Volatile element (B, Cl, F) behaviour in the roof of an axial magma

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Earth and Planetary Science Letters 213 (2003) 447^462
www.elsevier.com/locate/epsl
Volatile element (B, Cl, F) behaviour in the roof of an axial
magma chamber from the East Paci¢c Rise1
Kathryn M. Gillis a; , Laurence A. Coogan b , Marc Chaussidon c
a
School of Earth and Ocean Sciences, University of Victoria, P.O. Box 3055, Victoria, BC, Canada V8W 3P6
b
Department of Geology, The University of Leicester, University Road, Leicester LE1 7RH, UK
c
Centre de Recherches de Petrographiques et Ge¤ochimiques, BP 20, 15 rue Notre Dame des Pauvres,
54501 Vandoeuvre-le's-Nancy Cedex, France
Received 23 August 2002; received in revised form 23 April 2003; accepted 9 June 2003
Abstract
Understanding the behaviour of volatile elements at mid-ocean ridges is important for reasons ranging from their
influence on mantle viscosity through to their role as a food source for the deep biosphere. With the aim of
constraining what processes control the distribution of volatiles in the ocean crust at fast-spreading ridges, we present
a detailed study of the compositional variability in magmatic amphibole formed in the upper part of the plutonic
sequence at the East Pacific Rise (EPR). These amphiboles are massively enriched in chlorine (by more than an order
of magnitude), and moderately enriched in boron, with respect to magmatic amphiboles in cumulates from the MidAtlantic Ridge (MAR). Similar enrichments have been reported for basaltic glasses from the EPR and are interpreted
as indicative of assimilation. The greater enrichments observed in the plutonic section suggest both that assimilation
occurs at the roof of the axial magma chamber (AMC) and that lava compositions may record minimum amounts of
exogenic contamination. Amphiboles with compositions indicative of crystallisation from a contaminated magma
occur to depths of 800 m beneath the sheeted dyke complex. This is interpreted to indicate that at least this upper
portion of the plutonic section forms via crystallisation within the AMC followed by subsidence of a crystal mush.
Amphibole boron isotope compositions show that assimilation of altered sheeted dykes plus hydrothermal fluids
drives AMC magmas to heavier N11 B values (up to +5.8x). Subsequent degassing within a solidifying crystal mush
leads to a negative trend in N11 B^B with the most degassed magma having N11 B as low as 321.2x. This degassing
was associated with hydrofracturing of the partially molten crystal mush and could have facilitated a temporal link
with the overlying hydrothermal system.
: 2003 Elsevier B.V. All rights reserved.
Keywords: assimilation; East Paci¢c Rise; lower ocean crust; volatiles; magmatic degassing; amphibole trace elements; axial
magma chamber; B isotopes
* Corresponding author. Tel.: +1-250-472-4023; Fax: +1-250-721-6200.
E-mail addresses: [email protected] (K.M. Gillis), l[email protected] (L.A. Coogan), [email protected] (M. Chaussidon).
1
Supplementary data associated with this article can be found at doi:10.1016/S0012-821X(03)00346-7
0012-821X / 03 / $ ^ see front matter : 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0012-821X(03)00346-7
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1. Introduction
The geochemistry of mid-ocean ridge basalts
(MORB) and their plutonic counterparts provide
the foundation for estimating the chemical £ux
from the mantle along the global mid-ocean ridge
(MOR) system. The volatile elements are of particular interest as they in£uence the evolution and
di¡erentiation of the crust, mantle and exosphere,
and play a pivotal role in sustaining subsurface
microbial and active hydrothermal vent communities. The volatile £ux from the mantle can be
monitored using the compositions of MORB glass
(e.g. [1]), £uid inclusions trapped in plutonic and
basaltic rock [2], and hydrothermal £uids venting
at the sea£oor [3,4]. This £ux is primarily
achieved by magma transport and eruption, magma degassing, and leaching by hydrothermal £uids. The exogenic volatile £ux into the crust and
mantle is poorly constrained, but volatile constituents are known to reside in hydrous, hydrothermal phases and as £uids stored in pore spaces.
At fast-spreading MORs, mantle-derived and
exogenic volatile components meet at the sea£oor,
as lavas erupt and hydrothermal £uids carrying
components leached from the ocean crust vent.
They also meet within the crust at the magma^
hydrothermal boundary, located above the roofs
of axial magma chambers (AMCs). In a static
situation, mantle-derived material (sheeted dykes
and lavas) above this interface would be chemically modi¢ed by reaction with seawater-derived
£uids whereas below the AMC, volatiles and other components would be solely mantle-derived
due to the impermeability of AMCs. Geophysical
surveys along the East Paci¢c Rise (EPR), however, provide indirect evidence that the depth and
internal properties (e.g. melt volumes, proportion
of melt and crystals) of AMCs are not static [5,6].
Along the southern EPR, for example, the depths
of AMCs do not correlate with indicators of the
long-term magma budget of a ridge segment [7^9].
