High-pressure phase equilibria and element partitioning experiments on Apollo 15 green C picritic glass: Implications for the role of garnet in the deep lunar interior David S. Draper a, * , S. Andrew duFrane b , Charles K. Shearer Jr. a , Rachel E. Dwarzski a , Carl B. Agee a a Institute of Meteoritics, 1 University of New Mexico, MSC03-2050, Albuquerque, NM 87131, USA b Department of Earth and Planetary Sciences, 1 University of New Mexico, MSC03-2050, Albuquerque, NM 87131, USA Received 26 April 2005; accepted in revised form 13 January 2006 Abstract We report results of nominally anhydrous near-liquidus experiments on a synthetic analog to very low-titanium Apollo 15 green C lunar picritic glass from 2 to 5 GPa. Apollo 15 green C glass (A15C) is saturated with garnet and pyroxene on the liquidus at 3 GPa. However, such an assemblage is unlikely to represent the lunar-mantle source region for this glass, and instead an olivine + orthopyrox- ene-dominated source is favored, in accord with earlier lower-pressure experiments on A15C. Near-liquidus garnet has a slight but sig- nificant majorite component at 5 GPa in this iron-rich bulk composition, as expected from our previous work in ordinary-chondritic bulk compositions. Ion microprobe measurements of partitioning of Sr, Ba, Sc, Nd, Sm, Dy, Yb, Y, Zr, Hf, and Th between garnet and coexisting melt in these experiments are the first garnet partition coefficients (D values) available that are directly relevant to lunar com- positions. D values for these garnets differ significantly compared to D values for garnets grown in more magnesian, terrestrial bulk com- positions, which until now are all that have been available in modeling the possible role of garnet in the lunar interior. For example, D values for heavy rare earth elements are lower than are those from terrestrial basaltic systems. These partitioning values are well-de- scribed by the lattice-strain partitioning model, but predictive relationships for garnet partitioning using that model fail to match the measured values, as was the case in our earlier work on chondritic compositions. Using our new D values in place of the ‘‘terrestrial’’ values in a variety of models of lunar petrogenesis, we suggest that garnet is unlikely to be present in the source regions for very titanium- poor lunar liquids despite its appearance on the liquidus of A15C. Ó 2006 Elsevier Inc. All rights reserved. 1. Introduction Insights on petrogenetic processes occurring in the lunar interior have been gained from decades of study of samples returned by Apollo and Luna missions. These earlier stud- ies have been augmented in recent years by increasing inter- est in lunar science prompted in part by results from robotic orbiters such as Lunar Prospector and Clementine, and by consortia such as the New Views of the Moon. Not- withstanding these intensive efforts, fundamental questions remain regarding the nature both of the source regions for lunar magmas and of the pressure–temperature conditions over which melting took place that gave rise to them, e.g., Shearer and Papike (1999) and Hess (2000). Lunar picritic glasses are thought to be the result of fire- fountain eruptions on the Moon, more or less during the period of peak lunar magmatism (Papike et al., 1998). Their compositions span the range of nearly all lunar bas- alts. As probes of the lunar interior, picrite glasses are un- ique in that they are quenched near-primary liquids that originated at greater depths in the Moon compared to dominantly crystalline mare basalts. Picrite glass composi- tions range from a very low-Ti end member having 48 wt% SiO 2 , <0.5 wt% TiO 2 , Mg# (molar ratio of Mg/ Mg + Fe) 65, and CaO/Al 2 O 3 1.1 to a very high-Ti www.elsevier.com/locate/gca Geochimica et Cosmochimica Acta 70 (2006) 2400–2416 0016-7037/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.gca.2006.01.027 * Corresponding author. Fax: +1 505 277 8803. E-mail address: [email protected](D.S. Draper).
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www.elsevier.com/locate/gca
Geochimica et Cosmochimica Acta 70 (2006) 2400–2416
High-pressure phase equilibria and element partitioningexperiments on Apollo 15 green C picritic glass: Implications
for the role of garnet in the deep lunar interior
David S. Draper a,*, S. Andrew duFrane b, Charles K. Shearer Jr. a,Rachel E. Dwarzski a, Carl B. Agee a
a Institute of Meteoritics, 1 University of New Mexico, MSC03-2050, Albuquerque, NM 87131, USAb Department of Earth and Planetary Sciences, 1 University of New Mexico, MSC03-2050, Albuquerque, NM 87131, USA
Received 26 April 2005; accepted in revised form 13 January 2006
Abstract
We report results of nominally anhydrous near-liquidus experiments on a synthetic analog to very low-titanium Apollo 15 green Clunar picritic glass from �2 to 5 GPa. Apollo 15 green C glass (A15C) is saturated with garnet and pyroxene on the liquidus at �3 GPa.However, such an assemblage is unlikely to represent the lunar-mantle source region for this glass, and instead an olivine + orthopyrox-ene-dominated source is favored, in accord with earlier lower-pressure experiments on A15C. Near-liquidus garnet has a slight but sig-nificant majorite component at �5 GPa in this iron-rich bulk composition, as expected from our previous work in ordinary-chondriticbulk compositions. Ion microprobe measurements of partitioning of Sr, Ba, Sc, Nd, Sm, Dy, Yb, Y, Zr, Hf, and Th between garnet andcoexisting melt in these experiments are the first garnet partition coefficients (D values) available that are directly relevant to lunar com-positions. D values for these garnets differ significantly compared to D values for garnets grown in more magnesian, terrestrial bulk com-positions, which until now are all that have been available in modeling the possible role of garnet in the lunar interior. For example, D
values for heavy rare earth elements are lower than are those from terrestrial basaltic systems. These partitioning values are well-de-scribed by the lattice-strain partitioning model, but predictive relationships for garnet partitioning using that model fail to match themeasured values, as was the case in our earlier work on chondritic compositions. Using our new D values in place of the ‘‘terrestrial’’values in a variety of models of lunar petrogenesis, we suggest that garnet is unlikely to be present in the source regions for very titanium-poor lunar liquids despite its appearance on the liquidus of A15C.� 2006 Elsevier Inc. All rights reserved.
1. Introduction
Insights on petrogenetic processes occurring in the lunarinterior have been gained from decades of study of samplesreturned by Apollo and Luna missions. These earlier stud-ies have been augmented in recent years by increasing inter-est in lunar science prompted in part by results fromrobotic orbiters such as Lunar Prospector and Clementine,and by consortia such as the New Views of the Moon. Not-withstanding these intensive efforts, fundamental questionsremain regarding the nature both of the source regions for
0016-7037/$ - see front matter � 2006 Elsevier Inc. All rights reserved.
lunar magmas and of the pressure–temperature conditionsover which melting took place that gave rise to them, e.g.,Shearer and Papike (1999) and Hess (2000).
Lunar picritic glasses are thought to be the result of fire-fountain eruptions on the Moon, more or less during theperiod of peak lunar magmatism (Papike et al., 1998).Their compositions span the range of nearly all lunar bas-alts. As probes of the lunar interior, picrite glasses are un-ique in that they are quenched near-primary liquids thatoriginated at greater depths in the Moon compared todominantly crystalline mare basalts. Picrite glass composi-tions range from a very low-Ti end member having�48 wt% SiO2, <0.5 wt% TiO2, Mg# (molar ratio of Mg/Mg + Fe) �65, and CaO/Al2O3 � 1.1 to a very high-Ti
endmember having �34 wt% SiO2, �16 wt% TiO2, Mg#�50, and CaO/Al2O3 � 1.35 (Delano, 1986). Along withlunar mare basalt compositions, the glasses were extensive-ly studied experimentally (largely during the 1970s) inapplications of the inverse experimental approach, i.e.,mapping liquidus mineralogy as a function of pressure(Basaltic Volcanism Study Project, 1981), in order to con-strain their depths of origin (Longhi, 1992a). Most of thesepicrite glass studies, summarized below, found points ofnear-liquidus multiple saturation in the pressure range1.5–2.5GPa, and these findings have contributed to thevariety of interpretations for lunar magmagenesis, e.g., ex-tents of magma ocean processing, role of cumulate over-turn, degree of source region refertilization by Ti- andincompatible-rich materials such as KREEP (K-, rare earthelement-, and P-rich component), and the role of assimila-tion (Delano, 1986; Longhi, 1992b; Hess and Parmentier,1995; Shearer and Papike, 1999; Hess, 2000).
