Precambrian Research 113 (2002) 323–340

Partial melting in Archean subduction zones: constraintsfrom experimentally determined trace element partition

coefficients between eclogitic minerals and tonalitic meltsunder upper mantle conditions

Matthias G. Barth a,*, Stephen F. Foley b, Ingo Horn c,1

a Department of Earth and Planetary Sciences, Har�ard Uni�ersity, 20 Oxford Street, Cambridge, MA 02138, USAb Institut fur Geologische Wissenschaften, Uni�ersitat Greifswald, F.-L.-Jahnstrasse 17a, 14787 Greifswald, Germany

c Department of Earth Sciences, Memorial Uni�ersity of Newfoundland, St. John’s, NF AIB 3X5, Canada

Accepted 3 September 2001

Abstract

The partitioning of an extensive suite of trace elements between garnet, clinopyroxene, and hydrous tonalitic meltshas been studied experimentally at 1.8 GPa and 1000–1040 °C. The knowledge of trace element partitioning betweenminerals and coexisting tonalitic melts is essential for geochemical models in order to explain the genesis of Archeantonalite-trondhjemite-granodiorite suites (TTGs). Our experiments were performed on a natural tonalite doped withtwo different trace element mixtures and with TiO2 to stabilize titanates. Trace element concentrations of crystals andcoexisting melts were measured by laser ablation microprobe. The partition coefficients (D ’s) are independent of thetrace element concentration over the concentration ranges used. The partition coefficients reported in this paper area considerable extension of the few available data sets in intermediate to silicic systems. The D-values mostly agreewell with previously published data and with predictive models. The garnet partition coefficients show steeper patternswith higher D ’s for heavy REE, lower D ’s for highly incompatible elements and lower La/Yb-ratios than previouslypublished data for basaltic systems, whereas clinopyroxene partition coefficients show similar patterns but highervalues than for basaltic systems. A set of calculated partition coefficients based on best-fit parabolas was used tomodel trace element fractionation in melts in equilibrium with rutile-bearing eclogitic residues. The calculated meltsreproduce the overall trace element patterns of Archean TTGs, i.e. the melts show strongly fractionated REEpatterns, depletions in Nb and heavy REE, and high Zr and Hf concentrations relative to Sm on normalized plots.The calculated residues have depleted trace element patterns with positive Nb anomalies and flat heavy rare earthelement patterns similar to rutile-bearing xenolithic eclogites reported from Siberia and West Africa. Restites fromeclogite melting are complementary to Archean TTGs and may be an important reservoir of Nb and Ta. © 2002Elsevier Science B.V. All rights reserved.

Keywords: Eclogite; Tonalite; Trace element partitioning; Garnet; Clinopyroxene

www.elsevier.com/locate/precamres

* Corresponding author. Present address: Petrology Group, Faculty of Earth Sciences, University of Utrecht, Budapestlaan 4,3584 CD Utrecht, The Netherlands. Tel.: +31-30-253-5063; fax: +31-30-253-5030.

E-mail address: barth@post.harvard.edu (M.G. Barth).1 present address: Laboratory for Inorganic Chemistry, ETH Zurich, Universitatsstrasse 6, CH-8092 Zurich, Switzerland

0301-9268/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.

PII: S 0301 -9268 (01 )00216 -9

M.G. Barth et al. / Precambrian Research 113 (2002) 323–340324

1. Introduction

Tonalites, trondhjemites, and granodiorites(TTG) are widespread in the Archean crust andare considered to be products of partial melting ofbasic protoliths. The trace element concentrationsof typical TTG exhibit enrichments in incompat-ible elements and negative Nb–Ta anomalies. Therare earth element (REE) patterns are stronglyfractionated, with low heavy REE contents andno significant Eu anomaly (Martin, 1994).Whereas several recent experimental studies havedemonstrated that partial melting of basalts andamphibolites result in tonalitic and trondhjemiticliquids (Beard and Lofgren, 1991; Rushmer, 1991;Winther and Newton, 1991; Sen and Dunn, 1994;Wolf and Wyllie, 1994; Rapp and Watson, 1995),typical TTG are only obtained when garnet is aresidual phase and for pressures �1.6 GPa(Drummond and Defant, 1990; Martin, 1994).Studies of trace elements in TTGs suggest thatresidual titanates may play a crucial role to ex-plain the negative Nb–Ta anomalies (Foley andWheller, 1990). The knowledge of trace elementpartitioning between minerals and coexistingmelts and fluids, respectively, is essential for pet-rogenetic and geochemical models that investigatethe genesis and the source of TTG. In spite of thesignificance and the widespread occurrence ofTTG, experimental studies on the trace elementbehavior in tonalitic systems are rare (Klein et al.,1997, 2000). Basaltic systems in general are muchbetter studied than intermediate and silicic sys-tems. Clinopyroxene is already studied well(Green, 1994; Gaetani and Grove, 1995; Woodand Blundy, 1997), but the data sets for garnetand titanates are still full of gaps (Green, 1994).Therefore, there is a need for precise and system-atic experimental determined partition coefficients(D ’s).

