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Earth and Planetary Science Letters 223 (2004) 365–379
High-pressure melting experiments on garnet clinopyroxenite and
the alkalic to tholeiitic transition in ocean-island basalts
Shantanu Keshava,*, Gudmundur H. Gudfinnssonb,1, Gautam Sena,2, Yingwei Feib,3
aDepartment of Earth Sciences, Florida International University, Miami, FL 33199, USAbGeophysical Laboratory, Carnegie Institution of Washington, Washington DC 20015, USA
Received 26 November 2003; received in revised form 28 February 2004; accepted 2 April 2004
Available online
Abstract
It has been suggested, on the basis of recent high-pressure melting experiments, that high-Mg garnet clinopyroxenite is the
most important lithology controlling the major-element budget of ocean-island lavas [Geology 31 (2003) 481–484]. To clarify
these claims and further our understanding of the petrogenesis of ocean-island basaltic (OIB) lavas, we present the results of a
high-pressure (2.0–2.5 GPa) melting study on a high-Mg garnet-clinopyroxenite mantle nodule (77SL-582) from Hawaii.
Major-element compositions of the partial melts as a function of pressure (P), temperature (T), and degree of melting (F),
mineral chemistry of the coexisting crystalline phases, and the solidus/liquidus brackets of this particular garnet clinopyroxenite
are reported. The solidus of 77SL-582, which resembles a tholeiitic picrite in terms of its bulk composition, is bracketed at
1295F 15 and 1335F 15 jC at 2.0 and 2.5 GPa, respectively, which is f 60–70 jC lower than the solidus of mantle
lherzolite at identical pressures [Geochem. Geophys. Geosystems 1 (2000) 2000GC000070]. The solidus of 77SL-582 is also
lower by f 30–40 jC than reported for a slightly alkalic, high-Mg garnet clinopyroxenite [Geology 31 (2003) 481]. At both
pressures, the moderate degree (f 18–20%) partial melts of 77SL-582 are strongly alkalic with f 8–12 wt.% nepheline in
the norm. Even at a degree of melting as high as f 60%, moderately alkalic basaltic liquids are produced. With further melting,
however, partial melts become hypersthene-normative. In the CaO–MgO–Al2O3–SiO2 (CMAS) system, the eclogite surface is
restricted to the tholeiitic part of the basalt tetrahedron [D.C. Presnall, Effect of pressure on fractional crystallization of basaltic
magma, in: Y. Fei, C. Bertka, and B. Mysen (eds.) Mantle Petrology: Field observations and High Pressure Experimentation: A
Tribute to Francis (Joe) R. Boyd, The Geochemical Society Special Publication no. 6, 1999, pp. 209–224.]. A comparison with
high-pressure melting experiments in the CMAS system at 2.0–3.0 GPa indicates that the alkalic to tholeiitic transition in our
experiments can be explained by a rapid expansion of the eclogite ‘‘surface’’ from the tholeiitic part to well into the alkalic part
of the basalt tetrahedron in natural systems. Importantly, the partial melting trends of 77SL-582 are transverse to those seen in
ocean-island basalts. In addition, although the alkalic to tholeiitic basalt transition observed in ocean-island basalts is well
reproduced in our experiments, there is very little overlap between the partial melts of 77SL-582 and ocean-island basalts. It
appears that most of the major-element systematics of the ocean-island basalts considered here can be explained by combined
0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsl.2004.04.029
* Corresponding author. Now at: Geophysical Laboratory, Carnegie Institution of Washington, Washington DC 20015, USA. Tel.: +1-202-
478-8929; fax: +1-202-478-8901.
E-mail addresses: s.keshav@gl.ciw.edu (S. Keshav), g.gudfinnsson@gl.ciw.edu (G.H. Gudfinnsson), seng@fiu.edu (G. Sen),
fei@gl.ciw.edu (Y. Fei).1 Tel.: +1-202-478-8931; fax: +1-202-478-8901.2 Tel.: +1-305-348-2299; fax: +1-305-348-3877.3 Tel.: +1-202-478-8936; fax: +1-202-478-8901.
S. Keshav et al. / Earth and Planetary Science Letters 223 (2004) 365–379366
contributions of melts from anhydrous and carbonated lherzolite [Geophys. Res. Lett. 24 (1997) 2837; G.H. Gudfinnsson, D.C.
Presnall, Continuous gradations among primary kimberlitic, carbonatitic, melilititic, and komatiitic melts in equilibrium with
garnet lherzolite at 3–8 GPa, Proc. 8th Int. Kimber. Conf. Extended abstracts.] at high pressures with possible contributions
from melts from garnet clinopyroxenite of bulk composition similar to 77SL-582.
D 2004 Elsevier B.V. All rights reserved.
Keywords: ocean-island basalts; garnet pyroxenite; high-pressure melting; alkalic to tholeiitic transition
1. Introduction [14,15]. In addition, a careful analysis of such
Partial melting of upper mantle peridotite has
been recognized as the primary source of basaltic
lavas at mid-ocean ridges (MOR) and ocean-island
basalt localities [6]. The upper mantle, in terms of its
trace elements and isotopes, appears to be heteroge-
neous [7], and even in parts of the mantle subject to
convective stirring, isotopic variability is observed in
mid-ocean ridge basalts (MORB) and ocean-island
basalts (OIB). Major-element variability on a global
scale is also seen in MORB and has been ascribed
either to variations in the potential temperature (Tp)
[8] or inherent chemical heterogeneity in the mantle
source of these basalts [9,10]. Trace element and
isotopic variability is also distinctly seen in OIB
[7,11] and has been mostly explained by the pre-
sence of variable amounts of subducted oceanic crust
in the form of garnet clinopyroxenite in the mantle
source region [9,10]. It has been recently proposed
that this trace element and isotopic heterogeneity in
OIB is also correlated with the major-element con-
centrations of the erupted basalts [1,9,12]. However,
the possibility of garnet-clinopyroxenite component
in the source of OIB remains controversial [13].
Based on high-pressure melting experiments on a
mildly alkalic, high-Mg garnet clinopyroxenite, it
has been argued recently that garnet clinopyroxenite
is the most important lithology in the mantle source
regions of OIB [1]. This aforementioned study
demonstrates that the high-pressure partial melts of
some garnet clinopyroxenites are alkalic in nature
and could act as the parental magmas of OIB, and
thus trace element and isotopic variability in OIB
can also be suitably tied to the major elements,
making garnet clinopyroxenite the most plausible
candidate for OIB melts [1]. However, such studies
have not demonstrated the alkalic to tholeiitic basalt
transition, a transition that is documented in OIB
studies reveals that there is actually very little
overlap between partial melts of garnet clinopyrox-
enite and OIB.