Most of the pre-1990s studies of mare basalts concludedthat garnet was not in their mantle sources. This conclusionwas based on (1) the assumption that mare magmatism rep-resented melting in the shallow lunar mantle outside the sta-bility field of garnet, (2) the observation that the traceelement data for the mare basalts did not exhibit typical‘‘garnet signature’’ trace-element fractionations, and (3)the prediction from lunar magma ocean models that thesource region of the mare basalts were essentially low-Alcumulates. In ensuing years, reinterpretation of experimen-tal studies on the lunar pyroclastic glasses by Delano(1986) and Longhi (1992b) expanded the mantle pressure re-gimes over which mare magmatism may have occurred(�1000 km). More recent trace element and isotopic data(see below) have been interpreted as indicating the presenceof garnet in the source regions for some of the mare basalticmagmas represented by the pyroclastic glasses. These recentdata present a dilemma for mare magmatism because sourc-es for these magmas are still thought to be Al-poor (Longhi,1995) and the stability of garnet as a liquidus or near-liquidusphase for picrite glasses has not been documented.
Because essentially all experimental studies on picriteglasses performed to date were carried out at pressures low-er than those at which garnet is likely to become stable inthese bulk compositions, phase equilibrium evidence rele-vant to this question is scant. We have begun to redressthat lack by performing near-liquidus inverse-approachexperiments on several lunar picrite glass compositions atgarnet-relevant pressures in order to constrain that miner-al’s possible role in their source regions. The experimentsare intended both to identify the high-pressure liquidusmineralogy of a range of picrite glass compositions andto measure trace element partitioning between those liqui-dus minerals and coexisting picrite melts. Garnet-liquidtrace element partition coefficients that have been used toevaluate the role of garnet in lunar magmagenesis haveheretofore, by necessity, come from experimental studieson terrestrial compositions, which are substantially moremagnesian than are lunar compositions. As we will show
below and as indicated by previous determinations of gar-net-liquid partitioning in Fe-rich chondritic liquids (Draperet al., 2003), partition coefficients for some important ele-ments change markedly in such Fe-rich systems.
In this contribution, we report results of high pressureexperiments on Apollo 15 green C glass (A15C hereafter),the picrite glass poorest in TiO2. These experiments docu-ment both anhydrous high-pressure near-liquidus phaserelations of this composition and determine trace elementpartitioning between low-Ti lunar liquids and coexistingnear-liquidus garnet using secondary ion mass spectrome-try (SIMS, or ion microprobe). Understanding the con-straints on the presence or absence of this mineral hasimportant implications for the physical and chemical struc-ture of the lunar interior.
In the following sections, we review previous experimen-tal work on lunar basaltic compositions and the more re-cent geochemical evidence pointing to a possible role ofgarnet in their source regions. We then outline our experi-mental and analytical approach before reporting the resultsof our experiments and the applications of these results toconstraining the nature of the deep lunar mantle.
1.1. Review of previous experimental work
Papike et al. (1998) summarized experimental studies per-formed on both mare basalt and picrite glass compositions atpressures from �1.0 to 2.5 GPa. These include Walker et al.(1972, 1975, 1976, 1977); Green et al. (1971a,b, 1975); Kushi-ro et al. (1972); Hodges and Kushiro (1974); Chen et al.(1982); Chen and Lindsley (1983); Kesson (1975); Groveand Vaniman (1978); O’Hara et al. (1970); Ringwood andEssene (1970); Delano (1980); Longhi (1992b,a, 1995); andLonghi et al. (1974). More recent work includes experimentsby Wagner and Grove (1997), Elkins-Tanton et al. (2000;2003), and Van Orman and Grove (2000). Most of thesestudies showed, in general, that multiple saturation pressuresfor the picrite glasses were greater than were those for marebasalts (2.0–2.5 GPa for most glasses compared to 0.5–1.5 GPa for most mare basalts). In addition, the saturatingassemblage was olivine + low-Ca pyroxene ± spinel for awide variety of lunar compositions. These include the low-Ti picrite glasses studied by Chen et al. (1982), Chen andLindsley (1983), and Grove and Vaniman (1978); for thehigh-Ti glass compositions studied by Wagner and Grove(1997); Delano (1980); and Walker et al. (1975); and forthe low-Ti mare basalts studied by Ringwood and Essene(1970). In contrast, almost all the high-Ti crystalline marebasalt compositions studied experimentally were saturatedwith olivine + clinopyroxene + a Ti-rich phase such asilmenite (Ringwood and Essene, 1970; Longhi et al., 1974;Green et al., 1975; Kesson, 1975).
There are a variety of interpretations for these results.Source regions for lunar mare basalts could simply be dif-ferent from those for the picrite glasses; their differing mul-tiple-saturation assemblages and pressures wouldcorrespond to the different lithologies of their respective
2402 D.S. Draper et al. 70 (2006) 2400–2416
sources and/or the differing pressures at which melting oc-curred. Alternatively, the multiple-saturation pressuresshown by these compositions could be averages of a setof polybaric processes, as argued by Longhi (1992b). Inthis view, multiple saturation pressures for the mare basalts(assuming they represent liquid compositions) could repre-sent minimum depths of melting, such that the actualdepths were similar to those for the picrite glasses. A lowerapparent saturation depth for crystalline mare basaltscould result, for example, from extensive olivine fraction-ation from a picritic magma parental to both the mare bas-alts and picrite glasses. Papike et al. (1998) favored thislatter interpretation, although they did not directly addressthe implications of the different saturating assemblages be-tween the high-Ti basalts and the other three sample types.
The existence of a low-pressure multiple-saturationpoint does not by any means preclude the existence of addi-tional points at other pressures. Therefore, it is quite possi-ble that picrite glasses could show additional points ofmultiple saturation, perhaps with garnet as part of the sat-urating assemblage, at pressures above those shown in thestudies cited above. The results reported here constitute thefirst phase equilibrium data on picritic glasses at pressuresabove �2.3GPa, allowing this notion to be directly tested.
1.2. Geochemical evidence for garnet
Geophysical evidence for garnet is ambiguous largelybecause data from Apollo-deployed seismometers are ofinsufficient resolution (Khan et al., 2000; Lognonne et al.,2003). Geochemical evidence for garnet in lunar magmasources is recent compared to most of the huge amountof lunar research that began before the end of the Apolloprogram, and includes isotopic (Beard et al., 1998) andtrace element (Neal, 2001; Neal and Shearer, 2004) data.Beard et al. (1998) measured Lu–Hf isotopic systematicsin crystalline mare basalts and argued that fractionationsof Lu/Hf and Sm/Nd in low-Ti magmas may have beenproduced by garnet and orthopyroxene in the sources,although garnet was thought to have been consumed dur-ing polybaric melting that began in the garnet stabilityfield. However, as shown by Neal (2001), such a signaturedoes not extend to other trace elements in mare basalts,such as Y, Yb, Zr, and Sc. Neal (2001) illustrated, for cer-tain lunar picrite glasses, a clear signature of garnet reten-tion in the source (e.g., Zr/Y and chondrite-normalizedSm/Yb above those for KREEP while having lowSc/Sm), and agreed that if garnet was present at some pointin mare basalt sources, it was consumed by comparativelyhigh degrees of partial melting to generate those basalts.
Neal (2001) further argued that only a garnet-bearingsource rock could produce melts having both lowerSc/Sm than the source and higher chondrite-normalizedSm/Yb, assuming the bulk Moon composition of Taylor(1982, 1992), and that this garnet-bearing source representsa remnant of ‘‘primitive Moon’’ that did not take part inthe wholesale melting that produced the early lunar magma
ocean. The logical extension of this conclusion is that notall crystalline mare basalts and picrite glasses originatedfrom similar sources. The glasses cited by Neal (2001) thathave the required compositional features outlined aboveare Apollo 12 red, Apollo 14 green A and yellow, Apollo17 type I, and Apollo 17 74220-type. Additional analysisby Neal and Shearer (2004) focused on the comparativelyTi-rich glasses Apollo 14 orange and black and Apollo12 red as the most likely candidates for garnet in thesource, accounting for Apollo 14 green A via the involve-ment of KREEP.
In contrast to the reasoning of Neal (2001) and Neal andShearer (2004), Longhi (1995) has argued that garnet isunlikely to be a residual phase for melting that producedmare basalts and picrite glasses. His polybaric meltingmodels predict that source regions for these magmas wouldbe too poor in Al for an aluminous phase to be stable. Ourexperimental program should be able to help resolve thisapparent conflict, both from the perspective of high-pres-sure phase relations and trace element partitioning. Ourfirst step is to examine the A15C composition. Althoughthis glass is not included in the candidate group definedby Neal (2001), it remains of interest because it constitutesone endmember of the lunar picrite glass suite (e.g., highestSiO2 and Mg#, lowest TiO2 and CaO/Al2O3). In addition,its lower-pressure phase relations are already well-knownfrom the work of Elkins-Tanton et al. (2003). Our prelimin-ary results were reported in abstract form (Shearer et al.,2003; Draper et al., 2004) and are fully documented here.