Indeed, the depths of AMCs may vary by as
much 500 m over short distances (2^20 km) [10],
implying that AMCs likely migrate vertically on
short timescales (presumably linked to eruption;
i.e. timescales of years to centuries). Thus, as part
of the evolutionary cycle of a ridge segment, an
AMC could advance upwards by the instantaneous injection of a new lens at a shallower level in
the crust, the progressive assimilation of its roof
and the overlying sheeted dykes, or a combination
of these processes [11]. A newly injected lens
would intrude into the root-zones of an earlier,
deeper hydrothermal system, leading to a shoaling
of the hydrothermal system. This in£ux of magmatic heat would cause partial melting of the new
roof and ingress of roof material into the magmatic system [12]. In a similar fashion, progressive
upward migration of an AMC would cause stoping of hydrothermally altered material.
There are some clues in MORB chemistry that
point to assimilation being an important process
at fast-spreading ridges. The best evidence comes
from the over-enrichment of Cl in normalMORBs (n-MORB) from the EPR relative to
those from the Mid-Atlantic Ridge (MAR),
when corrected for mantle source variability and
di¡erentiation (Fig. 1) [1,13,14]. The Cl content of
magmatic amphibole also supports this (documented below; Fig. 1). The existence of a
steady-state AMC at fast-spreading ridges provides a much greater capacity for assimilation
than is available at slow-spreading ridges, where
magmatic activity is ephemeral and short-lived.
Contamination of n-MORB by assimilation has
also been called upon to explain the B- and Clisotope systematics for EPR glass [15,16], although with much more limited data.
The magma^hydrothermal interface is the best
place to examine the processes associated with the
cycling of exogenic components as it is only here
that the thermal conditions would allow for signi¢cant assimilation. We present major element,
trace element and B-isotopic data for magmatic
amphibole hosted in high-level plutonic rocks
formed at the EPR to examine how mantle-derived MORB dykes and exogenic components
(Cl, B) are cycled through the magmatic system
and back into the upper crust and hydrosphere.
Our samples come from the Hess Deep area in the
equatorial Paci¢c where tectonic unroo¢ng has
exposed sections of young EPR crust and, in particular, the critical sheeted dyke^gabbro boundary
where assimilation must be focussed [17,18]. This
is a unique sample suite in that gabbroic samples
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449
Fig. 1. Histogram of Cl contents normalised to unity for (a) Mid-Atlantic Ridge glass [14]; (b) East Paci¢c Rise glass [14];
(c) Mid-Atlantic Ridge magmatic amphibole [27]; and (d) East Paci¢c Rise magmatic amphibole (this study). Note that fastspreading EPR glass and, especially, magmatic amphibole display extreme Cl enrichment relative to the slow-spreading MAR.
Note that MAR glass data exclude samples from near hot spots (e.g. AMAR).
are known, unequivocally, to have resided immediately beneath a sheeted dyke complex, and thus
must have formed within an AMC. We show that
the assimilation of volatile components stored in
the basal sheeted dyke complex is required to explain the amphibole chemistry, and that magma
degassing plays a key role in the late-stage evolution of AMC magmas.
2. The Hess Deep sample suite
The Hess Deep is the deepest part of a rift
valley that formed by the propagation of the Cocos^Nazca spreading centre into the eastern side
of the Galapagos microplate, rifting young (0.5^
1.2 Ma) crust that formed at the fast-spreading
(130 mm/yr) EPR [19]. The samples used in this
study were selected from three localities in the
Hess Deep area (see ¢gure 1 in [20] for map).
The primary location is the northern rift valley
wall where two Alvin dive programmes examined
a well exposed sheeted dyke complex and upper
gabbro sequence (P. Lonsdale, unpublished data,
1992; [18,21]). Here, gabbroic rocks have been
sampled from outcrops up to 800 m beneath the
base of the sheeted dyke complex [18]. The second
location is the western end of an intrarift ridge
that is 15 km southwest of the ¢rst area. These
exposures, explored by the Nautile submersible
(Dive 10) [17] and drilled by the Ocean Drilling
Program (ODP) during Leg 147 (Site 894), are
interpreted to have formed near the top of the
gabbroic sequence [22^24]. The third locality, located south of Site 894 along the southern slope
of the intrarift ridge, was explored by the Nautile
submersible (Dives 9 and 18)[22]. Samples from
this area are representative of deeper levels in
the gabbroic sequence than those recovered at
the former two localities [11,22].
The dominant lithology in the upper gabbros is
gabbronorite with less abundant amphibole gabbro, olivine gabbronorite, gabbro, olivine gabbro,
Fe^Ti gabbronorite, and Fe^Ti oxide amphibole
gabbro [23,25,26]. Lithologies from the southern
slope of the intrarift ridge are generally less
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Fig. 2. Photomicrographs showing typical magmatic amphibole textures. (a) Brown granular amphibole rim on clinopyroxene in
gabbronorite (sample 3369-1042). (b) Brown granular interstitial amphibole in Fe^Ti oxide amphibole gabbro (sample 33701408). (c) Brown magmatic vein cutting gabbronorite (sample NZ 10-15), indicative of supra-solidus brittle deformation. Field of
view for panel a is 1.5 mm and for panels b and c, 5 mm. A = amphibole; C = clinopyroxene; Pl = plagioclase; Ox = Fe^Ti oxide.
evolved and include olivine gabbro, amphibole
gabbro, gabbro, gabbronorite, troctolite, and
anorthosite [20,22]. Magmatic amphibole is prevalent in most samples from the upper gabbros but
is rare in the lower gabbros.