In order to further our understanding of the petro-
genesis of ocean-island basalts, and to verify the
recent claims about the suitability of garnet clinopyr-
oxenite as an important source of magma generation
at OIB [1], we report the results of partial melting
experiments on a high-Mg garnet clinopyroxenite at
2.0–2.5 GPa. In this respect, some relevant questions
we address are: (1) What is the high-pressure, anhy-
drous solidus of garnet clinopyroxenite, and how is it
different (if at all) from that of anhydrous mantle
lherzolite at identical pressures? (2) What is the
melting temperature interval of the garnet clinopyr-
oxenite? (3) What is the effect of pressure (P),
temperature (T), and degree of partial melting (F)
on the composition of partial melts? Finally, (4) Is
garnet clinopyroxenite, as recently claimed [1] a
suitable source for ocean-island basalts?
With the above questions in mind, we present the
results of 2.0–2.5 GPa melting experiments on a high-
Mg garnet-clinopyroxenite mantle nodule from Salt
Lake Crater, a post-erosional vent on the island of
Oahu, Hawaii. Major-element composition of the
partial melts as a function of pressure (P), temperature
(T), and degree of melting (F), mineral chemistry of
the coexisting crystalline phases, phase relations, and
the anhydrous solidus and liquidus brackets of this
particular garnet clinopyroxenite are reported. In the
concluding part, the data are applied to the petro-
genesis of a few ocean-island basalt lavas. On the
basis of these experiments, we conclude that although
the partial melts of this garnet-clinopyroxenite resem-
ble ocean-island basalt lava compositions in some of
their traits, garnet clinopyroxenite of similar bulk
composition plays (if any) a minor role in dictating
the major-element budget of OIB.
Table 1
Composition (wt.%) of starting material 77SL-582 used in this
study
77SL-582 Mix1Ga
This study
SiO2 46.38(13)b 45.60
TiO2 0.63(3) 0.90
Al2O3 16.42(9) 15.20
Cr2O3 0.14(2) 0.11
FeO* 7.64(10) 7.80
MnO 0.22(3) 0.15
MgO 16.48(12) 16.67
CaO 10.74(6) 11.48
Na2O 0.99(3) 1.40
K2O 0.09(4) 0.04
P2O5 – 0.01
Sum 99.73 99.32
Mg# 79.36 79.30
CIPW (wt.%)
Qz – –
Co – –
Or 0.53 0.24
Ab 8.37 12.00
An 40.07 34.88
Lc – –
Ne – 0.11
Di 10.53 17.57
Hy 13.52 –
Ol 24.27 31.85
Mt 1.11 1.02
Il 1.20 1.71
Hm – –
Ap – –
a Hirschmann et al. [1].b Number in parentheses in all tables is uncertainty in the least
significant figures.
S. Keshav et al. / Earth and Planetary Science Letters 223 (2004) 365–379 367
2. Starting material and experimental details
The starting material (77SL-582) used in the present
set of high-pressure melting experiments was made
from a naturally occurring garnet-clinopyroxenite xe-
nolith from Salt Lake Crater, Oahu, Hawaii. This and
similar xenoliths from Oahu have been shown to be
high-pressure (1.6–3.5 GPa) cumulates [16–18]. This
xenolith was chosen because it presents direct evi-
dence that garnet clinopyroxenite is present in the
oceanic upper mantle, and thus may be an appropriate
source material for magmas at ocean-island basalt
localities.
The xenolith is made up of clinopyroxene (cpx)
and garnet (gt). Cpx and garnet range in size from
0.5–2 to 0.8–2.5 mm, respectively. Exsolved lamel-
lae of orthopyroxene (opx) and blebs of spinel are
present in the host cpx. The mineral separates of cpx
and garnet were cleaned in an ultrasonic bath, and
were initially ground in a tungsten carbide ball mill.
Fine grinding was done in an agate mortar in ethanol.
This mix was dried at 300 jC for 24 h, and the process
was repeated once more, before finally passing it
through a fine (f 20 Am) nylon screen in order to
ensure a uniform grain size. This mix was fused at 2.0
GPa and 1500 jC for 10 h (experimental procedure as
explained below), and the bulk composition of the
starting mix was determined by electron-microprobe
analysis of the glass (Table 1) at the Geophysical
Laboratory, Carnegie Institution of Washington. The
bulk composition of 77SL-582 resembles a tholeiitic
picrite (Table 1). This starting mix resembles the one
used in the petrogenetic models for mid-ocean ridge
basalts (MORB) in a previous study [19], except for
slightly higher MgO, SiO2, FeO*, and lower Al2O3,
Na2O, and CaO in the latter study. However, 77SL-
582 is different from the starting material used in a
recent study [1] that was a garnet clinopyroxenite of
mildly alkalic picrite bulk composition (Table 1).
Phase equilibrium experiments were performed in
a half-inch (1.27 cm) Boyd–England [20] type end-
loaded piston-cylinder device at the Geophysical
Laboratory. The experimental assembly consisted of
talc-pyrex sleeves, straight graphite furnaces, and
internal spacers of crushable MgO (Fig. 1). To prevent
loss of iron from the starting mix to the capsule
material, all the experiments used a double-capsule
technique in which the charge was loaded into an
inner graphite capsule and then sealed in a welded
platinum (Pt) outer capsule. In order to ensure ‘‘bone-
dry’’ conditions, before packing the starting mix in the
graphite capsule, the capsules were fired at f 600 jCfor 2–3 s, using a hand-held torch. The double
capsules with the starting material were vacuum dried
at 110 jC for 12–20 h. All the experiments were done
using the ‘hot piston-in’ technique, with a friction
correction of � 10% [21]. The precision in pressure
measurements is estimated to be F 0.05 GPa. Tem-
peratures were measured using W26Re–W5Re ther-
mocouples, and were automatically controlled to
within F 3 jC. The precision in temperature measure-
ments is F 15 jC. The thermocouple emf was not
corrected for the effect of pressure. The oxygen
Fig. 1. A cross-section of the piston-cylinder assembly [20] used in
the experiments.