2. Experimental and analytical procedures
2.1. Experimental
2.1.1. Starting material
Experiments were performed on a synthetic powderedglass with a bulk composition (Table 1) very close to thatof A15C as reported by Delano (1986). This powder wassynthesized by L. Elkins-Tanton for her study of greenglass phase relations (Elkins-Tanton et al., 2003) and kind-ly provided for our use in this study. Because our experi-ments were performed on the exact same powder used inthat study, there are no discrepancies between the two setsof results that might occur from slight differences in start-ing material bulk composition.
In order to measure trace element partitioning, wedoped the A15C powder (for experiments beginning withrun A67 and higher numbers; those with lower numberswere undoped) with a ‘‘cocktail’’ of nitric and hydrofluoricacids in which were dissolved reagent-grade solutions of Sr,Ba, Sc, Nd, Sm, Dy, Yb, Y, Zr, Hf, and Th. These elementswere doped at �50–300 times chondritic abundance levelsso that good-quality SIMS data would be straightforwardto obtain. The particular elements were chosen for two rea-sons: first, to eliminate isobaric interferences during SIMSanalysis; and second, to allow meaningful applicationof the lattice-strain model for trace element partitioning
Table 2Run conditions for experiments on Apollo 15 green C
Values in parentheses are weight percent modes calculated by least-squaresmass balance.
a Intended T for this run was 1775 �C; phase assemblage and compo-sitions (Table 3) reflect lower temperature, probably owing to problem ofcapsule placement in heater. Reported T is estimate consistent with otherresults.
b This run’s temperature fell to �1000 �C after 20 min at targetconditions; it was returned to target T but then suffered a furnace failureafter another 15 min during which T was much less stable than in otherruns. Thus, the temperature reflected by the assemblage and mode in thisrun is likely substantially less than 1800 �C. This is denoted by adownward arrow on this run’s symbol in Fig. 2.
c Run A228 was likely slightly higher T than Run 221; Mg# of melt is 66vs. 63 for A221 (see Table 3).
Table 1Composition of starting material
Oxide 1 2 Trace elements (ppm)
SiO2 48.3 48.0 Sc 763 (13)TiO2 0.23 0.26 Sr 54.3 (8.6)Al2O3 7.77 7.74 Y 232 (17)FeO 16.12 16.50 Zr 688 (6)MnO 0.19 0.19 Ba 423 (46)MgO 18.27 18.20 Nd 17.6 (1.9)CaO 8.59 8.57 Sm 25.3 (2.9)Na2O 0.00 0.00 Dy 21.8 (1.9)Cr2O3 0.55 0.57 Yb 16.4 (1.7)
Total 100.01 100.03 Hf 37.4 (4.5)Th 1.24 (0.07)
Mg# 66.9 66.3CaO/Al2O3 1.11 1.11
1, composition of starting glass from Elkins-Tanton et al. (2003).2, composition of Apollo 15 green C from Delano (1986).Trace element composition is average of 3 ion microprobe analyses, 1 � rstandard deviations in parentheses.Mg# = molar Mg/Mg + Fe · 100.
Apollo 15 green C experiments 2403
promulgated by Blundy and Wood (1994) for clinopyrox-ene and applied in recent years to garnets relevant to terres-trial (van Westrenen et al., 2001b) and martian (Draperet al., 2003) bulk compositions. Another purpose of ourhigh-pressure work on lunar picrite glasses is to isolateimportant crystal-chemical factors controlling garnet-melttrace element partitioning (e.g., effects of Fe, Ti, Cr, andmajorite), and these applications will be developed in fu-ture contributions.
2.1.2. Run conditions
Run conditions and resulting phase assemblages are list-ed in Table 2. Experiments at 2.5 Pa and above were rununder nominally anhydrous conditions in a Walker-typemultianvil press at the Institute of Meteoritics, Universityof New Mexico, using cell assemblies, procedures, and cal-ibrations identical to those described by Agee et al. (1995),where complete details can be found. Tungsten carbidecubes with corners truncated to an 8 mm edge length wereused with cast, finned octahedra of Aremco Ceramacast�584 ceramic. These octahedra were fired at 1100 �C priorto being drilled out for assembly fabrication, and storedin a drying oven at �110 �C until ready for use. The pow-dered starting material, also stored in the drying oven be-tween experiments, was placed inside graphite capsuleswith press-fit lids and insulated from the rhenium foil heat-er walls by hard-fired alumina sheaths. We positioned sam-ples in the center of the heater and bracketed the samplecapsules with crushable alumina spacers. Temperatureswere measured by radially-inserted W/Re thermocouples,with the junction near the capsule at the center of the heat-er, and controlled by a Eurotherm 930P programmablecontroller. Pressures were applied at 3–5 GPa/h. RunsDD504-6 and DD504-2, at 1.8 GPa, were conducted usinga 1/2-in., endloaded piston-cylinder at the University ofOregon. These two reconnaissance experiments were run
in talc-pyrex cells. Temperatures were monitored usingW5Re-W26Re thermocouples and controlled with an Ome-ga electronic ice point and a Eurotherm Model 808 digitaltemperature controller. We applied no pressure correctionto thermocouple emf. Reported pressures are nominal(Heise gauge) and incorporate �10% correction for fric-tion. The samples were quenched in 1–2 s by cutting powerto the graphite furnace tubes.
Most experiments were held at target conditions for30–120 min. Multianvil and piston-cylinder runs werequenched by cutting power to the heater, which causedtemperatures to fall below the glass transition in 610 sand to room temperature in less than 5 min. Successfulrun products were mounted in epoxy, ground flat, and pol-ished for electron microprobe and SIMS analyses.
2.1.3. Approach to equilibrium
Achieving chemical equilibrium in multianvil experi-mentation is more difficult than it is in other forms ofexperiments (e.g., piston-cylinder runs), due in large partto the strong thermal gradients that are unavoidable inmultianvil runs. Vagaries in the precise placement of thethermocouple junction with respect to the sample, andpotential displacements of the sample capsule from the
2404 D.S. Draper et al. 70 (2006) 2400–2416
heater’s midsection, aggravate the difficulty. Although wedo not claim to have achieved perfect equilibrium in theseexperiments, the following considerations give us confi-dence that our results are acceptable.
First, in all subliquidus experiments reported here, gar-net and pyroxene grains grew to relatively large sizes (50–400 lm in longest dimension) with well-formed faces, inthe region closest to the assembly’s ‘‘hot zone’’ (seeFig. 1 for an example). Mineral compositions are reason-ably homogeneous, as shown both by standard deviationson electron probe microanalyses (Table 3) and by backscat-tered electron images with fairly uniform grey levels, likethose in Fig. 1. Appreciable zoning occurs only in thin (afew lm) rinds that we infer to have formed during quench-ing, and these rinds were avoided during electron and ionmicroprobe analysis. Second, quench melt compositionsalso display reasonable homogeneity (Table 3) given thatthe melt in all experiments quenched to an intergrown mass
Fig. 1. Backscattered electron photomicrographs of run products. Gt,garnet; Quench, quenched melt. Top, run A67 with large garnet grains.Large black voids are decompression fractures. Bottom, run A229 withsmaller garnet grains.
of microlites rather than to a glass (Fig. 1), although theseanalyses are less homogeneous than are those of crystallinephases. In addition, this felty texture resulted in electronmicroprobe totals of 95–98% in some experiments, a situa-tion not uncommon in multianvil work on ultramaficcompositions.
2.2. Analytical
2.2.1. Electron microprobe conditions
Polished run products were analyzed for major elementsusing the JEOL 8200 Superprobe in the Institute of Mete-oritics operating at 15 keV accelerating potential and usingstandards of natural and synthetic minerals and glasses.Mineral phases were analyzed with a focused, 1-lm beamand microcrystalline quenched melt was analyzed with adefocused beam 20–30 lm in diameter. Major elementswere counted for 20 s on peak and 10 s on background,and minor elements for 30 s on peak and 15 s on back-ground. All electron microprobe data were obtained witha 20-nA beam current. Because this bulk composition isnominally alkali-free, we did not operate at low beam cur-rent for analyses of quenched melt as we customarily dowhen alkalies are present. Major element compositionsreported below are averages of 5–20 spots on each phasein each run product.