3. Analytical methods
Electron and ion microprobe analyses were determined on the same thin sections. Major element mineral compositions were determined by
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wavelength dispersive techniques at the University
of Alberta; standard ZAF corrections and natural
standards were used.
Trace elements were determined using a Cameca IMS-4f ion microprobe at the University of
Edinburgh following a similar methodology to
that described in [27]. Prior to analysis samples
were washed individually in an ultrasonic bath
in petroleum ether for s 5 min, dried on a hotplate and re-washed twice in de-ionised water,
prior to gold-coating. After a few minutes of sputtering to remove the gold coat and surface contamination, analyses were performed as two analyses within the same pit with the light (Li, Be, B,
F, Cl, K, Sc, Ti, V, Cr) and heavy elements (Sr,
Y, Zr, Nb, Ba, rare earth elements (REEs), Hf)
analysed separately with all counts ratioed to 30 Si.
Heavy elements were run ¢rst to diminish possible
surface contamination of the light elements. A
primary 16 O3 beam of 15 keV net energy was
focussed on a 10^20 Wm spot with an V8 nA
current and standard energy ¢ltering techniques
were used to reduce molecular interferences. Barium and light (L)REE oxide corrections on Eu
and the heavy (H)REEs were performed o¥ine as
were major element molecular interferences on V
and Cr. Calibration was achieved via ion yields
calculated based on NIST 610 assuming 500 ppm
of all elements in this glass except for F (295 ppm
[28]). Hoover Dam amphibole was analysed daily
as a standard to monitor accuracy and precision
and the results are compared with INAA data [29]
(Table 11 ).
The boron isotope compositions of selected amphiboles were analysed by ion microprobe (Cameca IMS-3f) at the CRPG-CNRS in Nancy over a
3-day period, following techniques described in
[30]. Analyses were immediately adjacent to those
for the trace elements. Examination by SEM
showed that the major element compositions of
amphibole are homogeneous in the vicinity of
trace element and B-isotope spots. The boron isotope data are reported as N11 B values
(N11 B = 1000U(11 B/10 Bsample 611 B/10 Bstandard 31)
relative to a NBS 951 borate standard (11 B/
1
See online version of this paper.
451
10
B = 4.04558). Instrumental mass fractionation
was monitored using GB4 glass. The analytical
precision of the N11 B values is S 1.7x (1 c).
4. Results
Major element, trace element, and B-isotopic
data have been determined for magmatic and hydrothermal amphibole in the Hess Deep gabbros.
We report data for magmatic amphibole only (see
Table 11 ), as the purpose of this paper is to examine the evolution of volatiles in AMCs. We describe the criteria used to distinguish between
magmatic and hydrothermal amphibole in Section
4.1; only magmatic amphibole is discussed in subsequent sections.
4.1. Amphibole characteristics
Amphibole analyses are divided into three populations on the basis of their texture and Nb content. Magmatic amphiboles are brown to greenish-brown and granular with Nb s 1 ppm;
hydrothermal amphiboles ¢ll veins and replace
primary silicate phases and have Nb 6 1 ppm;
and amphiboles with equivocal origins meet one
criterion each for magmatic and hydrothermal
amphibole. Niobium contents are used to distinguish magmatic from hydrothermal amphibole
because Nb is relatively immobile during hydrothermal alteration and is highly incompatible in
the other igneous silicate phases (olivine, orthopyroxene, clinopyroxene, plagioclase contain 6 1
ppm Nb and generally I1 ppm), meaning that
amphiboles with high Nb contents are almost certainly magmatic in origin (see [27] for discussion).
High Nb/La was used by [27] as a criterion for
discriminating magmatic amphiboles in the gabbroic rocks from the MARK area of the MAR. In
the Hess Deep suite, there is no di¡erence in Nb/
La between amphiboles which have clear textural
discrimination as magmatic or hydrothermal. We
interpret this as being due to a greater role of
ilmenite in controlling the bulk partitioning of
Nb, as magmatic amphiboles with low Nb/La
only occur in oxide-rich samples. Major element
compositions cannot be used as unequivocal cri-
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terion because major and trace elements can be
decoupled during subsolidus interactions with seawater-derived hydrothermal £uids [31]. However,
we note that magmatic amphiboles de¢ned in this
way generally have higher Ti, Na and K abundances and lower Si abundances and Mg#’s (Mg/
Mg+Fe). Plagioclase^amphibole thermometry, using the calibration of Holland and Blundy [32],
indicates equilibration temperatures for magmatic
amphibole of 850^925‡C, similar to temperatures
for magmatic amphibole from the MAR [27]. Hydrothermal amphibole equilibrated at lower temperatures (610^814‡C) [20].
The samples used in this study include seven
Fe^Ti oxide amphibole gabbros recovered from
the northern rift valley wall within 200 m of the
sheeted dykes; ¢ve gabbronorites that are either
intermixed with, or lie beneath, the evolved upper
gabbros ; and three deeper amphibole gabbros
from the southern slope of the intrarift ridge.
Magmatic amphibole is a common phase in the
upper gabbros but is rare in the deeper gabbros.