Fig. 2. Backscattered electron images of the run products at
various P–T conditions showing (a) low-degree ( F) melts (2.5
GPa/1345 jC) along with clinopyroxene (cpx) and garnet (gt). The
light phase in the center is an unreacted cpx core from the starting
mix; (b) quenched liquid (qnch) and cpx–gt assemblage (2.5 GPa/
1370 jC) in one of the run products, and (c) clear liquid (glass),
quenched liquid (qnch), and orthopyroxene (opx). The heteroge-
neous distribution of clear glass and quench is to be noted (2 GPa/
1440 jC).
S. Keshav et al. / Earth and Planetary Science Letters 223 (2004) 365–379368
fugacity of the experimental conditions is estimated to
be close to the CCO buffer. Run durations ranged
from f 16–70 h. Experiments were automatically
quenched (quench rate f 200–250 jC/s), while
maintaining the run pressure. After recovery of the
assembly, the capsule position was checked to verify
the correct positioning of the sample with respect to
the furnace hot spot and the thermocouple. The
retrieved capsules were sectioned longitudinally and
were mounted in epoxy for optical and microprobe
examination.
Compositions of the experimental quenched run
products were determined at the Geophysical Labora-
tory and also at the Florida Center for Analytical
Electron Microscopy (FCAEM), FIU, using a JEOL
JXA-8900R electron microprobe. Analyses were done
using a 15-kV, 20–30-nA beam current, and 30–40-s
peak acquisition time for all elements, except Na and
K, which were analyzed for 20 s to minimize loss due
to volatilization. The spot-size for mineral phases
ranged from 1 to 2 Am and for quenched melt regions
5–10 Am. A combination of mineral and oxide stand-
ards was used. The CITZAFR program was used to
reduce counts to oxide concentrations. Uncertainties
for major (z 5%) and minor (V 5%) oxides analyzed
by microprobe are better than 2% and 5% of quoted
values, respectively. Low-degree melts ( < 18–20%)
are not reported here owing to quench modification
problems and analytical difficulties.
3. Experimental results
3.1. Run products
Thermally induced compaction of melt away from
the solids is promoted due to the presence of thermal
gradients. Thermal compaction is a rapid process and
Fig. 3. Pressure– temperature isopleth for the equilibrium melting of
garnet-clinopyroxenite 77SL-582. Experimental conditions are
provided in Table 2. Also shown are the solidus brackets for
S. Keshav et al. / Earth and Planetary Science Letters 223 (2004) 365–379 369
it cannot be avoided as long as such thermal gradient
exists [22]. However, it is possible to achieve a
steady-state condition where coexisting phase com-
positions represent the average equilibrium condi-
tions for the entire charge. Because of such thermal
gradients, pools of melt often segregate near the hot
portion of the charge, typically near the capsule
walls. In the present set of experiments, the grain
size of the mineral phases ranges from 5 to 50 Amclose to the solidus and also below the solidus (Fig.
2a). Melt pools, roughly half way between the
bracketed solidus and liquidus were mostly com-
posed of a fine mat of quenched crystals (Fig. 2b),
while glass (along with mineral phases) was found at
relatively higher degrees of melting (Fig. 2c). As
mentioned above, because of thermal gradient, melt
pools near the capsule walls were quite common,
and were large enough to be analyzed. Near-solidus
melts exist in local pockets (f 2 Am across) and are
connected to each other through a complex network
of narrow ‘‘channels’’. Below the solidus, minerals
are heterogeneously distributed and seldom have
well-developed grain junctions, as opposed to the
minerals where a melt phase is present. Near solidus
mineral phases exhibit some zoning. The experimen-
tal conditions and run products are summarized in
Table 2.
Table 2
Experimental conditions and run products
Run no. P
(GPa)
T
(jC)Duration
(h)
Assemblagea
376 2.0 1280 53 cpx + sp + gt
373 2.0 1310 52 L + cpx + sp + gt
371 2.0 1340 67 L + cpx + sp + gt
369 2.0 1360 36 L + cpx + sp + gt
370 2.0 1385 34 L + cpx + sp
368 2.0 1410 26 L + cpx + sp + opx
367 2.0 1440 36 L + opx
366 2.0 1470 20 L
380 2.5 1320 51 cpx + gt
381 2.5 1345 25 L + cpx + gt
378 2.5 1370 26 L + cpx + gt
377 2.5 1400 22 L + cpx + gt
375 2.5 1430 27 L + cpx + gt
372 2.5 1460 36 L + cpx + gt
383 2.5 1478 16 L + cpx + gt
374 2.5 1495 36 L
a L—liquid; opx—orthopyroxene; cpx—clinopyroxene; sp—
spinel; gt—garnet.
3.2. Attainment of equilibrium and modal calculations
Although the present set of experiments was not
reversed, the following observations indicate that
chemical equilibrium was closely approached: inter-
nally consistent phase relations, lack of significant
zoning in individual grains, compositional homoge-
neity of glasses, and Fe–Mg partitioning between
minerals and melt, i.e., KD ([Femin/Femelt]� [Mgmelt/
Mix1G [1] and the parameterized lherzolite solidus [2].
Fig. 4. Temperature (jC)–melt percent diagram for the melting of
77SL-582. Triangles and squares are melt percent at 2.0 and 2.5
GPa, respectively. Dashed and solid lines represent smoothing
functions fit to the data points.
S. Keshav et al. / Earth and Planetary Science Letters 223 (2004) 365–379370
Mgmin]) values of 0.28F 0.08 for cpx and 0.45F 0.09
for garnet, which resemble equilibrium values estab-
lished in previous studies [23–25].