2.2.2. Ion microprobe conditions
After electron microprobe analysis, carbon coats wereremoved from run products and then gold-coated forSIMS analysis. Analyses were made using primary O� ionsaccelerated through a nominal potential of 10.0 kV. A pri-mary beam current of 20 nA was focused on the sampleover a spot diameter of 20 lm. Sputtered secondary ionswere energy filtered using a sample offset voltage of105 V and an energy window of ±25 V. Analyses involvedrepeated cycles of peak counting. The analytical procedureincluded counting on a background position to monitordetection noise. Absolute concentrations of each elementwere calculated using empirical relationships, derived fromdaily calibrations, of the measured ratios of trace elementions to 30Si+ ions (normalized to known SiO2 content) toelement concentrations. Calibration curves were construct-ed using two or three glass standards for each element.Calibration curves for each element have correlation coef-ficients of greater than 0.97. Analytical precision is ad-dressed in Table 4 by comparison between the nominalcomposition of one of our glass standards and its compo-sition when measured as an unknown using our calibra-tions. These compare very well with the exception of Th,for which the nominal composition was 1.1 ppm but whichwe measured at 3.1 ppm. In any case, D values listed areratios of Si-normalized intensity ratios, to avoid propaga-tion of any uncertainty arising from applying calibrationcurves.
We examined our SIMS analyses in detail to investi-gate the possibility that patches of melt may have been
Table 3Electron microprobe analyses of subliquidus run products
Values are averages of 5–10 spots per phase. Values in parentheses are 1 � r standard deviations. Quench, quenched melt; Pyx, pyroxene; Gar, garnet; andb.d.l., below detection limits.
Apollo 15 green C experiments 2405
Table 4Partition coefficients calculated from ratios of Si-normalized SIMS intensities
Values in parentheses are uncertainties on D values propagated from standard deviations on SIMS intensities (10 cycles per point, three points per phaseper experiment). AH83- KL2 is a SIMS standard basaltic glass; Meas. is composition measured treating standard as an unknown, Std. is the nominalcomposition. E, D0, and r0 are lattice-strain parameters from fits to Eq. (1) with calculated uncertainties in parentheses; r2 is multiple regression coefficientof determination for these fits. Refits are fit parameters calculated without including DNd and DSm as described in text. DNd-refit and DSm � refit are Ds forNd and Sm calculated from refit parameters.
2406 D.S. Draper et al. 70 (2006) 2400–2416
incorporated along with some of the garnet analyses,thereby compromising the utility of measured D values,particularly those for highly incompatible elements suchas the light REE. Melt could be incorporated either fromoverlap of the ion beam with large areas of melt adjacentto garnet crystals, or as submicron inclusions trappedwithin the grains themselves (any such inclusions wouldhave to be smaller than what is resolvable using the elec-tron microprobe’s optics, as none were observed in back-scattered electron imaging). This possibility was examinedin two ways. First, SIMS-analyzed run products were re-examined optically to verify the locations of pits formedby the beam, and any spots found to show visible overlapwith adjacent melt were discarded (only two such pointswere found). Second, each SIMS analytical ‘‘cycle’’ wasscrutinized to ensure that count rates were homogeneousthroughout the analysis. As the analysis cycles proceed,the beam might have ‘‘drilled through’’ the garnet graininto melt lying beneath the plane of section of the mount-ed run product. We included Al and Mg in our list ofanalyte elements in order to monitor for this situation, be-cause the large difference in content of these elements be-tween garnet and coexisting melt would have been readilyidentifiable. We found that only a few individual sets ofanalytical cycles showed very minor evidence of suchincorporation, and those likewise were omitted from fur-ther consideration.
Despite these precautions, we find that some meltincorporation did take place when analyzing some ofthe smallest garnet grains attempted (15–20 lm in longestdimension). Fortunately, the systematics of lattice-strainmodeling of partitioning allow us to correct for thisproblem. These issues will be detailed in Section 3below.
3. Results
3.1. Phase relations
Assemblages from experiments on A15C listed in Table2 are plotted on a pressure–temperature diagram in Fig. 2.This plot also includes the P–T points and mineral-incurves from the experiments of Elkins-Tanton et al.(2003). Our experiments require the existence of a pointof multiple saturation with garnet and pyroxene at a pres-sure between 3.0 and 3.5 GPa (600–700 km lunar depth)and a temperature of �1750 �C. Our results require a slightinflection or curvature to the shape of the liquidus surfacewith increasing pressure, which is an occurrence observedin many high pressure studies when garnet becomes anear-liquidus phase. Overall, the position of the liquiduslocated by our experiments is quite compatible with thatdefined by the experiments of Elkins-Tanton et al. (2003).However, as described below, pyroxene in our experiments
0 1 21300
1400
1500
1600
1700
1800
1900
gt + px
gt
px
T (
˚C)
P, GPa
Elkins-Tanton et al. ( 2003) Liq Liq+px Liq+gt Liq+px+gt
olopx
opx + cpx
Liq
Fig. 2. Pressure–temperature diagram summarizing near-liquidus phaserelations of A15C. Liq, quenched melt; ol, olivine; opx, orthopyroxene;cpx, clinopyroxene; px, supersolvus pyroxene; and gt, garnet. Low-pressure experiments (grey boxes) replotted from Elkins-Tanton et al.(2003). High-pressure point of multiple saturation is at 3–3.5 GPa,�1750 �C.
Apollo 15 green C experiments 2407
is a single, supersolvus phase in contrast to the two coexis-ting low- and high-Ca pyroxenes occurring in theirexperiments.
3.2. Phase compositions
Table 3 lists electron microprobe analyses of mineralsand melts in all subliquidus experiments reported here.Garnet in our highest-pressure runs, A229 and A225, hasa small but distinct majorite content, manifest by 3.06–3.12 Si and 1.70 to 1.61 Al, respectively, per 12-oxygen for-mula unit in the stoichiometry calculated from electronmicroprobe analyses. In this garnet, some Fe in octahedralcoordination is required to fill the garnet octahedral (Y)site, as the only other trivalent cation available to enterY is Cr3+, and only 0.080–0.086 Cr per formula unit isavailable in those garnets. These changes reflect the en-hanced solubility with increasing pressure of a pyroxenecomponent, R2
2þSi2O6, in the Y site of garnet,R3
2þR23þ½SiO4�3, via the substitution VI[R2+ + Si4+] =
2VIR3+ (Xirouchakis et al., 2002; Draper et al., 2003). Inother words, majorite garnet is typified by some Fe2+ andSi4+ entering the Y site in octahedral coordination, causingAl per 12 oxygens to fall below 2.0 and Si per 12 oxygens toexceed 3.0, with some divalent cation(s) also entering Y forcharge balance. Even small majorite contents probablyplay a role in affecting garnet-melt trace element D values,in similar fashion to data reported by Draper et al. (2003).
As mentioned above, pyroxenes formed in ourexperiments are single phase clinopyroxenes (pigeonite tolow-Ca augite), compared to coexisting orthopyrox-ene + clinopyroxene at lower pressures as documented byElkins-Tanton et al. (2003). Pyroxene compositions aremore a function of temperature, and hence melt fraction,
at a given pressure than of pressure alone. These composi-tions at 3 GPa are En66Fs18Wo15 at 55% melt (run A8) andEn56Fs30Wo14 at 22% melt (run A41); at 3.5 GPa,En65Fs22Wo13 at 65% melt (run A42); and at 5 GPa,En55Fs24Wo22 at 10% melt (melt fractions calculated bymass balance). Al contents of these pyroxenes range from0.1 to 0.2 per 6-oxygen formula unit. Our pyroxenes com-pare favorably to those reported by Chen et al. (1982) fromtheir experiments on Apollo 14 low-titanium green glass;relative to A15C this composition is 3 wt% poorer inSiO2 and has lower Mg# (56 vs. 67) and CaO/Al2O3
(0.96 vs. 1.10). Their pyroxenes have ranges of En55–
81Fs14–27Wo4–19 and 0.1–0.45 Al per formula unit at�2 GPa. We account for the presence of a single pyroxenein our experiments, instead of two pyroxenes as in those ofElkins-Tanton et al. (2003), by noting that our experimen-tal temperatures are likely to be well above the high-pres-sure solvus in the pyroxene system. For comparison,experiments by Lindsley (1980) in the iron-free enstatite–diopside system show peak solvus temperatures at 3.0GPa of �1525 �C, more than 100 �C cooler than the low-est-temperature run reported here, and the presence of Fein the system would be expected to lower the solvus tem-perature even further. Lindsley (1980) also showed broadfields of a single clinopyroxene in iron-bearing systems at3.0 GPa. Therefore, the appearance of only one pyroxeneat our experimental conditions is not unexpected.