It forms granular rims on clinopyroxene (Fig. 2a),
discrete granular grains (Fig. 2b), and, in one
sample (NZ 10-15), ¢lls fractures (Fig. 2c). Magmatic amphibole comprises a signi¢cant part of
the mineral mode in gabbronorites ( 9 5%), in
contrast to the much smaller amounts generally
observed in samples from the MAR (generally
6 0.5% [27]). In more evolved lithologies (e.g. Fe^
Ti oxide amphibole gabbros) magmatic amphibole can form up to 20% of the mode where it
occurs in association with Fe^Ti oxides, quartz,
apatite, zircon, and/or sul¢des.
4.2. Trace element systematics
Magmatic amphiboles from the Hess Deep plutonics display a wide range of abundances; for
example, chondrite-normalised REE concentrations range from 6 10 to s 1000 (Fig. 3). Amphiboles hosted in Fe^Ti oxide amphibole gabbros have the highest REE contents whereas
amphibole in gabbronorites and amphibole gabbros generally have lower values. The LREEs are
variably fractionated ((La/Sm)n = 0.08^1.67) and
show no correlation with rock type. The most
LREE-enriched analyses show the greatest rela-
Fig. 3. Chondrite-normalised [59] trace element plot for magmatic amphibole. Bold lines represent amphibole gabbro and
gabbronorite samples; thin lines represent Fe^Ti oxide amphibole samples.
tive depletions in Eu, Sr, Ti, Zr and Nb, which
is indicative of co-precipitation of plagioclase (Eu,
Sr) and ilmenite S magnetite S zircon (Ti, Zr, Nb)
with amphibole. Magmatic amphibole from the
MARK area di¡ers from the Hess Deep in that
it formed by an interstitial melt^crystal mush reaction which consumed plagioclase, not by coprecipitation of plagioclase and amphibole [27].
The REE data fall within the compositional range
seen in previous study of Hess Deep magmatic
amphiboles [31,33].
In order to distinguish elements that behave
conservatively during progressive crystallisation
from those that have more complex histories,
Fig. 4 shows how selective incompatible elements
vary with respect to Nb. Crystallisation vectors
are shown to illustrate expected compositional
trends for a magma crystallising various assemblages, beginning at amphibole saturation (see ¢gure caption for details). Kinks in these trends
mark the onset of either Fe^Ti oxide (Fig. 4a)
or accessory apatite (Fig. 4b^d) crystallisation.
Titanium, Ce, (Eu/Eu*)n , and, to a lesser extent,
F follow expected crystallisation trends (Fig. 4a^
d), as do most other elements, including Be, K,
Zr, Y, Ba, Sr, La, Pr, Nd, Sm, and the heavy
REEs. By contrast, Cl is decoupled from other
similarly incompatible elements as its abundance
does not increase as rapidly as predicted (Fig. 4e)
and B displays no correlation with Nb over the
entire compositional range (Fig. 4f), suggesting
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K.M. Gillis et al. / Earth and Planetary Science Letters 213 (2003) 447^462
453
Fig. 4. Selected trace element abundances plotted versus Nb: (a) Ti, (b) Ce, (c) (Eu/Eu*)n (Eun * = (Smn +Gdn )/2), (d) F, (e) Cl,
(f) B. Open symbols represent amphibole gabbro and gabbronorite samples; closed symbols represent Fe^Ti oxide amphibole
samples. Vectors indicate the compositional trends for a melt crystallising silicates (40% plagioclase, 35% clinopyroxene, 15% orthopyroxene, 10% amphibole) and silicates (42% plagioclase, 7.5% clinopyroxene, 5% orthopyroxene, 35% amphibole), Fe^Ti oxides (4% magnetite, 4% ilmenite), and apatite (2.5%). The modal proportions of the crystallising phases are from the melting experiments of [60^62]. For the elements shown, crystallisation of zircon does not signi¢cantly change the vectors. See Table 21 for
the distribution coe⁄cients used.
that Cl and B concentrations are controlled by
other factors.
Hess Deep amphiboles are s 10 times more
enriched in Cl (Figs. 1 and 5) and up ¢ve times
more enriched in B than amphiboles from the
MARK area of the MAR [27]. Moreover, F/Cl
ratios are signi¢cantly lower in the Hess Deep
amphiboles (0.1^4.2) than for those from the
MAR (9^226), highlighting a very di¡erent evolution for these volatile elements in fast- versus
slow-spreading magmatic systems (Fig. 5). The
thermodynamics of halogen partitioning between
amphibole and melt are poorly understood and
amphibole major element composition is known
to be an important factor [34]. However, the
MARK and Hess Deep amphiboles have generally similar major element compositions and F
contents, but have dramatically di¡erent chlorine
contents, demonstrating that the origin of the
chlorine enrichment cannot be due to di¡erent
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4.3. B-isotope systematics
Fig. 5. Cl versus F contents in magmatic amphibole. Note
the extreme enrichment of Cl with respect to F in the EPR
amphibole relative to those from the MAR. MAR amphibole
data from [27].
partitioning. Instead, the melt from which the
Hess Deep amphibole grew must have been massively enriched (by Vone order of magnitude) in
chlorine. By analogy with the enrichment of Cl in
MORB from the EPR, this enrichment is interpreted to come from assimilation of Cl-rich material (see Sections 1 and 5.2).