In earlier studies it was necessary to add traceelements at the percent level to the starting mate-rials on order to measure the trace element con-centrations of experimentally producedmineral/melt pairs in situ by electron microprobe(EMP). Due to great improvements of the mi-crobeam techniques we now can measure traceelements at or much closer to natural levels (e.g.

Klein et al., 2000 used ion microprobe to measurethe trace element contents of their experimentalcharges.) where the partition coefficients are inde-pendent of the trace element concentration (Hen-ry’s Law; Watson, 1985; Beattie, 1993).

In this paper, we report an extensive data set ofexperimentally determined partition coefficientsbetween garnet, clinopyroxene (cpx), and coexist-ing hydrous (water-undersaturated) tonalitic meltsunder upper mantle conditions, forming an exten-sion of the study of Jenner et al. (1993). Thisstudy includes many more trace elements thananalyzed by Jenner et al. (1993), reflecting theimprovements in the LAM technique over the lastyears. Rutile/melt partition coefficients are pre-sented in Foley et al. (2000a). We present aninternally consistent set of partition coefficientsfor trivalent cations and model trace element frac-tionation during eclogite melting in Archean sub-duction zones.

2. Experimental and analytical methods

All experiments were run on a natural tonalitefrom Lalkaldarno, Victoria, Australia, as previ-ously reported in Jenner et al. (1993) and Foley etal. (2000a). To optimize trace element concentra-tions for electron microprobe (EMP) and laserablation microprobe (LAM) analyses, trace ele-ments were added to the tonalite. We chose sev-eral transition elements, high field strengthelements (HFSE), large ion lithophile elements(LILE) and light, medium and heavy REE thatallowed us to define typical trace element patternsand possible anomalies (e.g. Eu- and Nb-anoma-lies). The trace elements were divided into twogroups to avoid interferences with other elementsduring EMP and LAM analysis and to limit thetotal doped concentration of trace elements. Dop-ing mix 1 contained about 2000 ppm each of Y,Ba, La, Nd, Sm, Dy, Er, and Hf; and doping mix2 about 2000 ppm of each of V, Rb, Nb, Eu, Gd,Yb, Lu, and Pb. To ensure saturation in TiO2 andto stabilize rutiles large enough to analyze, thetonalite mixtures used in the experiments containadditional 5 wt.% TiO2. The starting materialscontain some trace elements at doped levels and

M.G. Barth et al. / Precambrian Research 113 (2002) 323–340 325

others at natural levels (Table 1). None of thetrace elements is doped in both mixtures; there-fore, we could analyze trace elements at bothnatural and doped concentrations and couldconfirm that Henry’s Law is fulfilled in all cases.

All trace elements were added as oxides, exceptBa (carbonate) and Rb (chloride). Preliminarymixtures containing 1 wt.% of each trace elementwere prepared by mixing the elements and thepowdered tonalite under acetone in an agate mor-tar. The mixtures were fused at 1250 °C (wellabove the liquidus), quenched to glass andground. Final mixtures DM 1 and DM 2 contain-ing 2000 ppm of each element were obtained bymixing preliminary trace element enriched mix-tures with powdered tonalite and TiO2, and thenfusing and quenching to a glass. Both startingmaterials were ground and sieved under ethanolto ensure a small and uniform grain size (�20�m). The addition of the trace elements and TiO2

had only a minor influence on the composition(Table 1); namely, the major element concentra-tions were lowered slightly due to dilution by theextra TiO2. The TiO2 concentration in the melt isbuffered by the TiO2 saturation curve in silicatemelts (Green and Pearson, 1986; Ryerson andWatson, 1987) and does not exceed 1.6 wt.%(Table 2). The powdered starting materials weredried and then stored in a desiccator.