Electron microprobe totals for our quenched melts donot sum to 100% in most cases (Table 3). This deficiencyresults largely because ultramafic compositions like A15Care virtually impossible to quench to a glass at the highpressures of these experiments. Instead, quenched meltsform extremely fine-grained (submicron to micron size),felty mats of ferromagnesian crystals that resemble komat-iitic spinifex textures (Fig. 1). Textures like these have beenreported from a wide variety of experimental studies ofultramafic compositions. For this reason we analyzedquenched melts with defocused beams, as indicated above.We assume that low totals reflect deficiencies in X-raycounts of all elements equally and are not due to a singleelement being incorrectly analyzed. This assumption is bol-stered by repeated checks of microprobe standards and sec-ondary standards (i.e., materials not used in calibrationthat were analyzed in multiple previous microprobe ses-sions), and by successful least-squares mass-balance modecalculations that yield quite good fits of the mineral andquenched melt compositions to that of the starting material(Table 2). For example, re-running these mass balancemodes using melt compositions renormalized to 100%yields exactly identical modes.
3.3. Trace element partitioning values
Table 4 lists trace element partition coefficients from fivedoped experiments. Runs A221 and A225, althoughgarnet-bearing, were too fine-grained to attempt SIMSanalysis, with garnets ranging in size only to �15 lm. Note
0 5 10 15 20 250.1
1
Sc Zr
Di,
Gar
net-
mel
t
P, GPa
0.1
1
Y Hf
Di,
Gar
net-
mel
t
0.01
0.1
1
10
Sm Nd Yb
This study Nicholls Salters WvW Maj Draper
Di,
Gar
net-
mel
t
a
b
c
Fig. 3. Summary diagram comparing trace element D values as a functionof pressure between this study and literature. (a), data on Sm, Nd, and Yb;(b), on Y and Hf; (c), on Sc and Zr. Symbol shapes keyed to variouselements as given in legends (e.g., diamonds for Sm, inverted triangles forY, circles for Zr), and symbol fills for different studies as denoted inlegend; data from this study in light grey shading. Nicholls, Nicholls andHarris (1980); Salters, Salters et al. (2002); WvW, van Westrenen et al.(1999, 2000, 2001a); Maj, experiments producing majoritic garnet in Mg-rich systems (Kato et al., 1987; Ohtani et al., 1989; Yurimoto and Ohtani,1992; Inoue et al., 2000; Suzuki et al., 2000); and Draper, mildly majoriticgarnet in Fe-rich chondritic system (Draper et al., 2003). Note that most D
values from A15C garnets are lower than virtually all previously publishedvalues, or fall at the low end of previously published ranges.
2408 D.S. Draper et al. 70 (2006) 2400–2416
that the D values listed in Table 4 were calculated directlyfrom the garnet/melt SIMS intensity ratios, as indicatedabove, in order to avoid propagation of any uncertaintyarising from construction of calibration curves. Table 4also includes a SIMS analysis of a standard basaltic glassthat was treated as an unknown, compared to the nominalstandard composition, as a measure of the accuracy of ourSIMS data.
As mentioned above, some minor melt incorporationlikely occurred in SIMS analyses of garnets, despite ourbest efforts to minimize it. For example, D values for theleast garnet-compatible elements (Sr, Ba, Nd, and Th) ingarnet from run A67 are significantly lower than they arein garnets from the other runs (Table 4). Those garnetsare substantially smaller than are the garnets in run A67(25–50 lm in A138 and A140 compared to several hundredlm in A67). We infer that this contamination from meltincorporation was minor, except for run A138, becausethe D values for other elements such as Sc, Y, Zr, Hf,Dy, and Yb are quite similar between these experiments.Run A138 appears most strongly affected by melt incorpo-ration, with the highest D values for Sr, Ba, and Th. As willbe seen below, D values for the least compatible elementsfrom the other four experiments can be well-constrainedusing fits to the lattice-strain partitioning model of Blundyand Wood (1994).
We compare the D values measured in our experi-ments with previously reported literature values in Figs.3 (against experimental pressures) and 4 (against garnetcomposition). Fig. 3 illustrates significant differences be-tween our D values and those from studies performedon magnesian compositions appropriate for terrestrialapplications, in particular for Yb and Y. D values forSm are slightly lower, whereas those for Nd, Hf, Sc,and Zr are similar. All these values are quite similar toD values measured in a chondritic composition by Drap-er et al. (2003) and to other, higher-pressure garnets inwhich garnets manifest a significant component of major-ite. In contrast, in Fig. 4 our D values largely fall ontrends of D vs. garnet composition established by previ-ous studies of both magnesian, terrestrial compositions atlow to high pressures and by our previous work on Fe-rich, chondritic compositions. These relationships are aconsequence of garnet compositions in the experimentsreported here being generally less pyropic at a given tem-perature and pressure than are garnets grown in moremagnesian systems.
3.4. Lattice-strain partitioning model and correction for melt
incorporation
3.4.1. Lattice-strain model
The lattice-strain model of Blundy and Wood (1994)has been successfully applied to garnet-melt partitioning,in particular by van Westrenen and coworkers (van Wes-trenen et al., 1999, 2000, 2001a,b). The lattice-strain mod-el fits partitioning data expressed in terms of D vs. ionic
0 10 20 30 400.1
1
Di , garnet-m
elt
Mol % Grossular
0.01
0.1
1
10
Di , garnet-m
elt
30 40 50 60 70 80 90 1000.1
1
Di, g
arne
t-m
elt
Mol % Pyrope
0.01
0.1
1
10
Di, g
arne
t-m
elt
a c
db
Fig. 4. Comparison of garnet-melt D values as a function of garnet composition between this study and literature values. (a, b) D vs. pyrope content; (c, d)D vs. grossular content. Data sources and symbols as in Fig. 3. Data from this study, although again falling at the lower end of ranges for previous data,generally fall on pre-existing trends of D values against garnet composition. See text for additional discussion.
Apollo 15 green C experiments 2409
radius; the pioneering work of Onuma et al. (1968) firstdescribed the parabolic trends of partitioning data onsuch plots. Blundy and coworkers drew on work by Brice(1975) to describe Onuma-type partitioning trends using
Di ¼ D0 � exp�4pEN r0
2ðri � r0Þ2 þ 1
3ðri � r0Þ3
h i
RT
24
35; ð1Þ
where Di is the measured partition coefficient for element i,ri is the ionic radius of that element, R is the gas constant,N is the Avogadro’s number, and T is in Kelvins. This rela-tionship is appropriate for describing partitioning of cat-ions having the same charge into a given crystallographicsite, for example trivalent cations partitioning into the gar-net X site. Fits of partitioning data from a given experi-ment to Eq. (1) yield values for D0, the maximum D
value for a cation of that charge into that site; r0, the idealsite size for strain-free partitioning of a cation of that char-ge; and E, the bulk (Young’s) modulus, which can be con-sidered a measure of site compressibility. Mathematically,these parameters correspond, respectively, to the y-inter-cept of maximum on the parabola; its x-intercept; andthe width of the parabola (tighter curves yield higher E).It is important for a meaningful fit that D values are avail-able for elements having ionic radii both greater than and
less than the value of r0 so that both limbs of the parabolaare constrained. For the trivalent cations partitioning intothe garnet X site, this requirement is met when Sc is includ-ed in the collection of elements used in measuring partition-ing, because its ionic radius (0.870 A at coordinationnumber 8; Shannon, 1976) is significantly below typical r0
values, while ionic radii for the rare earths are all greater.In general, if it can be shown that partitioning data for aset of isovalent cations follow this relationship, then Eq.(1) can be used to solve for the appropriate values of E,D0, and r0, and these can in turn be used to calculate Di
for some unmeasured cation with radius i.van Westrenen et al. (2001b) used data from several pre-
vious partitioning studies to form a predictive expressionfor trivalent cation partitioning in garnet. They related par-titioning to temperature, pressure, garnet major-elementcomposition, and garnet-melt DMg, to produce predictedvalues of D0, r0, and E for use in Eq. (1) to calculate D val-ues for systems in which partitioning has not been directlymeasured. This predictive relationship (termed ‘‘WvW pre-dictive model’’ hereafter) was intended to cover pressuresup to 7.5 GPa for largely basaltic compositional systems,and recovered published partitioning data not included intheir parameterization quite successfully, to within �20%relative. Applications of the WvW predictive model to gar-net-melt partitioning in an Fe-rich, chondritic bulk compo-
Fig. 5. Plot of trivalent D values (Table 4) against ionic radii from Shannon (1976) showing well-defined parabolic trends. Solid black curves are fit tolattice-strain model of Blundy and Wood (1994) Eq. (1), yielding parameters and uncertainties listed in legend. Solid grey curves are refits omitting DSm
and DNd as described in text, with revised values for these, calculated from refit curves, shown labeled ‘‘Refits.’’ Dashed curves are partitioning predictedby formulation of van Westrenen et al. (2001b). Note large mismatches between observed and predicted partitioning, especially in D0.
