of shallow intrusive high-silica rocks are exposed (e.g. Zempoala
Ridge, Chalcatzingo trondhjemites) the emplacement of which is
linked to pre-Quaternary MVB arc volcanism (e.g. Gómez-Tuena et al.,
344
S.M. Straub et al. / Earth and Planetary Science Letters 303 (2011) 337–347
Fig. 3. A and B. Ni (ppm) vs Fo (mol%) of olivine phenocrysts analyzed for 3He/4He ratios in basaltic (left) and andesitic magmas (right). For clarity, only olivines from selected
samples are highlighted by colors, grey symbols denote other olivines. See Appendix for calculation of melts in equilibrium with mantle peridotite (green field) and pyroxenites
(ochre field). MORB olivines from Sobolev et al. (2007). C and D. Thick blue lines delineate olivine fractionation trends, with percentage of olivine loss given in italics. Numbers at
starting point indicate MgO wt.% and Ni ppm of initial mantle melts. Big black arrow indicates mixing between pyroxenite and peridotite initial component melts; red lines with
arrows indicate recharge mixing in the upper crust.
2008). These silicic intrusiva have similar or more primitive Sr–Nd–Pb
isotope ratios than the mafic MVB magmas (Gómez-Tuena et al.,
2008) (and our unpublished data). Recently, the Chalcatzingo
trondhjemites have been interpreted as slab melts modified by
reaction with the subarc mantle (Gómez-Tuena et al., 2008). Similar
silicic intrusiva, yet unexposed by erosion, may thus complement the
extrusive Quaternary MVB magmas, and directly contribute to crustal
growth.
4.5. Implications for global arc volcanism
Fig. 4. Air-normalized 3He/4He of olivine separates vs bulk rock143Nd/144Nd. MORB
range and MVB crustal xenoliths (upper and lower crust) from GeoROC (2009);
subducting sediment after Gomez-Tuena et al. (2007) . Mixing trends calculated with
143
Nd/ 14 4 Nd = 0.5131 (mantle) and 0.51238 (crust) for constant mantle
Nd = 1.25 ppm, He = 1.5 ⁎ 105 10−5 cm3 STP/g, but variable crustal He abundance
ranging from 1.1 ⁎ 10−7 cm3 STP/g (curve a), 1.1 ⁎ 10−6 cm3 STP/g (b), 1.1 ⁎ 10−5 cm3
STP/g (c) and 5.5 ⁎ 10−5 cm3 STP/g (d). MORB range after Farley and Neroda (1998).
The release of silicic slab melts or fluids at arc front depths and
formation of ‘reaction pyroxenites’ in the mantle wedge must be
expected in all subduction settings. Mafic series prevail in oceanic
arcs, but results of recent years demonstrated that high-silica dacitic
and rhyolitic volcanics are common as well, forming a secondary
abundance high next to the dominant basaltic-andesitic magmas, e.g.
in the Izu Bonin-Mariana, Tonga-Kermadec, South Sandwich and
western Aleutians arcs (e.g. Tamura and Tatsumi, 2002; Leat et al.,
2003; Smith et al., 2003; Straub, 2003; 2008; Tamura et al., 2010).
Mafic and evolved series are linked by melt mixing and are produced
in constant proportions through the 50 million year life of the Izu
Bonin and Mariana arcs (Lee et al., 1995; Straub, 2003, 2008). In the
Izu Bonin Mariana arc/trench system, high-silica plutonics (granites
and tonalites) may comprise up to 25 vol.% of the present-day arc
S.M. Straub et al. / Earth and Planetary Science Letters 303 (2011) 337–347
345
crust (Kodaira et al., 2007; Takahashi et al., 2007). It has been
suggested that silicic melts formed through assimilation of an
increasingly thickening crust, and that the production of silicic
melts gradually increased with time (e.g. Tatsumi et al., 2008).