For precise measurements by LAM, the ana-lyzed crystals must have a minimum size of �20�m, therefore, crystal growth conditions duringthe runs were important. The experiments wereheld well above the liquidus (�1200 °C) for 30min. Then the temperature was decreased (10 to15 °C/min) to the desired value.

Our experiments were performed at the Miner-alogisch-Petrologisches Institut, University ofGottingen, Germany. A single-stage 22 mm pis-ton-cylinder apparatus with a graphite furnaceand a CaF2 assembly was used for the experi-ments at 1.8 GPa. This assembly requires nopressure correction and pressures are believed tobe accurate to within 0.05 GPa (calibrated on thereaction albite= jadeite+quartz). Temperatureswere measured and controlled with a Eurotherm812 controller, using Pt/Pt90Rh10 thermocouples,and are accurate to within 15 °C (Klemme et al.,1995).

Graphite-lined Pt capsules were used to preventFe loss to the Pt capsule. The CaF2+BN+Cassembly resulted in very reduced conditions closeto the iron-wustite buffer (Foley et al., 2000a). 2

Table 1Major and trace element compositions of the tonalite and thestarting materials DM 1 and DM 2

DM 1 DM 2Wt.% Tonalite

60.8 60.8SiO2 65.4TiO2 5.485.480.52

15.6 15.616.8Al2O3

3.033.26 3.03FeO3.37MgO 3.13 3.134.48CaO 4.17 4.175.43Na2O 5.05 5.05

0.55K2O 0.550.590.11 0.10 0.10P2O5

Sum 97.9699.96 97.96

ppm430Mn 400 400

87.2Cr 81 81200073V 79

14.8Sc 14 1480Zr 74 74

3Nb 2.8 20000.38Ta 0.35 0.358Y 2000 7.4

2.32000Hf 2.51.5Th 1.4 1.4

12Rb 11 2000438Sr 407 407112Ba 2100 104

0.31Cs 0.29 0.29n.a.Pb n.a. 20007.8La 2000 7.3

Ce 15.1 14.0 14.09.03Nd 2000 8.4

Sm 2.1 2000 2.00.68 2000Eu 0.63

20002.1Gd 2.22.02Dy 2000 1.90.41Ho 0.38 0.38n.a.Er 2000 n.a.1.04Yb 0.97 20000.157 2000Lu 0.15

(n.a.=not analyzed). Analysis of undoped tonalite of majorelements, Y, Nb, and V by XRF (analyst P. Robinson, Uni-versity of Tasmania), trace elements by INAA (analyst B.Spettel, Max-Planck-Insitute, Mainz). Glasses DM 1 and DM2 analyzed by LAM-ICP-MS.

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Table 2Major element compositions of the experimental run products

Al2O3 (1�) FeO* (1�) MnO(1�) MgO (1�) CaO (1�) Na2O (1�) � TE (1�)Temperature [°C] sumn SiO2 (1�) TiO2 (1�)

melt17.8 (0.6) 2.05 (0.19) 0.02 (0.04) 1.38 (0.11) 3.17 (0.54)DM 3 4.40 (0.61)5 1.4 (0.3) 91.11000 69.0 (0.5) 1.39 (0.46)15.9 (0.3) 1.48 (0.05) b.d. 0.93 (0.04) 2.52 (0.35) 4.00 (0.58) 1.8 (0.2)1000 94.3DM 4 4 73.5 (0.9) 0.97 (0.06)17.2 (0.3) 2.30 (0.10) b.d. 1.70 (0.08) 3.95 (0.67) 4.35 (0.49)1.55 (0.04) 1.6 (0.4)68.0 (1.1) 93.61040 DM 3 517.5 (0.4) 2.19 (0.09) 0.04 (0.02) 1.61 (0.03) 3.65 (0.28) 4.84 (0.23) 1.5 (0.2) 95.41040 DM 4 5 68.1 (1.0) 1.43 (0.02)

garnet20.5 (0.1) 12.3 (0.3) 0.32 (0.04) 13.3 (0.2) 6.07 (0.28)DM 3 0.29 (0.03)5 5.8 (1.3) 96.91000 37.4 (0.2) 0.99 (0.02)