The rigorous algebraic schemes [26] that give
quantitative insights into phase relations in simple
systems cannot be applied to natural, multicomponent
systems. Thus, although phase relations from systems
with more than four components cannot be presented
graphically, they can be understood algebraically
[25–27]. Modal proportions (wt.%) of the phases
for the partial melting experiments on 77SL-582 were
calculated by mass-balance between the compositions
of various phases and the bulk composition of the
system. This is done by solving, Ax =B, where A is an
M�N matrix (called the composition matrix), x is the
modal vector (the unknown vector denoting the phase
Table 3
Melt compositions (wt.%)
Run no. 367 368 370 369 371
P (GPa) 2 2 2 2 2
T (jC) 1440 1410 1385 1360 1340
Na 10 13 12 9 8
SiO2 45.99(12) 45.64(11) 45.01(10) 44.43(19) 44.12(1
TiO2 0.69(3) 0.89(5) 1.07(3) 1.43(6) 1.51(8)
Al2O3 16.17(9) 15.78(11) 14.67(9) 13.97(17) 13.18(1
Cr2O3 0.14(3) 0.11(2) 0.09(4) 0.09(6) 0.08(5)
FeO* 8.97(12) 9.79(9) 11.51(11) 12.83(14) 13.37(1
MnO 0.21(2) 0.18(4) 0.18(3) 0.19(4) 0.19(6)
MgO 16.11(14) 15.70(11) 14.08(12) 13.39(21) 12.94(1
CaO 10.61(10) 10.43(11) 10.20(13) 9.91(16) 9.69(18
Na20 1.01(4) 1.30(3) 2.02(2) 2.17(7) 2.58(5)
K2O 0.21(3) 0.39(2) 0.73(1) 0.89(5) 1.02(7)
Sum 100.11 100.21 99.65 99.17 98.68
Mg# 76.20 74.08 68.56 65.04 63.31
Mode (wt.%) 99.54 87.11 69.14 34.83 21.34
CIPW (wt.%)
Qz – – – – –
Co – – – – –
Or 1.24 2.30 4.31 5.25 6.02
Ab 8.54 10.99 10.10 8.67 5.70
An 38.95 36.05 28.79 25.74 21.36
Lc – – – – –
Ne – – 3.78 5.24 8.73
Di 10.96 12.60 17.64 19.03 21.85
Hy 10.77 3.61 – – –
Ol 26.93 31.49 31.31 30.89 32.14
Mt 1.30 1.42 1.67 1.86 –
Il 1.31 1.69 2.04 2.47 2.87
Hm – – – – –
Ap – – – – –
a Number of spots analyzed and numbers in parentheses in all the tab
modes), and B is the known vector representing the
bulk composition of the system. The columns (M) in
M�N matrix in the A array represents the phases, and
rows N denote the compositions of various phases
obtained from microprobe analyses. Natural systems
are highly overdetermined, as they contain about 10
major oxides. Thus, in the present set of calculations,
assemblages were treated only in terms of four essen-
tial components, CaO, MgO, Al2O3, and SiO2.
Assemblages with two to three phases are still over-
determined because the matrix A is generally not
invertible, but as shown in an earlier study [25],
robust least-squares solutions can be quantitatively
gathered to capture the contribution of various phases
during partial melting. The modal vector x can be
solved using x=(ATA)� 1ATB.
383 372 375 377 378
2.5 2.5 2.5 2.5 2.5
1478 1460 1430 1400 1370
11 11 9 9 8
7) 46.01(11) 45.49(14) 44.71(20) 44.14(22) 43.82(20)
0.70(2) 0.87(5) 1.04(8) 1.19(7) 1.37(9)
7) 16.28(12) 15.61(13) 14.70(17) 13.61(16) 12.94(19)
0.15(2) 0.12(5) 0.08(2) 0.08(4) 0.06(3)
5) 8.12(14) 9.27(13) 12.09(11) 13.01(14) 13.91(19)
0.21(5) 0.18(4) 0.21(8) 0.19(7) 0.19(9)
9) 16.29(12) 15.81(18) 14.67(20) 13.69(19) 12.87(20)
) 10.49(9) 10.34(14) 10.17(12) 9.94(14) 9.85(19)
1.04(3) 1.10(6) 2.08(5) 2.58(8) 2.89(12)
0.21(4) 0.41(5) 0.69(8) 1.01(7) 1.27(10)
99.50 99.20 100.44 99.43 99.16
78.15 75.25 68.38 65.23 62.25
99.31 93.95 76.21 42.24 18.08
– – – – –
– – – – –
1.24 2.42 4.07 5.96 7.50
8.79 9.30 7.36 5.23 3.09
39.11 36.43 28.72 22.56 18.58
– – – – –
– – 5.54 8.98 11.57
10.32 11.92 17.62 21.73 24.70
11.39 7.64 – – –
26.01 28.42 34.08 30.82 29.14
1.18 1.35 1.76 1.89 2.02
1.33 1.66 1.98 2.26 2.61
– – – – –
– – – – –
les are two standard deviations, referring to the last digit(s).
Fig. 5. Partial melt compositions of 77SL-582 in the pseudo-ternary,
Olivine (Ol)–Nepheline (Ne)–Clinopyroxene (Cpx)–Quartz (Qz).
Bulk composition of 77SL-582 is shown as a star. Projection is from
plagioclase [Pl] after the scheme of Walker et al. [31]. Filled and
blank triangles denote data at 2.0 and 2.5 GPa, respectively.
Numbers with/without letters refer to the solid assemblage
coexisting with the liquid: 5a–6a, liq–cpx–gt– sp; 4a, liq–cpx–
sp; 3a, liq–opx–cpx–sp, and 2a, liq–opx. 2–6, liq–cpx–gt.
Abbreviations are: opx, orthopyroxene; cpx, clinopyroxene; sp,
spinel; gt, garnet; liq, liquid; F, melt%; T, temperature, and Pl,
plagioclase. Dotted line with arrow indicates how the melt
compositions of 77SL-582 change with increasing temperature.
ary Science Letters 223 (2004) 365–379 371
3.3. Phase relations of 77SL-582
The solidus of 77SL-582 is bracketed at 1295F 15
jC and 1335F 15 jC at 2.0 and 2.5 GPa, respectively,
which is f 60–70 jC lower than the anhydrous
mantle solidus at identical pressures [2]. Our reported
solidus is also lower by f 30–120 jC than some
previous studies on mafic lithologies at similar pres-
sures [1,28,29]. The liquidus of 77SL-582 is located at
1460F 15 and 1485F 15 jC at 2.0 and 2.5 GPa,
respectively. These brackets and the corresponding
run products with changing P–T conditions are shown
in Fig. 3.
At 2.0 GPa, the subsolidus of 77SL-582 consists of
cpx, spinel, and garnet. Garnet is the first phase to
disappear from the melting interval at 1385 jC (Fig.
3), and at 1385–1410 jC, cpx and spinel coexist with
the liquid (Fig. 3). At 1410 jC, orthopyroxene (opx)
is produced according to the reaction: cpx + sp =
opx + liquid (Fig. 3). This reaction has also been
documented in the previous studies [30,31]. Above
1410 jC, cpx and spinel disappear and only opx
coexists with the liquid (Fig. 3). It is not clear from
the experiments at 2.0 GPa, if spinel is in reaction
relation with the liquid in part of the melting interval
or is continually being consumed during melting. At
2.5 GPa, the subsolidus phase assemblage consists
only of cpx and garnet. Throughout the melting
interval, cpx and garnet are also the only phases that
coexist with the liquid; opx and spinel are not en-
countered, indicating that these fields pinch out by 2.5
GPa. At both pressures, the melt percentage (F)
initially increases gradually and then rapidly with
only a moderate increase in temperature (Fig. 4), a
melting behavior that is similar to that reported in a
recent study [1].