However, a time-progressive evolution is not supported by the
observed temporal trends that suggest continuous formation of the
silicic magmas together with the co-eval mafic counterparts (Straub,
2003, 2008; Straub et al., 2010). For these observations our model may
provide a potential genetic process, and focused geochemical studies
of contemporaneous mafic and silicic magmas in oceanic arcs may test
such mechanisms. However, in the oceanic environment, where the
subarc mantle volume is much larger, mantle melts from pyroxenite
veins may not invariably produce high-Ni olivines in the erupted
magmas. High-Ni olivines only crystallize if partial mantle melts from
pyroxenites ascend to surface nearly unmodified. To date, the
database of high-Ni olivine in arc magmas is still incomplete, but no
high-Ni olivines have thus far been reported from oceanic arcs. They
appear also to be missing from Central American volcano Irazu
constructed on ~40 km thick crust (Terry Plank, personal comm.,
2010). Failure to crystallize Ni-rich olivines can have several reasons.
For example, in oceanic arcs, Ni-rich pyroxenite melts formed in the
center of the wedge could react with shallow, olivine-bearing mantle,
or could be more strongly diluted by peridotite component melts.
More intense crustal processing than in the central MVB may also
lower the Ni content of the melt. Even if conditions conducive to the
crystallization of high-Ni olivine exist only in some arcs, their absence
does hence not imply the absence of mafic and silicic partial melts
from pyroxenite. As such, arc outfluxes and arc crustal growth may be
controlled by hybridized slab and mantle to a much larger extent than
perceived. Clearly, understanding how silicic intrusive and extrusive
magmas series relate to their co-eval mafic counterparts in all arcs
emerges as an important next step towards understanding arc magma
genesis and arc crustal growth.
5. Summary and conclusions
We propose that andesitic arc magmas are hybrids of initial highMg# basalt and dacitic mantle melts produced in a subarc mantle
modified by silicic slab components. The principal conclusions are as
follows:
(1) Initial arc melts in the central MVB are high-Mg# basalt and
dacite melts, and their hybrid andesite melts. A range of
parental arc magmas exists, rather than only one type (e.g.
basalt magmas). At A more silicic flux to the Moho requires
lesser crustal processing to produce the broad range of arc
magmas erupted in the central MVB.
(2) Crustal processing of initial mantle melts, such as moderate
fractional crystallization and recharge melt mixing, is essential
to produce the central MVB magmas with Mg# b70. The crust
may also entrap much of the silicic mantle magmas, which then
contribute directly to silicic arc crustal growth.
(3) The model proposed accounts for the contrast between large
variability of SiO2 at a given Mg#, and the strongly linear
mixing trends of SiO2, MgO and FeO* also typical of arc magmas
worldwide. Moreover, it explains why near-primary arc
magmas have relatively low Ni contents.
(4) The elevated silica abundances of central MVB arc magmas are
linked to silica additions from the slab. This implies a more
efficient turnover of slab materials in subduction zones
compared to models that attribute silica increase solely to
complex intra-crustal differentiation processes.
Supplementary materials related to this article can be found online
at doi:10.1016/j.epsl.2011.01.013.
Fig. 5. Cartoon of melt formation model. The mantle is considered as peridotite in which
segregation of silica-deficient and silica-excess pyroxenites are embedded. See text for
discussion.
Acknowledgements
Valentin Troll is thanked for enabling to link high-Ni olivines with
He isotope ratios, and Charlie Langmuir for the use of his laboratory
for some of the abundance work. Rick Conrey, Beth Goldoff, Wen-Yu
Hsu, Charles Mandeville, Ofélia Pérez-Arvizu and Dave Walker helped
with the analyses, Joshua Brown, Amy Knorpp and Gary Mesko with
sample preparation, and Jason Jweda, Laura Mori, Peri Sasnett and Jill
van Tongeren participated in the field work. Discussions with Terry
Plank, Greg Shellnut and Dave Walker, formal journal reviews by Gene
Yogodzinski and an unknown reviewer are greatly appreciated, as
well as comments and editorial handling by Rick Carlson. This study
was financially supported by the U.S. National Science Foundation
346
S.M. Straub et al. / Earth and Planetary Science Letters 303 (2011) 337–347
(grant EAR-07-38707 to SMS) and the National Science Council of
Taiwan (grants 96-2811-M-001-023 and 98-2811-M-001-052 to GFZ).
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