1.54 (0.08)1000 20.3 (0.3) 13.4 (0.3) 0.26 (0.04) 12.7 (0.5) 5.68 (0.84) 0.31 (0.02) 5.1 (1.1) 94.6DM 4 3 35.4 (0.1)21.0 (0.3) 12.0 (0.2) 0.31 (0.03) 14.9 (0.2) 5.75 (0.65) 0.19 (0.02)1.22 (0.15) 4.0 (1.4) 97.438.1 (0.3)7DM 3104020.5 (1.0) 11.8 (0.6) 0.30 (0.08) 14.5 (0.8) 5.56 (0.31) 0.23 (0.06) 4.6 (1.1) 96.51040 DM 4 10 37.6 (1.8) 1.33 (0.28)

clinopyroxene7.34 (0.01) 4.96 (0.04) 0.13 (0.01) 14.2 (0.1) 15.9 (0.3)DM 3 1.88 (0.06)5 1.0 (0.3) 97.71000 50.9 (0.1) 1.47 (0.07)

1.67 (0.06)1000 8.02 (0.26) 5.77 (0.25) 0.07 (0.03) 13.5 (0.2) 15.3 (0.2) 2.14 (0.06) 1.3 (0.2) 98.4DM 4 3 50.7 (0.1)9.60 (1.62) 4.89 (0.08) 0.11 (0.05) 13.7 (0.9) 14.5 (0.4) 2.08 (0.03)2.21 (0.09) 0.7 (0.3)49.9 (0.5) 97.61040 DM 3 3

10.3 (1.43) 5.83 (0.13)1040 0.15 (0.02) 17.5 (0.5) 8.43 (0.28) 1.70 (0.05) 1.5 (0.4) 98.4DM 4 5 49.1 (0.2) 4.01 (0.08)

All runs at 1.8 GPa, 2 wt.% water added. TE = sum of trace element oxides. n=number of separate analyses. All melt analyses recalculated on a water-free basis, the sum reflects the hydrous analytical microprobe total.b.d.=below detection limit. *FeO, total iron as FeO. Based on the correct stochiometry of the analyzed minerals, low totals probably were caused by low beam currents during electron microprobe analysis.

M.G. Barth et al. / Precambrian Research 113 (2002) 323–340 327

wt.% water was added as Mg(OH)2 to increase thestability field of garnet (Green, 1982), and toincrease the diffusion rate in the melt (Long,1978). The runs reported here were conducted at1.8 GPa and 1000–1040 °C; run times were 22 hat 1000 °C and 31 h at 1040 °C. After the exper-iments the samples were mounted in epoxy resinand polished.

2.1. Analytical techniques

The major element compositions of crystallinephases and coexisting glass were determined usingthe Jeol JXA-8600 wavelength-dispersive electronmicroprobe (EMP) at the University of Utrecht,The Netherlands. An accelerating potential of 15kV and a beam current of 10 nA was used.

Run products were analyzed for minor andtrace elements by laser ablation microprobe-in-ductively coupled plasma-mass spectrometry(LAM-ICP-MS) at Memorial University, St.John’s, Newfoundland. The ablation is achievedby a Q-switched Nd:YAG laser frequency-quadrupled to a wavelength of 266 nm. Thisenabled us to reduce the ablation pit size to 8-10�m in diameter in the silicate phases. Analyseswere made with a repetition rate of 10 Hz. Thepulse energy was controlled through the polarizer,resulting in pulse energies between 0.006 and 0.3mJ. The ablated material was swept by a continu-ous argon flow into a Fisons PlasmaQuad II+Sinductively coupled plasma-mass spectrometer.The amount of material ablated in LAM samplingis different for each analysis. Consequently, thedetection limits are different for each analysis andare calculated for each individual acquisition(Longerich et al., 1996). Detection limits of about0.5 ppm were achieved with small pit sizes (8–10�m); larger pit sizes (up to 30 �m in glass analy-ses) resulted in considerably lower detection limits(�0.05 ppm). The background is measured priorto each individual analysis. Thus, contaminationbetween samples of different concentrations anddrift in the background level can be excluded. 27Alwas used as internal standard. For our analyseswe calibrated against the spiked silicate glass ref-erence materials NIST 610 and NIST 612, andagainst the AGV-1 standard.