The evolution of Cl and F can also be traced
through apatite compositions, because apatite
forms ideal solid solutions between F^Cl^OH
endmembers at magmatic temperatures [35]. Since
the major element composition of apatite is essentially constant, halogen contents should record
melt composition variation even more faithfully
than amphibole compositions. Fig. 6 shows that
apatite compositions range from Cl-rich to F-rich,
with Cl-rich compositions being in the less
evolved samples and the F-rich compositions in
the more evolved samples. Apatite from the
MARK area of the MAR de¢nes a Cl^F trend
that parallels the Hess Deep trend but is o¡set
towards lower Cl values [36]. This is likely indicative of lower Cl contents in the MAR melts, providing additional evidence that assimilation plays
a signi¢cant role at the EPR [36]. During exsolution of a volatile phase from a magma Cl preferentially partitions into the £uid over F leading to
an increase magma F/Cl. Thus, the trend in apatite compositions is consistent with a model in
which Cl is progressively degassed from an evolving crystal mush during solidi¢cation.
Boron isotopic ratios show a wide range, from
+5.8 to 321.2x; analyses for individual samples
vary by up to 15x (see Table 11 ). There are no
systematic di¡erences between the N11 B of gabbronorite and Fe^Ti oxide amphibole gabbro. N11 B
values generally decrease with increasing B and K
contents. No correlation is observed between N11 B
values and elements that follow crystallisation
trends (e.g. Nb, Ce; Fig. 4). For comparison,
fresh n-MORB glasses from the EPR have N11 B
values from 31.5 to 36.5x [15,37] and are heavier than the best estimate of primitive mantle
values (N11 B = 310 S 2x [38]). The N11 B values
for seawater and hydrothermal £uids from unsedimented ridges are +40x [37] and +30 to
+36.8x [39], respectively. Hence, amphibole
N11 B values are signi¢cantly lower than seawater-derived £uids and extend to both higher
and lower values than average n-MORB.
Palmer and Swihart [40] provide a comprehensive review of B-isotope geochemistry. The most
relevant point for this study is that fractionation
between the isotopes of boron is largely controlled
by the preference of 11 B and 10 B for trigonal (£uid) and tetrahedral (minerals or melt) coordina-
Fig. 6. Cl versus F contents in apatites from Hess Deep
showing the average MgO and Nb contents of amphiboles in
the same samples. Note that the apatites in the less evolved
samples (higher MgO and lower Nb in amphibole) are more
chlorine-rich and these evolve to more F-rich compositions.
This is most readily explained as due to the loss of Cl from
the interstitial melt during degassing with Cl preferentially
partitioning into the vapour phase [63]. Sample symbols:
¢lled diamonds, 3369-1042; ¢lled squares, 3370-1418; ¢lled
triangles, 3370-1408; open squares, 3316-1431; ¢lled circles,
NZ 10-14.
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tion, respectively. This means that geological processes that involve £uid extraction, such as prograde metamorphism and magma degassing, act
to shift the N11 B for liberated £uids and residual
solids or melts towards heavier and lighter values,
respectively.
5. Discussion
In the previous section, it has been shown that
the Cl and B contents of magmatic amphibole
cannot be explained by closed system crystallisation of an uncontaminated n-MORB magma.
Here, we suggest that amphibole compositions
record the combined e¡ects of degassing of a crystallising melt within the solidifying margin of an
AMC (Section 5.1) and assimilation of hydrothermally altered sheeted dykes and hydrothermal £uids by an AMC (Section 5.2). Some of the implications of these processes for the volatile budget
of fast-spreading ridges are then discussed.
5.1. Open system degassing of the AMC
Amphibole compositions re£ect the combined
e¡ects of melt compositions at the time of amphibole saturation, and simultaneous crystallisation
and degassing of this melt. In this section, we
examine the e¡ects of the latter two factors on
amphibole compositions. Only after removing
these e¡ects can amphibole compositions be
used to investigate assimilation processes.
We assume throughout this discussion that the
magma would be well mixed with respect to the
mantle-derived and assimilated components at the
on-set of amphibole crystallisation ; i.e. assimilation will occur at high temperatures and amphibole saturation will occur some time later at lower
temperatures. At this stage, the melt would have
2^6 wt% H2 O, based on experimental studies of
amphibole stability (e.g. [41,42]), and would be
close to, or at, H2 O saturation at the low pressure
of an AMC (e.g. [43]). Assuming an initial H2 O
content of 0.1^0.2 wt% for n-MORB [44], 90^95%
closed system crystallisation would be required to
concentrate H2 O su⁄ciently to stabilise amphibole.
455
If, as is likely, water is added to the system
through assimilation, water saturation and amphibole stabilisation would occur at higher melt
fractions. This is consistent with the observation
of much higher proportions of magmatic amphibole in plutonic rocks from Hess Deep than samples from the MAR where assimilation is unimportant (e.g. [1]). Saturation of the magma with
amphibole as a cumulus phase in the Hess Deep
parental magma, as opposed to the formation of
amphibole as a product of melt^cumulate interaction as suggested for the MARK area samples
(see Section 4.2), is also consistent with the Hess
Deep parental magma being more volatile-rich.
Magmatic £uids exsolved during crystallisation
would be supercritical vapours and brines [45],
as demonstrated by the common occurrence of
co-genetic brine and vapour £uid inclusions in
evolved gabbroic rocks [2].