The solidus brackets for 77SL-582 reported here
are lower by f 30–40 jC than those of a slightly
alkalic, high-Mg garnet-clinopyroxenite (Mix1G) [1].
This difference is interesting since the Mg# of 77SL-
582 and Mix1G are virtually identical. In addition, if
the effect of total alkalies (in the bulk composition) in
depressing the lherzolite solidus is greater than the
Mg#, as suggested in a recent study [2], and if those
arguments are extended to garnet clinopyroxenite,
then the solidus of Mix1G [1] should be even lower
than that of 77SL-582, since Mix1G [1] has a greater
amount of Na2O than 77SL-582. The differences
S. Keshav et al. / Earth and Planet
between these two studies are not understood at the
present.
4. Melt compositions and the alkalic to tholeiitic
basalt transition in basaltic liquids
The melt compositions are reported in Table 3. Melt
compositions projected from plagioclase [Pl] onto the
Olivine (Ol)–Clinopyroxene (cpx)–Quartz (Qz)–
Nepheline (Ne) plane [32] are shown in Fig. 5.
Negative quartz was recalculated as nepheline and
then the liquid composition was recast in terms of
Ol–Ne–Cpx. At both pressures, partial melts of 77SL-
582 at moderate degree of melting (f 18–21%) are
strongly alkalic and nepheline (ne)-normative with up
tof 8–12 wt.% nepheline in the norm (Fig. 5).
Moderately alkalic liquids (f 3–5% ne-normative)
are generated at F as high as f 60–70% (Fig. 5).
Hypersthene (hy)-normative liquids are generated at F
greater than f 70% (Fig. 5). With increasing degree of
melting, the alkalic to tholeiitic basalt transition is
continuous (Fig. 5). Note that a previous melting study
at 2.0 GPa on a Hawaiian garnet websterite also
reported mildly alkalic (f 3% ne-normative) liquids
S. Keshav et al. / Earth and Planetary Science Letters 223 (2004) 365–379372
[29], as did a melting study at a lower pressure report a
similar alkalic to tholeiitic basalt transition using a
high-Mg eclogite xenolith from South Africa [28]. In
Fig. 6. Variation of SiO2, MgO, FeO*, Al2O3, CaO, and Na2O in the pa
triangles are data at 2.0 and 2.5 GPa, respectively. See text for discussion
the present case, with increasing temperature (at an
isobar), SiO2, Al2O3, and MgO contents of the partial
melts increase, while TiO2, FeO*, and Na2O +K2O
rtial melts of 77SL-582 as a function of temperature. Squares and
.
Table 4
Clinopyroxene compositions (wt.%)
Run no. 368 370 369 371 373 376 383 372 375 377 378 381 380
P (GPa) 2 2 2 2 2 2 2.5 2.5 2.5 2.5 2.5 2.5 2.5
T (jC) 1410 1385 1360 1340 1310 1280 1478 1460 1430 1400 1370 1345 1320
Na 13 10 12 8 14 12 11 9 14 15 7 13 9
SiO2 50.37(29) 50.78(38) 51.37(24) 51.12(36) 50.79(47) 49.99(43) 51.78(21) 50.39(37) 50.30(36) 50.98(41) 51.27(31) 50.15(45) 52.13(59)
TiO2 0.27(9) 0.44(2) 0.21(3) 0.30(5) 0.40(7) 0.50(6) 0.09(2) 0.20(5) 0.19(3) 0.34(7) 0.3(7) 0.37(5) 0.40(10)
Al2O3 9.39(37) 9.20(27) 8.89(18) 8.73(24) 8.37(39) 8.69(46) 9.14(19) 8.55(24) 9.77(57) 9.14(39) 8.94(41) 8.21(38) 7.98(49)
Cr2O3 0.22(2) 0.15(3) 0.25(4) 0.30(3) 0.14(5) 0.19(7) 0.14(3) 0.23(4) 0.20(4) 0.14(6) 0.14(2) 0.24(5) 0.30(9)
FeO* 6.78(19) 7.54(14) 8.19(24) 8.3(21) 8.64(34) 8.20(40) 5.39(21) 5.98(19) 6.34(28) 7.66(38) 7.75(28) 7.93(41) 8.31(60)
MnO 0.20(4) 0.14(2) 0.20(3) 0.20(4) 0.15(6) 0.12(8) 0.13(2) 0.28(5) 0.15(9) 0.12(6) 0.13(3) 0.24(8) 0.30(11)
MgO 19.64(27) 18.99(21) 18.16(30) 18.16(41) 16.15(32) 15.90(48) 21.69(30) 20.89(27) 19.71(39) 16.97(48) 16.71(52) 16.12(59) 15.78(53)
CaO 11.97(18) 12.89(16) 13.24(24) 13.84(27) 13.94(52) 14.44(41) 11.14(19) 11.54(28) 11.89(35) 12.78(39) 12.99(31) 13.45(47) 14.01(40)
Na20 0.49(5) 0.54(2) 0.80(7) 0.84(4) 1.36(8) 1.31(7) 0.52(6) 0.62(6) 0.66(9) 1.31(13) 1.29(9) 1.50(10) 1.58(9)
K2O – – – – – – – – – – – – –
Sum 99.33 100.67 101.31 101.19 100.13 99.35 100.08 98.68 99.21 99.44 99.52 98.21 100.79
Mg# 83.77 81.76 79.80 79.58 77.11 77.55 87.76 86.15 84.70 79.78 79.34 78.36 77.18
KDb 0.21 0.26 0.31 0.32 – – 0.15 0.18 0.22 0.30 0.33 – –
Mode 2.96 25.61 39.68 41.41 0.45 3.16 14.20 35.01 47.64
a Number of spots analyzed.b Fe–Mg exchange coefficient between cpx and melt.
S. Keshav et al. / Earth and Planetary Science Letters 223 (2004) 365–379 373
decrease (Fig. 6). CaO contents of the partial melts also
show a minor increase with increasing temperature
(Fig. 6). The compositions of the solid phases along
with their relative abundances in these experiments are
reported in Tables 4–7.
