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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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