As described in Section 4.3, boron isotopes are
good tracers of processes involving degassing due
to the strong partitioning of 10 B and 11 B between
melts and £uids. The boron isotopic shift of AMC
melts that results from the loss of volatiles during
crystallisation may be predicted using a Rayleigh
distillation model. Fig. 7 shows a calculated melt
evolution curve caused by coeval crystallisation
and £uid exsolution. The close ¢t of the model
trend with the amphibole data demonstrate that
magma degassing is a viable mechanism for the
observed 11 B depletion and B abundance increase
in magmatic amphibole during the latter stages of
AMC crystallisation. Varying parameters, such as
Dfluid melt and the relative rates of degassing and
crystallisation (see ¢gure caption), would modify
the slope of the curve but would not change the
nature of the N11 B^B correlation. Degassing could
also explain the relatively limited enrichment in Cl
compared with that of Nb in amphibole (Fig. 4e)
and the trend of decreasing Cl compared to F in
apatite with crystallisation (Fig. 6), although the
e¡ect of degassing on Cl systematics is complex to
predict, as the solubility of Cl is strongly dependent upon melt composition, pressure and oxygen
fugacity [46]. Moreover, the evolution of volatile
species is likely to be further complicated by, for
example, interaction of £uids exsolved in one area
reacting with magmas in di¡erent areas.
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K.M. Gillis et al. / Earth and Planetary Science Letters 213 (2003) 447^462
Fig. 7. Boron concentrations versus N11 B (x) for magmatic
amphibole. The model trend illustrates the change in amphibole N11 B and B values during simultaneous crystallisation
and degassing. The following parameters were used for this
Rayleigh fractionation model (e.g. equation 17 of [64]).
(1) An experimentally derived £uid^melt boron isotopic fractionation factor at 850‡C (K = 1.00527) [65]. (2) Dfluid melt = 3
[66] and Dsilicates melt = 0.01. (3) The ratio of exsolution to
crystallisation was set at 20:80. We further assume that boron is largely tetrahedrally coordinated in the melt [67],
which implies that amphibole^melt isotopic fractionation
should be minimal. Thus, during crystallisation the B becomes concentrated in the melt (bulk distribution coe⁄cient
V0.61 ( = 0.8U0.01+0.2U3) although slightly di¡erent for
each isotope) and thus amphiboles crystallised at lower temperatures when lower melt fractions remaining have higher B
abundances. At the same time, degassing fractionates the isotopes, driving the magma to more negative N11 B values because of the preference of 11 B for the £uid phase. Error bars
are S 1.7x.
An alternative model for the co-variation in
N11 B and B is that the lightest N11 B values with
the highest B abundances are formed from crystallisation from an uncontaminated magma and
that the heavier values result from interaction of
amphibole with hydrothermal £uids (Wolfgang
Bach, written communication, 2003). This requires that interactions with hydrothermal £uids
produce the largest isotopic shift in amphiboles
with the lowest primary B contents. Although
we cannot discount this model unambiguously
two lines of evidence lead us to favour the degassing model proposed above. Firstly, this would
require a signi¢cant revision of the N11 B of the
MORB mantle source from 310x S 2 [38] to
325x; which, amongst other things, would require signi¢cant contamination of all oceanic basalts with a high N11 B material that would pre-
sumably be seawater-derived. Secondly, both
seawater and hydrothermal £uids have relatively
high B concentrations, as do hydrothermal amphiboles from the MARK area of the MAR relative to magmatic amphiboles [27], making it unlikely that the low B abundance amphiboles are
formed by reaction with hydrothermal £uids.
Other lines of evidence indicate that degassing
is a signi¢cant process as magmas crystallise and
cool within the lower ocean crust. Fluid inclusion
data from the slow-spreading MAR and Southwest Indian Ridge show that magmatic £uids
evolve from being CO2 -rich to H2 O-rich [47,48],
consistent with theoretical predictions [44]. N13 C
values for these £uids are best explained by Rayleigh distillation of CO2 , followed by closed system respeciation and graphite precipitation during
cooling [49]. More limited study of gabbros from
the Hess Deep area show a di¡erent evolutionary
path for magmatic £uids in that they lack evidence for CO2 [50]. While more data are required
to understand these di¡erences, it is plausible that
at slow-spreading ridges £uids evolve in a closed
system whereas at fast-spreading ridges the system
is open [2].
5.2. Assimilation of the roof of the AMC
Normal-MORB glasses from the EPR are enriched in Cl and N11 B relative to expected values
for the fractionation products of mantle-derived
melts; characteristics that have been interpreted
to be related to interaction with seawater-derived
components [1,13,14,38]. Enrichment of B and Srisotope values in glass from the Galapagos
Spreading Center is also attributed to interaction
with hydrothermally altered material [51]. Where
this contamination occurs is uncertain; it could
result from £uid^melt interaction as magma is
transported to the sea£oor, interaction at the sea£oor during extrusion (e.g. [52]), or assimilation
of hydrothermally altered roof rock and/or seawater-derived £uid into a magma chamber. Assimilation has been proposed to be the most likely
explanation, at least for Cl, due to the large di¡erence between enrichment seen at the fast-spreading EPR and slow-spreading MAR [1]. Magmatic
amphiboles from the upper gabbros at Hess Deep
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K.M. Gillis et al. / Earth and Planetary Science Letters 213 (2003) 447^462
display similar compositional enrichments as EPR
glass (Fig. 1), con¢rming that assimilation must
be prevalent along the roof and margins of
AMCs.