One can attempt to understand the alkalic to tholei-
itic basalt transition in the partial melts of 77SL-582 by
Table 5
Garnet compositions (wt.%)
Run no. 369 371 373 376 383 37
P (GPa) 2 2 2 2 2.5 2.5
T (jC) 1360 1340 1310 1280 1478 14
Na 11 19 10 16 13 8
SiO2 41.59(29) 41.34(59) 41.27(24) 41.21(47) 41.59(31) 41
TiO2 0.28(11) 0.19(6) 0.29(3) 0.42(21) 0.10(3) 0.2
Al2O3 22.76(37) 22.81(41) 22.13(18) 22.01(24) 23.04(40) 22
Cr2O3 0.12(2) 0.14(3) 0.20(4) 0.18(8) 0.21(9) 0.0
FeO* 14.87(29) 14.71(21) 15.97(39) 16.29(37) 11.10(22) 12
MnO 0.28(8) 0.27(2) 0.27(11) 0.27(9) 0.28(12) 0.1
MgO 15.83(27) 15.61(34) 15.09(47) 14.61(41) 17.58(37) 17
CaO 5.09(18) 5.50(28) 5.02(24) 5.34(27) 5.25(52) 4.9
Na20 – – – – –
K2O – – – – –
Sum 100.82 100.57 100.24 100.33 99.95 99
Mg# 65.48 65.41 62.73 61.51 72.47 72
KDb 0.53 0.54 – – 0.35 0.3
Mode 28.10 30.94 0.24 2.8
a Number of spots analyzed.b Fe–Mg exchange coefficient between garnet and melt.
using as a guide the established liquidus phase relations
in the tholeiitic portion, the Forsterite (Fo)–Anorthite
(An)–Diopside (Di)–Quartz (Si) (FADS) join, of the
CaO–MgO–Al2O3–SiO2 (CMAS) system [33,34].
The compositions of garnet and clinopyroxene lie in
the aluminous-pyroxene plane in the CMAS system.
Isobarically, in the 2.0–3.0 GPa range, garnet and
2 375 377 378 381 380
2.5 2.5 2.5 2.5 2.5
60 1430 1400 1370 1345 1320
14 11 8 12 7
.65(40) 41.42(59) 41.48(52) 41.89(49) 40.94(51) 41.28(60)
4(9) 0.42(18) 0.28(9) 0.23(10) 0.17(9) 0.28(13)
.45(48) 22.78(39) 22.76(48) 22.14(57) 22.31(49) 22.01(58)
9(1) 0.14(3) 0.24(11) 0.30(6) 0.21(13) 0.23(10)
.01(40) 13.01(34) 14.09(43) 14.99(37) 15.34(38) 15.93(49)
7(8) 0.17(6) 0.17(5) 0.17(9) 0.17(6) 0.17(11)
.51(46) 16.17(50) 16.09(42) 15.89(39) 15.42(50) 15.49(67)
9(39) 5.10(28) 5.27(19) 5.21(28) 4.99(31) 4.87(49)
– – – – – –
– – – – – –
.11 99.54 100.68 100.82 99.55 100.25
.20 69.59 67.05 65.38 64.17 63.40
6 0.44 0.50 0.55 – –
9 9.09 22.76 34.28
Table 7
Spinel compositions (wt.%)
Run no. 368 370 369 371
P (GPa) 2 2 2 2
T (jC) 1410 1385 1360 1340
Na 5 7 4 3
SiO2 0.27(11)^ 0.18(8) 0.14(3) 0.31(13)
TiO2 0.03(1) 0.89(21) 0.23(9) 0.21(11)
Al2O3 66.83(39) 65.21(36) 64.89(41) 63.08(59)
Cr2O3 4.02(19) 1.35(11) 0.75(20) 1.50(33)
FeO* 7.15(21) 9.32(16) 13.88(31) 14.65(43)
MnO 0.09(2) 0.09(3) 0.08(1) 0.05(3)
MgO 22.67(16) 21.81(28) 19.90(39) 18.85(59)
CaO 0.12(13) 0.19(8) 0.04(1) 0.11(3)
Na2O – – – –
K2O – – – –
Sum 101.18 99.05 99.91 98.79
Mg# 87.14 83.90 77.52 75.58
Mode 3.26 5.24 5.38 6.31
a Number of spots analyzed.
S. Keshav et al. / Earth and Planetary Science Letters 223 (2004) 365–379374
clinopyroxene coexist with liquid along a divariant
surface in the CMAS system [33,34]. An interesting
aspect of the phase relations in CMAS is that the
garnet-clinopyroxenite surface all lies within the tho-
leiitic portion of the basalt tetrahedron. With increasing
pressure, this surface considerably expands, but still
lies within the tholeiitic portion. However, the effect of
adding Fe and Na to the CMAS system is to cause
shrinkage of the spinel volume, and relatively rapid
expansion of the garnet-clinopyroxenite ‘‘surface’’
[35–37], such that it penetrates well into the alkalic
portion of the basalt tetrahedron. Thus, the alkalinity of
the partial melts of 77SL-582 can be explained as a
result of the expansion of the garnet-clinopyroxenite
surface in natural systems relative to the CMAS sys-
tem. The relative alkalinity of the partial melts of 77SL-
583 at 2.0 GPa cannot be ascribed to the expansion of
the opx volume, because opx comes in at a very late
stage, at very high degree of melting, and when the
partial melts have already become hy-normative.
In addition, it is important to note that olivine is
absent in the experiments reported here, and thus
needless to say, the phase equilibria is dominantly
controlled by cpx and garnet. On the basis of our
experiments, we can also predict that with increasing
pressure, the partial melting relationship of garnet
clinopyroxenite is likely to be very different from that
Table 6
Orthopyroxene compositions (wt.%)
Run no. 367 368
P (GPa) 2 2
T (jC) 1440 1410
Na 9 13
SiO2 51.89(31) 52.31(40)
TiO2 0.08(1) 0.31(16)
Al2O3 9.01(21) 8.85(36)
Cr2O3 0.37(2) 0.26(6)
FeO* 6.01(19) 6.12(37)
MnO – –
MgO 30.13(41) 30.01(49)
CaO 2.05(13) 2.10(24)
Na2O 0.27(9) 0.21(11)
K2O – –
Sum 99.81 99.97
Mg# 89.96 89.80
KDb 0.21 0.16
Mode 0.46 6.67
a Number of spots analyzed.b Fe–Mg exchange coefficient between opx and melt.
of mantle lherzolite. With increasing pressure in the
garnet– lherzolite stability field, the primary phase
volume of garnet expands continuously, leading to
diminished Al2O3 in the partial melts [25]. Meanwhile,
the shrinkage in the olivine primary phase volume
results in increasing MgO contents of lherzolite partial
melts with increasing pressure [25]. Conversely, be-
cause of the absence of olivine in the melting interval of
77SL-582 at 2.0–2.5 GPa (and probably also at higher
pressures), the higher-pressure partial melts of 77SL-
582 are unlikely to show an appreciable change in their
MgO contents. However, the higher pressure partial
melts of 77SL-582-like bulk compositions and mantle
lherzolite may contain similar amounts of Al2O3.