2410 D.S. Draper et al. 70 (2006) 2400–2416
sition in which garnets had an appreciable majorite compo-nent yielded substantial mismatches between the predictedand measured partitioning behavior (Draper et al., 2003).Accordingly, we sought to determine whether their rela-tionship could successfully predict partitioning in our runson A15C, and in fact one aim of the experiments reportedhere is to provide additional data coverage on garnet-meltpartitioning in order to expand the applicability of theWvW predictive model beyond terrestrial compositions atcomparatively low pressures. This effort has already yield-ed revised predictive expressions that account for partition-ing in anhydrous, Fe-rich systems much better thanpreviously (Draper and van Westrenen, 2005).
Our trivalent partitioning data (Nd, Sm, Dy, Y, Yb, andSc, Table 4) yield good fits to Eq. (1), and once again ourmeasured values are not reproduced by the WvW predic-tive model. Fig. 5 summarizes these results for runs A67,A140, A228, and A229. (We do not consider further datafrom A138, which we judge experienced far more substan-tial melt incorporation than the other runs listed in Table4.) On all plots, solid curves are fits to Eq. (1) of our mea-sured D values from each experiment, and dashed curvesrepresent the partitioning predicted by the WvW model.
It is clear that the WvW model predicts substantially largerD values than those measured for virtually all trivalent cat-ions. Table 4 also lists the values of the fit parameters E, r0,and D0 resulting from application of Eq. (1) to the datafrom each experiment.
A major difference between D values in our garnets andthose from previous studies is that the parabolic curvesformed by our data on plots like those in Fig. 5 are broaderthan those from the earlier work. Broader parabolae corre-spond to lower values of E, indicating greater compressibil-ity of the X site in these garnets. Detailed crystal-chemicaltreatment of factors affecting how the lattice-strain fitparameters change with pressure, temperature, and compo-sition are the subject of separate contributions (e.g., Drap-er and van Westrenen, 2005), but briefly put, these lowapparent E values are probably anomalously so, even tak-ing into account the expectation from our previous work(Draper et al., 2003) that E is typically lower in more Fe-rich garnets. As described above, we suspect some meltincorporation took place in many of the SIMS analyses,and the effect of such incorporation would be to artificiallyelevate the D values somewhat for the least-compatible ele-ments. This would in turn lead to broader parabolae, and
0.01 0.1 1
1
10
100
DY
b
DSm
Draper Kato/Ohtani/Corgne N & H S & L WvW A15C A15C-corr Barth Green Pertermann
Fig. 6. Plot of DYb vs. DSm comparing measured and corrected DSm. Datasources: Draper, majoritic garnets in Fe-rich chondrite melt, Draper et al.(2003); Kato/Ohtani/Corgne, majoritic garnets in more magnesian melt,Kato et al. (1988), Ohtani et al. (1989), Corgne and Wood (2004); N andH, garnets in basaltic to andesitic melt, Nicholls and Harris (1980); S andL, garnets in basaltic melt, Salters and Longhi (1999) and Salters et al.(2002); Barth and Green, garnet in hydrous basaltic melt, Barth et al.(2002) and Green and Blundy (unpublished); Pertermann, garnet ineclogitic melt, Pertermann et al. (2004). Original DSm measurements fromthis study are connected to their revised values by horizontal arrows. Notethat revised values fall closer to overall trend than do measured values.
Apollo 15 green C experiments 2411
hence lower values of E. The lattice-strain model, fortu-nately, provides a powerful tool for quantifying the extentof that incorporation and for correcting for it.
3.4.2. Correcting for melt incorporation in garnet SIMS
analysesTo assess the possible effects of melt incorporation in the
SIMS analyses of experimental garnets, we refit our D
values from each of the four experiments cited above toEq. (1), but omitted DSm and DNd from the fits. In otherwords, the refit parabolae were constrained by D valuesfor Sc, Y, Yb, and Dy. Because these four elements spana sufficient range of ionic radius, and in particular providepoints on both limbs of the curves (i.e., at ionic radii bothhigher and lower than the likely range of D0 values), amathematically robust fit still results. The refit curves aredocumented in the lower portion of Table 4, where thenew fit parameters are listed along with their calculateduncertainties. Fig. 5 also includes curves (light grey) gener-ated from these refits. Note that the refit curves account forthe D values for Sc, Yb, Y, and Dy at least as well as, if notbetter than, the original fits, as judged by the r2 values thatare in most cases higher than those from the original fits.Table 4 also includes values of DSm and DNd calculatedusing Eq. (1) from the refit parameters as outlined in thepreceding section. For runs A140, A228, and A229 the re-vised values are significantly lower than the original mea-surements, whereas the difference for A67 is much less(recall that A67 had substantially larger garnets than didthe other experiments). The revised DSm and DNd valuesare also plotted in Fig. 5. Finally, note that the effect ofminor melt incorporation on the four elements used inthe refits is expected to be very small, given that those ele-ments are either compatible in garnet or only slightlyincompatible. In other words, it takes only a very smallamount of melt incorporation to compromise D valuesfor highly incompatible elements, but that same amountof incorporation has a comparatively trivial effect onmore-compatible element Ds because melt contains com-paratively less of the compatible elements, by definition.
Another check on the quality of our partitioning datais shown in Fig. 6. Here, we plot DYb vs. DSm for gar-nets from this work and many previously published stud-ies (Nicholls and Harris, 1980; Kato et al., 1988; Ohtaniet al., 1989; Salters and Longhi, 1999; van Westrenenet al., 1999, 2001a; Barth et al., 2002; Salters et al.,2002; Draper et al., 2003; Corgne and Wood, 2004; Per-termann et al., 2004). Overall, a linear relationship isexpected on this log–log diagram, and this is broadlytrue of many, but not all, of the data shown in Fig. 6.D values for these elements as measured in the fourexperiments mentioned above are connected to their re-vised values with horizontal arrows; that is, DYb remainsessentially unchanged while DSm decreases as a result ofthe refit procedure outlined above. Although the as-mea-sured values were not greatly displaced from the maintrend, their revised positions are significantly closer to
it. This situation lends additional confidence that therevisions to our DSm and DNd values are reasonable.The downward revisions of these two values may seemlarge at first glance, being a factor of two to ten smaller;however, as we show below, these changes do not affectthe conclusions drawn from use of these data in petroge-netic models of low-Ti lunar liquid genesis.
How much melt was incorporated in these ‘‘suspect’’garnet analyses? We can assess this question in aqualitative sense using the batch-melting equation,CL/C0 = 1/(F + D � FD), where CL is the element con-centration in the liquid, C0 is the starting compositionfor that element, F is the melt fraction of an experi-ment, and D is the bulk partition coefficient. If we as-sume that C0 for Sm and Nd = 25.3 and 17.6 ppm,respectively (Table 1), we can calculate how CL/C0
would differ when using the as-measured D values, asopposed to the corrected values. Using run A229 asan example (because it consists of only garnet and li-quid; we did not measure D values in pyroxene so can-not calculate a bulk D for runs containing bothpyroxene and garnet), with F = 0.89 (by mass balance,Table 2), we find that for Sm, CL/C0 = 1.098 if the as-measured DSm is used, and 1.118 if the corrected valueis used. For Nd, the corresponding CL/C0 = 1.110 foras-measured DNd and 1.122 for the corrected DNd.These differences correspond to the quenched melt inthis experiment having 27.8 ppm Sm and 19.5 ppmNd using the as-measured D values compared to
2412 D.S. Draper et al. 70 (2006) 2400–2416
28.3 ppm Sm and 19.7 ppm Nd for the corrected D
values. These slight changes, well below the analyticaluncertainties for these elements cited in Table 1, sug-gest that the amount of melt incorporation in garnetSIMS analyses was in fact very small.