Quantitative modelling of the assimilation process is complicated by a number of factors. First,
amphibole^melt distribution coe⁄cients for B and
Cl are imperfectly known and must be a function
of temperature, and amphibole and melt composition. Second, amphibole saturation within the
AMC probably occurs within a solidifying crystal
457
mush into which assimilation is unlikely to occur
(although partially molten assimilated blocks
could continue to dehydrate and/or dissolve).
Thus, amphibole compositions would have to be
backtracked not only into a melt composition but
also for the crystallisation interval between the
end of assimilation and amphibole saturation.
When the uncertainties related to these processes
are combined with the assumptions inherent in
any modelling of assimilation into an open system, crystallising magma body, quantitative modelling of this process becomes uninformative.
Due to the complications in quantitative modelling discussed above, we have instead opted to
qualitatively assess the assimilation process. We
do this in Fig. 8 by comparing the compositions
of the Hess Deep amphiboles with those from the
MARK area gabbros. The latter are assumed to
represent the composition of amphibole formed in
the late stages of crystallisation from a n-MORB
parental magma which was una¡ected by assimilation. To illustrate the relative changes in melt
(and thus amphibole) composition derived through
6
Fig. 8. Cl^B^N11 B systematics in magmatic amphiboles from
Hess Deep gabbros. Simple two-component mixing curves
are shown to illustrate the compositional e¡ects of possible
endmember assimilants (see text for description). The starting
point (black circle) is based on the most primitive amphibole
compositions from the MARK area, which are unlikely to
have been in£uenced by assimilation [27]. Solid vectors illustrate four endmember assimilants. (1) Hydrothermally altered
sheeted dykes (average values for ODP Hole 504B sheeted
dykes: 49^650 ppm Cl (ave. 240 ppm) [68], 0.2^1.1 ppm B
(ave. 0.4 ppm) [69], N11 B ratios of 30.1 to 1.0x [69]).
(2) Dehydrated, high-grade sheeted dykes with no porosity
(assumptions: B is partitioned into £uids so high-grade rocks
(restites) are depleted in boron (e.g. [70,71]); dehydration residues have light N11 B values (e.g. [72]), due to the preferential
partitioning of 11 B into the £uid phase [40]; Cl is not extensively partitioned into the metamorphic £uids because Cl is
favoured over OH in amphibole structures at high temperatures [34]). (3) Hydrothermal £uids (average EPR vent £uid
composition; data from [39]). (4) Brine (B and Cl data from
450‡C experiments of [39], assumes no isotopic fractionation
with phase separation [73]). Dashed vectors are illustrative of
crystallisation and magma degassing (see Section 5.1 for an
in-depth discussion of degassing). Open symbols represent
amphibole gabbro and gabbronorite samples; closed symbols
represent Fe^Ti oxide amphibole samples.
EPSL 6736 5-8-03
458
K.M. Gillis et al. / Earth and Planetary Science Letters 213 (2003) 447^462
the assimilation of di¡erent materials, we show
mixing lines between MARK amphibole and different possible assimilants. Possible assimilated
materials in an AMC roof may include any combination of hydrothermally altered sheeted dykes
and/or gabbros, hydrothermal £uids, and/or
brines. As an AMC advances upward, altered
sheeted dykes could be incorporated into the magmatic system as unmodi¢ed blocks (through stoping) or dehydrated, high-grade metamorphic
rocks. In the former case, whereby large blocks
of dykes containing pore £uids are stoped into
AMCs, dykes would undergo a series of petrochemical changes and the metamorphic £uids
would be released within the AMC. Constraints
from the Troodos ophiolite show that in the latter
case, dykes would recrystallise to amphibolite to
granulite facies assemblages and partially melt
prior to their assimilation into the magmatic system [12,53]. This is because dehydration reactions
are driven by progressive upward migration and/
or thickening of the conductive boundary layer
within AMC roofs. Direct ingress of seawater-derived hydrothermal £uids, or brines that formed
by phase separation, into AMCs could plausibly
be achieved during episodic fracturing events of
the conductive boundary layer, due to dyke intrusion or other mechanisms.
When comparing the mixing lines with the amphibole data (Fig. 8), it is important to note that
amphibole probably crystallised after assimilation
was complete. This means that the compositional
range displayed by amphibole re£ects changes in
melt composition due to crystallisation and magma degassing (see Section 5.1), in addition to the
assimilation-in£uenced melt composition at the
time of amphibole saturation. Keeping this in
mind, it is possible to place some constraints on
the nature of the assimilated material. The heaviest amphibole N11 B values occur at the lowest B
abundances (Figs. 7 and 8), suggesting that these
re£ect the magma composition after assimilation.