5. Ocean-island basalts: garnet clinopyroxenite in
the source?
The continuous transition from low-F, Si-poor,
alkalic to high-F, relatively Si-rich, tholeiitic partial
melts reported in our experiments is remarkable (Figs.
5 and 7), and appears to correspond well with some
OIB rock suites (Fig. 7). This transition was not
observed in a recent similar study [1], a difference
that could be ascribed to the slightly alkalic bulk
composition used in that study.
On the basis of our experiments, the proposal that
garnet clinopyroxenite is the most important source of
S. Keshav et al. / Earth and Planetary Science Letters 223 (2004) 365–379 375
OIB [1] can now be critically evaluated. We evaluate
the origin of some OIB by comparing their major-
element compositions with our experimental results
and partial melts produced in other published volatile-
free [25,38] and volatile-bearing [4] experiments on
natural lherzolites. The three OIB suites chosen are
from Polynesia [39], Samoa [14], and Hawaii [40,41].
Fig. 7A shows that the partial melts from 77SL-582 in
our experiments are initially alkalic, smoothly cross-
ing into the tholeiitic basalt field. Partial melts from
two natural lherzolite starting mixes labeled W [25]
and HK [38] are tholeiitic at 2.0–4.0 GPa. However,
we note that melts produced by very low degree
melting of these lherzolites could lie within the
alkalic-basalt field. The partial melts in lherzolite-
basalt sandwich experiments [42] at similar pressures
are all alkalic, similar to the partial melts of 77SL-582,
and overlap the field of Samoan lavas. Partial melts of
a carbonated lherzolite at 3.0 GPa are alkalic to
transitional [4], and partially overlap the field of
Hawaiian lavas. Polynesian lavas are different from
almost all the experimental melts shown in Fig. 7A,
and there is partial overlap of these lavas with the
melts of 77SL-582. However, low-degree melts of
anhydrous lherzolite are also likely to overlap some of
the Polynesian lavas. The partial melts of carbonated
lherzolite [4] are distinct from the anhydrous lherzo-
lite and garnet clinopyroxenite-derived melts in being
richer in MgO, CaO, alkalies, and poorer in Al2O3,
SiO2 (Fig. 7). Most noticeably, the partial melts of
77SL-582 are positively correlated in the CaO–SiO2
and MgO–Al2O3 diagrams (Fig. 7B,C), and are
transverse to the OIB trend. Partial melts from vola-
tile-free lherzolite are distinct from OIB (Fig. 7B,C),
but as mentioned earlier, low-to-moderate degree
melts of volatile-free lherzolite [24] mixed with those
of carbonated lherzolite [4] are likely to completely
overlap the OIB data. It further indicates that volatile-
free melting of either lherzolite or garnet clinopyrox-
enite at the pressures of investigation cannot repro-
duce the OIB trend. In Fig. 7, Samoan and Polynesian
lavas appear to be broadly similar to the low-F melts
Fig. 7. Major-element compositions of OIB compared to partial melt
compositions of 77SL-582 in terms of (A) Na2O+K2O–SiO2, (B)
CaO–SiO2, and (C) MgO–Al2O3. Abbreviations are: Lh +CO2,
partial melts of carbonated peridotite [4]; HK and W are partial melt
compositions of anhydrous lherzolite at 2.0–3.0 GPa [38], and
3.0–4.0 GPa [25], respectively; Gpx, partial melting trend of 77SL-
582. Field labeled ‘‘Lh + basalt sandwich’’ refers to partial melt
compositions of basalt – lherzolite sandwich experiments [42].
Blank and filled triangles are partial melts compositions of 77SL-
582 at 2.0 and 2.5 GPa, respectively. Lines with double-headed
arrows are putative mixing trends between partial melts of
carbonated lherzolite and 77SL-582, with thin, diagonal lines
showing proportion of the two melts in the mixed melts. In these
mixed melts, Gpx is the partial melt of 77SL-582 (this study) and
CLh is the partial melt of a carbonated lherzolite [4].
S. Keshav et al. / Earth and Planetary Science Letters 223 (2004) 365–379376
from 77SL-582, suggesting that these lavas may have
tapped a garnet-clinopyroxenite component similar to
77SL-582. The relatively high SiO2 and CaO of the
Polynesian and Hawaiian lavas may be attributed to
source rock differences. However, the partial melting
trends of 77SL-582 are completely opposite to the
trends shown by Samoan and Polynesian lavas. These
lavas are unlikely to have been produced by low
degrees of melting of a lherzolite source inasmuch
the latter plot distinctly away from the Samoan and
Polynesian lavas in Fig. 7B,C. Hawaiian lavas show a
much wider spread that partially overlaps the partial
melts of 77SL-582, and extends toward the melts
generated from carbonated lherzolite at 3.0 GPa [4].
Although this observation suggests that the Hawaiian
lavas require melt contributions from mixed sources
(i.e., garnet clinopyroxenite and carbonated lherzo-
lite), they could also be explained by mixing between
carbonated and volatile-free lherzolite.
We do not favor a major role for garnet clino-
pyroxenite in the generation of these lavas because
of the clearly opposing major-element trends. Since
we do not have experimental data on garnet-clino-
pyroxenite melting in the presence of CO2, we
cannot evaluate whether Hawaiian and Polynesian
lavas could be produced by garnet-clinopyroxenite
melting in the presence of CO2. We note that the
differences between the experimental melts and the
lavas may all be due to the limited nature of the
starting mixes used in the experiments. In addition,
since the partial melts of 77SL-582 do not appear to
correspond well with the OIB, we evaluate a hybrid
model, whereby partial melts of carbonated lherzo-
lite mix with those of 77SL-582 to produce the OIB
trend. Examples of the hypothetical mixing curves
are shown in Fig. 7. In the alkalies-silica diagram
(Fig. 7A), mixed melts are positively correlated, and
are thus transverse to the OIB trend. In the CaO–SiO2
diagram (Fig. 7B), mixed melts not only have the
OIB trend, they also overlap most of the OIB;
although mixing of partial melts between anhydrous
and carbonated lherzolite also holds the potential to
explain the OIB data. In the MgO–Al2O3 diagram
(Fig. 7C), although the mixed melts have the OIB
trend, there is a lack of direct overlap. Qualitatively, it
appears that mixing between partial melts of anhy-
drous and carbonated lherzolite could explain most of
the OIB data.