Fits of our tetravalent data to Eq. (1) are unfortunatelynot feasible, because the range of ionic radii of the 4+ cat-ions present in our experiments (Ti, Hf, Zr, and Th) is notsufficiently large to yield a robust fit. The expected ideal sitesize r0 for the garnet Y site is on the order of 0.7 A, and theionic radii of our 4+ cations are all approximately equal toor larger than this value. As a result, all data would fall onone limb of the parabola, up to its peak, but without a da-tum on the opposite limb, so no meaningful fit to Eq. (1) ispossible. There is a potential solution to this dilemma forgarnets having a significant majorite component. In sucha case, a D value for Si in octahedral coordination (VISi)could be calculated, and because the ionic radius of 6-coor-dinated Si4+ is 0.40 A (Shannon, 1976), a fit to Eq. (1)would be mathematically obtainable. However, it is notclear whether a putative D for VISi would use in its denom-inator the total amount of Si in the melt or only some frac-tion that could be shown to be in octahedral coordinationin the liquid phase. Such considerations are well beyondthe scope of this contribution, and future work will takedoped A15C to higher pressures in an effort to apply thelattice-strain model to tetravalent element partitioning bygrowing garnets having a significant majorite componentwhile evaluating the role of Si coordination in coexistingmelt.
In any case, we are now in a position to supply parti-tioning values for low-Ti lunar compositions for any triva-lent cation entering the garnet X site by using the D valuesmeasured and revised from use of Eq. (1). We can usedirectly the values we actually measured in our experi-ments, and can derive any unmeasured trivalent D valueby using the values of r0, D0, and E that result from fitsof our partitioning data to Eq. (1) and calculating D giventhat unmeasured element’s ionic radius. This study there-fore provides the first garnet-melt partitioning data fromcompositions directly relevant to the lunar interior, in con-trast to having to rely on D values taken from experimentson very different, ‘‘terrestrial’’ bulk compositions. In thefollowing sections, we apply these new D values to previ-ously published work in which the role of garnet was as-sessed in part by making use of garnet partitioning valuestaken from these earlier ‘‘terrestrial’’ studies.
4. Discussion: Garnet in low-Ti picrite glass source?
4.1. Model of Beard et al. (1998)
Beard et al. (1998) (‘‘B98’’ hereafter) measured Lu–Hfisotopic systematics for a range of mare basalts and showedthat low-Ti and high-Ti lunar magmas likely originatedfrom distinct source regions, with the source for low-Tibasalts having chondrite-normalized Lu/Hf approximately
four times that for high-Ti basalts. They argued that theseresults could best be explained by polybaric melting of aninitially garnet-bearing source, with garnet being consumedas the melting regime moves to shallower levels and intothe spinel stability field. Such a model generated liquidshaving the required Lu/Hf fractionations while having lastequilibrated with a residuum consisting dominantly of oliv-ine and orthopyroxene. However, that model was calculat-ed using garnet D values from the available literature at thetime, and as indicated above the experimental partitioningdata for garnet all came from studies performed on terres-trial compositions that are quite different from lunar ones(see Beard et al., 1998, for references). Specifically, the gar-net D values they used for Sm, Nd, Lu, and Hf were 0.25,0.15, 8.05, and 0.27, respectively, yielding DSm/DNd = 1.67and DLu/DHf = 29.8.
Here, we recalculate the B98 non-modal batch meltingmodel, which employed data on Sm, Nd, Lu, and Hf, usingD values obtained from this study. First, we simply recalcu-lated the B98 model 2A, which those authors suggested bestaccounted for their low-Ti mare basalt results and was set upwith 2% garnet in the source. In each set of calculations, weused both the as-measured D values from runs A67, A140,A228, and A229 and the corrected values for DSm and DNd
as outlined in the preceding section and listed in Table 4.Also, DLu was calculated from fit parameters to Eq. (1), bothwith and without as-measured DSm and DNd to generate thefits. For runs A67, A140, A228, and A229, respectively, theseDLu values are 2.14–2.38; 2.05–2.73; 1.27–1.67; and 0.86–1.24. We therefore have, for each of the four experimentsused, a pair of D values for each of the four elements of inter-est. The corresponding D ratios for these four runs rangefrom 1.2 to 6.3 (average 3.93) for DSm/DNd and 3.3 to 9.6(average 6.52) for DLu/DHf. It is immediately evident fromthese values, compared to those used in the original B98model, that calculated melting paths should differ substan-tially using our new partitioning data.
Recalculated model results are plotted in Fig. 7, whichconsists of modified versions of Fig. 10 of Beard et al.(1998) and focuses on the model they designated ‘‘2A,’’ thegarnet-bearing model (2% garnet in source) that appears tobest fit their measured data. The parameters d[Lu/Hf]n andd[Sm/Nd]n in Fig. 7 are ways of normalizing measured ratiosin a lava or melt to values for the lava’s source region calcu-lated from isotopic data, similar to the definition of Saltersand Hart (1989). d[Lu/Hf]n is defined as ð½Lu=Hf �source
n �½Lu=Hf �lava
n Þ=½Lu=Hf �sourcen Þ, and d[Sm/Nd]n is defined in
analogous fashion; see Beard et al. (1998) for full details.In Fig. 7A, their original model 2A curve is plotted in opengrey circles (‘‘Original model’’ in legend), up to 1% melting,by which point the garnet initially present is held to havebeen consumed. This curve reaches the high d[Lu/Hf]n endof the set of low-Ti mare basalt compositions plotted (greydiamonds), and subsequent melting to higher melt fractionsyields trends that closely mimic those data; that part of themelting curve is omitted for clarity. For comparison, thesame model curve is recalculated changing nothing but the
a
b
c
Fig. 7. Results of batch melting calculations similar to those summarizedin Fig. 10 of Beard et al. (1998). Grey diamonds are low-Ti lunar basalts.(a) Identical model to Beard et al. (1998) with 2% garnet in source, butusing the D values for Sm, Nd, Lu, and Hf from this study rather thanthose available previously from experiments on terrestrial compositions.Original model is Model 2A curve calculated by Beard et al. (1998). Ourcurves calculated using data from individual experiments (‘‘Ind. Exps.’’)A67, A140, A228, and A229 shown with both measured and revised valuesof DSm and DNd; heavy curve is result when using average of revised D
values. All curves depict 0–1% partial melting. (b) Model with sourcegarnet content increased in proportion to lower DLu/DHf ratio from our D
values compared to previous values. Curves are shown using corrected D
values used for individual experiments and heavy curve uses average ofrevised values; all curves depict 0–1% melting as in (a). (c) Models usingdifferent source modes in attempt to reach similar compositions as 1% meltin original Beard et al. (1998) Model 2A. Dots signify 1% melt increments.Only garnet-rich, clinopyroxene-poor or -absent sources reach the targetvalues. See text for discussion.
Apollo 15 green C experiments 2413
D values for Sm, Nd, Lu, and Hf as indicated above. Theseresults are shown in families of curves, one pair from thedata for each individual experiment from both the as-mea-sured and corrected D values (‘‘Ind. Exps.’’ in legend);those curves also terminate at 1% melting. Note that thepositions of curves calculated from each individual exper-iment shift only to a small extent by using the corrected D
values. This observation lends support to our inferencethat the magnitude of melt incorporation in our garnetSIMS analyses is minor; no change to model conclusionswould ensue from use of one or the other set of data.For the remainder of the discussion, then, we will focuson models calculated using the corrected values, given thatthey yield more realistic inferences about garnet crystalchemistry as mentioned above. Accordingly, we also plotmodel results in Fig. 7A calculated using the average ofthe corrected D values, shown as the heavy curve. All ofthe new curves intersect the set of low-Ti data at far lowervalues of d[Lu/Hf]n than in the original model, and subse-quent melting produces trends that intersect the field oflow-Ti data points only at the lowest values of d[Lu/Hf]n(also not shown for clarity). From these results, we con-clude that the source lithology used in the original B98model, with 2% garnet, cannot produce liquids havingthe Sm/Nd and Lu/Hf signatures measured by Beardet al. (1998) because the garnet that can actually be inequilibrium with A15C melt does not partition Sm, Nd,Lu, and Hf correctly.
Could the modeling be reconciled with our new D valuesif the amount of garnet in the source being melted is in-creased? One might suppose that increasing source garnetcontent in proportion to the change in DLu/DHf, which isa factor of 3.1–9.0 (average 4.6) lower using our new D val-ues than the value used by Beard et al. (1998) in their mod-els, might succeed in doing so. Fig. 7B examines thispossibility, again comparing the original B98 model in greycircles with melting trends calculated by scaling up the gar-net content of the source in proportion to those differingratios (i.e., garnet contents of 6–18%, average 9.2%), reduc-ing the modes of olivine and pyroxene in proportion totheir mode in the original model. Here, we plot curves onlyfor the corrected D values from each of the four experi-ments we have been using, again terminating at 1% melt-ing, along with a heavy curve calculated using averagecorrected D values. Clearly, these trends also do not comeclose to the 1% melt endpoint of the original model 2A, andin order to intersect the trend of low-Ti data points, melt-ing must proceed to 2–3%, higher than the value at which itwas assumed that garnet would be consumed. Also, in-creased degrees of melting yield curves that range to valuesof d[Sm/Nd]n much too low to match the low-Ti maretrend. In addition, contents of garnet in the deep lunarmantle as high as 6–18% do not seem realistic.