To achieve these N11 B values requires addition of
N11 B-enriched material, which rules out dehydrated roof-rock from being the sole component
assimilated (Fig. 8b,c). If, instead, enough brine
or hydrothermal £uid is added to achieve the positive N11 B values, much higher Cl and B contents
result than are observed (Fig. 8b,c), suggesting
that £uids cannot be the only components assimilated. Addition of hydrothermally altered rock
would not be able to produce the high B or Cl
contents. Thus, the most likely assimilated material is a combination of altered rock and £uid¢lled porosity requiring a signi¢cant £ux of assimilated material into the AMC [11]. The porosity of the AMC roof zone and lowermost sheeted
dykes is not well known, but is likely 6 2% and
probably much lower (see [11] for review of available data).
5.3. Implications for volatile evolution at
fast-spreading ridges
The magmatic amphibole data from Hess Deep
require that exogenic components are taken up by
the axial magmatic system through the assimilation of altered rock plus seawater-derived £uid.
This indicates that the newly formed, and as yet
unaltered, crust may be a sink for some volatile
elements such as Cl. This contrasts with the generally held view that, prior to hydrothermal alteration, the generation of oceanic crust would act as
a source of volatiles to the exosphere through
magmatic degassing. Both mantle-derived and
exogenic components that pass through AMCs
can be returned to the oceans through magma
degassing or through the leaching of these components from crystallised rocks by hydrothermal
£uids. This suggests that the fast-spreading crust
may act as both a source and sink for exogenic
volatile components.
The distribution of amphiboles with a contaminated signature can be used to trace the extent of
this contamination within the magmatic system.
At Hess Deep, amphiboles with these compositions have been traced to depths up to 800 m
beneath the base of the sheeted dyke complex,
showing that mass transport of contaminated material occurred over at least this distance. This is
consistent with models in which at least the upper
portion of the lower crust forms by crystallisation
within the AMC followed by some form of crystal
subsidence (e.g. [54,55]).
Because assimilation will concentrate volatile
components in an AMC, the amount of direct
EPSL 6736 5-8-03
K.M. Gillis et al. / Earth and Planetary Science Letters 213 (2003) 447^462
degassing into the hydrothermal system during
solidi¢cation will be much greater than predicted
for closed system crystallisation of a primary
n-MORB magma. This could have a direct impact
on hydrothermal systems by increasing the probability that volatile build-up within the AMC
would intermittently lead to brittle failure of the
conductive boundary layer separating the magmatic and hydrothermal systems, providing a temporal link between these regions. Direct evidence
for brittle failure at supra-solidus conditions within AMCs comes from the upper gabbros at Hess
Deep where fractures can be ¢lled with magmatic
amphibole (Fig. 2c). A link from the magmatic
system to the overlying hydrothermal root-zone
has not yet been made at Hess Deep but has
been observed at the sheeted dyke^plutonic transition zone in the Troodos ophiolite. Here, crosscutting vein and £uid inclusion constraints show
that a contact aureole that separated a magmatic
and hydrothermal system was periodically fractured [53]. In the modern MOR system, cracking
events at depths appropriate for the magma^hydrothermal transition, inferred from microseimicity data, have been documented along the EPR at
9‡50PN [56] and the Endeavour segment of the
Juan de Fuca Ridge [57]. Although these events
occurred above AMCs within the brittle regime,
they con¢rm that episodic cracking does occur in
the vicinity of hydrothermal root-zones. If such
cracking events tap into the magmatic system,
vent £uids should be enriched in magmatic volatile components over steady-state values. Indeed,
short-lived enhancements in the magmatic volatile
£ux (CO2 ) at active vent sites has been linked to
dyke-injection [58].
459
the assimilated material is more likely to be a
mixture of altered rock and pore £uid. Magma
degassing occurs during the solidi¢cation of the
plutonic section due to the concentration of
both mantle-derived, and assimilated, volatile species. This can lead to hydrofracture in the surrounding cumulates and, perhaps, to the direct
addition of volatile elements to the hydrothermal
system. Amphiboles that crystallised from contaminated magma are found throughout the
upper 800 m of the plutonic section. This is
most readily explained by subsidence of a contaminated crystal mush from the AMC, consistent
with thermal models for lower crustal accretion
(e.g. [54]).
Acknowledgements
Thorough and informative reviews by Catherine Me¤vel and Wolfgang Bach are gratefully acknowledged. We also thank R. Hinton and J.
Craven for their invaluable guidance with the Edinburgh ion probe analyses and T. Chadko for his
assistance with the electron probe. P. Lonsdale, J.
Karson, E. Klein, and S. Hurst are thanked for
inviting K.M.G. to participate in the Alvin cruises
and R. He¤kinian for the Nautile samples. The
Edinburgh ion probe time was supported by Natural Environmental Research Council (NERC)
Grant IMP/137/1098. K.M.G. acknowledges a
Natural Sciences and Engineering Research
Council Discovery Grant. The B-isotope data
were collected while K.M.G. was a Chercher Associate¤ at the Centre de Recherches de Petrographiques et Ge¤ochimiques in Nancy, France.
[BOYLE]
6. Conclusions
Magmatic amphiboles in plutonic rocks from
the EPR are enriched in chlorine with respect to
magmatic amphiboles from slow-spreading ridges,
indicating that AMCs assimilate large quantities
of exogenic components. These enrichments cannot be accounted for if the assimilant is a pure
brine or if altered dyke-rock has undergone prograde metamorphism and dehydration. Instead,
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