Alternatively, rather than resulting from mixing of
melts from different kinds of lherzolite sources, the
OIB trends could potentially be produced by varying
degrees of melting of lherzolite with variable
amounts of CO2 [5]. It is important to note that
the experimental data from melting of volatile-free
garnet lherzolite [25,38] do not extend to the low
degrees of melting likely to be relevant to the
generation of alkalic OIB. Experimental data at
lower pressures [37] indicate that such melts could
be alkalic in the 3.0–4.0 GPa pressure range. One
problem with this model is the high magnesium
content of the partial melts of carbonated peridotite
compared to compositions of OIB. Possible explan-
ations for this discrepancy include, firstly, that crys-
tal fractionation causes the compositions of erupted
OIB lavas to become considerably less magnesian
than their primary magmas, and, secondly, that the
experimentally produced CO2-bearing melts [4] are
highly magnesian because of the large amount of
magnesite added to the starting composition, which
notably led to the exhaustion of cpx at a low degree
of melting.
Higher-pressure (3–7.5 GPa) partial melting re-
sults have recently been reported on Mix1G [43].
These results indicate that the near-solidus partial
melts of garnet pyroxenite are low in alumina, an
effect that can be ascribed to the increasing stability of
garnet in the solid residue. However, it is to be noted
that the partial melting trends at these higher pressures
do not change, and remain transverse to the OIB trend
shown in Fig. 7. These partial melts (mostly in their
alumina contents) also partially overlap with those
generated from melting of anhydrous lherzolite at 3–5
GPa [25]. Thus, when the data of Hirschmann et al.
[1] and Kogiso et al. [43], and the present study are
combined, it appears that melting of garnet pyroxenite
of the bulk compositions chosen in these studies does
not have the potential to explain the major-element
trends shown by the OIB discussed in the present
study. Melting of a CO2-bearing mantle source,
similar to that of Hirose [4], can explain most of
the OIB trend.
In the discussion above, it is assumed that partial
melts of garnet pyroxenite could potentially segregate
from their source(s), are transported directly to the
surface, and are also able to maintain their distinct
petrologic character without much interaction with the
S. Keshav et al. / Earth and Planetary Science Letters 223 (2004) 365–379 377
surrounding lithosphere. This appears highly improb-
able. Undersaturated compositions similar to those
reported here might react with orthopyroxene in the
ambient mantle lherzolite. This reaction process pro-
duces olivine, and thus the partial ,melts of garnet
pyroxenite, after/during this reaction, may become
similar to partial melts of mantle lherzolite. This
process has been, to some extent, explored by Yaxley
and Green [44] and Yaxley [3]. Thus, these melt–wall
rock reactions are likely and perhaps would also lead
to a range of refertilized garnet lhezolite sources, that
are further capable of yielding an interesting array of
possible liquid compositions. However, we note that in
sandwich experiments reported by Kogiso et al. [43],
the melt compositions, although similar in some
respects, are quite far removed from the OIB trend,
that further leads us to believe that CO2-bearing mantle
source holds the right potential to explain most of the
OIB major-element trends.
6. Conclusions
In order to understand the melting processes re-
sponsible for the generation of OIB and also to clarify
the role of garnet clinopyroxenite in the existing
debate as a lithology controlling the major-element
budget of the OIB, we have reported partial melting
experiments at 2.0–2.5 GPa on a high-Mg garnet-
clinopyroxenite xenolith (77SL-582) from Oahu, HI.
The bulk composition of 77SL-582 resembles a tho-
leiitic picrite. The solidus of 77SL-582 is f 60–70
jC lower than that of mantle lherzolite at identical
pressures. Partial melts of 77SL-582 at moderate to
relatively high degree of melting are strongly to
moderately Si-poor, ne-normative in composition,
becoming relatively Si-rich, hy-normative with further
melting. These melts display the alkalic to tholeiitic
basalt transition that has also been recognized in
ocean-island basalts. This transition in the partial
melts of 77SL-582 can be explained by a relatively
rapid expansion of the garnet-clinopyroxenite liquidus
boundary in natural systems with increasing pressure.
In most of the melting interval, the partial melt
compositions of 77SL-582 at the pressures of inves-
tigation are quite far removed from the ocean-island
basalts, and display transverse trends displayed by the
ocean-island basalts. In the context of the petrogenesis
of OIB, it appears that garnet clinopyroxenite of
composition similar to 77SL-582 plays a relatively
minor role (if any), and that most of the major-element
compositions can be adequately explained by mixing
of melts mostly generated from carbonated and anhy-
drous mantle lherzolite at high pressures, or possibly
by variable degree of melting of lherzolite with
variable amounts of CO2.
Acknowledgements
SK thanks Heather Watson for her help with the
high-pressure experiments. Bert Collins and Steve
Coley at the Geophysical Lab are thanked for
providing various assembly parts used in the high-
pressure experiments. Dave George and Chris Hadi-
diacos at the Geophysical Lab and Thomas Beasley at
FCAEM, FIU are acknowledged for their help in
acquisition of the probe data using the instruments at
these two places. SK remains grateful to Merri Wolf
and Shaun Hardy at Carnegie for making available
rare papers from obscure volumes. These experiments
were done while SK was a visiting investigator at the
Geophysical Lab in the spring of 2003, and during
this tenure the Dissertation Year Fellowship award
from the University Graduate School, FIU supported
him. Discussions on various aspects of mantle pet-
rology and magma genesis with Michael Bizimis, Mel
Borges, Grenville Draper, Steve Haggerty, Andrew
Macfarlane, Jim Van Orman, Dean Presnall, Max
Tirone, and Michael Walter are gratefully acknowl-
edged. Special thanks go to Dean Presnall for sharing
his mantle wisdom and for collecting and catalogu-
ing the sample 77SL-582 used in this study. Cons-
tructive official reviews by Paul Asimow, Peter Ulmer,
and Greg Yaxley lead to improved clarity and focus of
this presentation. Bernie Wood and his staff at EPSL
are acknowledged for their editorial efforts. This
research was supported by the US NSF grants OCE-
9810961, OIA-9977642, and OCE-0241681 to Gau-
tam Sen and EAR-0106645 to Dean Presnall. [BW]
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