Finally, in Fig. 7C we calculate melting curves for avariety of arbitrarily chosen source lithologies to try toreproduce the 1% melt endpoint of the original model2A curve. In these four curves, we assume a source
2414 D.S. Draper et al. 70 (2006) 2400–2416
consisting of 50% olivine and 20% orthopyroxene, withthe remainder some combination of clinopyroxene andgarnet (assuming that only those two phases have thecapacity to impart fractionations in Sm, Nd, Lu, andHf); the four modes chosen are given in the legend.The closest matches to the target value come from sourc-es having �30% garnet that are either devoid of clinopy-roxene or have only a few percent of that mineral,melting to �6%. Clearly such lithologies are inconsistentwith the olivine- and orthopyroxene-dominated assem-blages thought to typify the lunar mantle.
4.2. Ramifications for possible role of garnet
There are essentially three possibilities regarding the roleof garnet in the formation of lunar liquids. Either garnetwas present in the source initially, and remained in the res-idue after melting; or it was present initially but was con-sumed during low-degree melting; or it was never presentin the source rocks at all. We can use our experimental re-sults to assess these three possibilities for very low-Ti lunarliquids in the following ways.
If garnet was in the source for A15C and was notconsumed upon melting, then we would expect to find,at the pressure–temperature conditions at which the li-quid was separated from its residue, that (a) garnetwas on the liquidus and (b) that it partitioned trace ele-ments in such a way as to yield reasonable matches tothe trace-element compositions of low-Ti lunar liquids.For the experiments reported here, the first of these con-ditions is met: there is a broad P–T range of near-liqui-dus garnet stability. However, as outlined above, thesecond condition is not met: these garnets do not havetrace element D values that can produce such a matchfor the trace element compositions of very low-Ti lunarbasalts or picrite glasses.
The expected experimental observations if garnet wasinitially present in the A15C source but was consumedupon melting are slightly more complicated. In such a case,we would neither expect nor require that trace-elementcontents of low-Ti lunar liquids match the patterns thatwould arise from partitioning imposed by garnet, becauseat melt separation, there would be no garnet surviving toimpose such signatures. Furthermore, at the conditions ofmelt extraction, we would expect NOT to find garnet asa liquidus phase, because by that stage, garnet would havebeen consumed and therefore not present to saturate the li-quid at that point in its petrogenesis. This is a fundamentalrequirement of the near-liquidus, inverse-approach experi-ments performed in this study. Again, the first of these con-ditions is met: there is no match between the trace elementpatterns in low-Ti lunar liquids and those that would beimposed by garnet that partitions trace elements as mea-sured here. However, the second condition is not met: wedo in fact observe liquidus garnet, and over a substantialrange of P–T conditions. It is likely, although we cannotprove it solely from the work documented here, that this
range of P–T conditions encompasses that at which garnetcould have been present in lunar source lithologies. Fur-thermore, we cannot rule out the possibility that garnetwas present in the source rocks at pressures between �2.0and 2.5 GPa, where we have no experimental coverage(Fig. 2), although we note that rather restricted conditionswould be required in order for this to be possible.
Finally, if garnet took no role in the formation of low-Tilunar liquids, either by having once been present in thesource or by persisting in the residue after melting, thenfinding near-liquidus garnet would be essentially irrelevant,and a result of simply taking a liquid of basaltic (sensulato) composition to high enough pressure to form garnet(which virtually every basalt will eventually do if the pres-sure is sufficiently high). Given that A15C displays a lower-pressure multiple saturation point (Elkins-Tanton et al.,2003) with a quite reasonable lunar mantle mineral assem-blage, we can infer (although we cannot prove) that ourexperiments are consistent with garnet having played norole whatever in the formation of A15C melt.
We therefore make the following conclusions from inter-pretation of our high-pressure experimental results onphase relations and element partitioning. Although garnetis the liquidus phase for A15C at high pressure, the pointof multiple saturation defined by our experiments requiresa residual lithology that is unlikely to be appropriate for lu-nar mantle, e.g., Hess (2000). Were olivine to occur alongwith garnet and pyroxene near the liquidus of A15C, amuch more convincing case for a garnet-bearing sourcecould be made. In addition, the �3 GPa liquidus tempera-ture of �1750 �C, although not incompatible with most lu-nar thermal models, is at the high end of the range of likelytemperatures for depths of 600–700 km at the time of peaklunar magmatism, e.g., models summarized in Basaltic Vol-canism Study Project (1981). Accordingly, we concludethat the multiple saturation point identified by Elkins-Tan-ton et al. (2003) remains the more appropriate, represent-ing at least the minimum depth of melting for thiscomposition. In principle, our multiple saturation pointcould then represent a maximum depth of melting. In addi-tion, the occurrence of single-phase pyroxenes, althoughstraightforward to understand thermochemically, is not inagreement with the clinopyroxene + orthopyroxene expect-ed in the lunar mantle. We therefore do not believe thehigh-pressure point of multiple saturation found for liquidA15C with garnet and pyroxene at �3 GPa represents theconditions at which, or source lithology in which, meltingoccurred to produce the most titanium-poor lunar glasses.
Taken together, considerations of both the phase rela-tions and element partitioning in the Apollo 15 green Ccomposition lead us to conclude also that garnet wasnot in the residue of melting that produced the A15C lu-nar picrite glass, nor, by extension, in residua for othervery low-Ti lunar liquids. Even though A15C is saturatedwith garnet at high pressure, the garnet that is in equilib-rium with such liquids simply does not partition Sm, Nd,Lu, and Hf in such a way as to explain low-Ti lunar
Apollo 15 green C experiments 2415
compositions. In addition, REE patterns for Apollo 15low-Ti picrite glasses are generally flat to slightly LREEdepleted (Steele et al., 1992; Shearer and Papike, 1993;Shearer et al., 2003), without the light REE enrichmentsthat would be produced by residual garnet of any type,both ‘‘terrestrial’’ (using D values from previous litera-ture) and lunar (D values from this study). By elimina-tion, then, and applying the inferences outlined above,our experimental results are consistent with garnet neverhaving been present in the source rocks of this lunar li-quid. This conclusion is, however, at odds with the obser-vation by Beard et al. (1998) that a Lu-retentive phaseplayed a role in the formation of the source regions forlow-Ti lunar compositions. We could speculate that thefeatures identified in the isotopic measurements of Beardet al. (1998) were imparted on the low-Ti source regionsthrough early magma ocean crystallization processes orresult from an initial lunar composition that is not chon-dritic with respect to Lu and Hf; evaluating such possibil-ities is beyond the scope of this contribution.
5. Summary and conclusions
The Apollo 15 green C picritic glass is saturated withgarnet and single-phase pyroxene between 3.0 and3.5 GPa (600–700 km lunar depth) and a temperatureof �1750 �C. This olivine-free assemblage is not likelyto represent the lithology of the lunar mantle. However,the coexistence of garnet with A15C liquid allows us tomeasure partitioning values for important trace elements,and thus assess the geochemical signatures that shouldarise if A15C were in fact derived from a garnet-bearingsource. The results of these calculations do not supportthe notion that garnet was present in the residues ofmelting that produced very low-Ti lunar liquids likeA15C. We infer, furthermore, that garnet probably wasnot involved at any stage of the formation of thesemelts.
Acknowledgments
We are very grateful to Dr. Linda Elkins-Tanton forgenerously providing us with the A15C powder from herearlier study. D.S.D. thanks Dr. A. Dana Johnston forhis hospitality while visiting the University of Oregonexperimental petrology laboratory and Dr. Lars Borg forhelpful lunar and isotopic discussions. We thank Mr. PaulBurger for constructing the trace element solution used inthese and other experiments. Drs. Linda Elkins-Tanton,John Jones, and John Longhi provided very useful journalreviews. This research was supported by National ScienceFoundation Grant EAR-0337237 to D.S.D. and byNational Aeronautics and Space Administration Cosmo-chemistry Grant 133388 to C.B. Agee.
Associated editor: Clive R. Neal
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