Fig. 1. Quantitative variations in weight percent across a 60�m garnet crystal from sample 31. Distance between spots isabout 10 �m.

Major and trace elements can be analyzedsimultaneously by LAM, which enables easy iden-tification of the phase being ablated on a second-by-second timescale. Thus, sampling integrity isensured; phase boundaries and inclusions can berecognized (Jenner et al., 1993), an importantadvantage over other microbeam techniques. Fur-thermore, ablation of progressive layers allowsanalysis of small crystals and zoning profiles onthe 5–20 �m scale.

3. Results

The experimental products consist of melt,clinopyroxene (from a few �m up to 60–30 �m),garnet (10–30 �m, max. 60 �m), orthopyroxene(up to 50–15 �m), and rutile (5-15 �m, rarely upto 30 �m) at 1040 °C. At 1000 °C run productsconsist of melt, cpx (up to 50–30 �m), garnet (upto 25 �m), Ti–silicates (max. 20 �m), rutile, andplagioclase. Larger garnet crystals contain manymelt and rutile inclusions, whereas smaller crystalsmostly lack inclusions. All garnet data presentedare for inclusion-free ablation pits. Major elementcompositions for glass and crystals are given inTable 2. None of the analyzed phases show sig-nificant major or minor element zonation (Fig. 1).Glass is homogeneous with respect to Al2O3, theelement used as internal standard (RSD 1.9 to3.6%, Table 2), allowing accurate quantification

M.G. Barth et al. / Precambrian Research 113 (2002) 323–340328

of the trace element contents by LAM-ICP-MS. Onthe other hand, irregular distribution of tiny ti-tanate and quench crystals caused apparent compo-sitional variations for CaO in some of the glassmatrices (e.g. 16% RSD in sample c33); the spotsize for EMP and LAM-ICP-MS (�30 �m) wastoo small to reproduce the overall melt compositionin such cases. However, CaO concentrations mea-sured by EMP and LAM-ICP-MS agree withinerror (e.g. 3.65�0.28 wt.% by EMP vs. 3.67�0.36wt.% by LAM in sample c31). Furthermore, nosystematic compositional gradients in the experi-mental charges were observed.

The following lines of evidence demonstrateattainment of equilibrium during the experiments:1. The phase assemblages are at or near equi-

librium (Barth, 1996); that is, the liquidus phasesare garnet and cpx analogous to previous studiesin systems with similar bulk compositions (e.g.Green, 1982; Carroll and Wyllie, 1990)

2. The run durations were long enough to producea homogeneous glass and euhedral to subhedralcrystals that exhibit no or only slight zoning inmajor and minor elements (Fig. 1), suggestinga balance between crystal growth rate and ionicdiffusion rate.

3. Partition coefficients from both tonalite mix-tures, which have several orders of magnitudedifference in trace element contents of the dopedelements, show similar values (Fig. 2).

The addition of TiO2 to the starting materialresulted in �4% rutile in the run products, takingmuch of the Nb (and Ta) from the melt, andpotentially causing non-equilibrium (unexpectedlyhigh) partition coefficients for Nb for the garnet/melt and clinopyroxene/melt pairs. That is, a traceelement enriched accessory mineral is capable ofsequestering the particular trace elements it favors,leaving the melt with lower contents of these traceelements. This may be lower than the true equi-librium content of the melt that gave rise to thesilicate phases accommodating very small amountsof the same trace element. No results were obtainedwithout rutile being present but the low DNb

(0.0016–0.0045 and 0.0029–0.017 for garnet andcpx, respectively,) suggest that sequestering of Nbby rutile did not cause non-equilibrium partitioningin this study.

3.1. Garnet

Major element compositions are presented inTable 2. Garnet compositions range fromPy58Gr16Alm26 at 1040 °C to Py53Gr17Alm30 at1000 °C.

The trace element concentrations and partitioncoefficients for garnet are listed in Tables 3 and 4,respectively, and partition coefficients are shown inFig. 2a. The pattern of the partition coefficients isvery steep for garnet, with LILE being highlyincompatible (DBa=0.00041) and heavy REE hav-ing very high D-values (DLu=9.0–21.6). The rela-tive fractionation of the heavy REE is low; DYb/DEr

is always �2. DSr is much lower than DNd, sogarnet fractionates the Sr/Nd ratio.

Fig. 2. Partition coefficients of garnet (A) and clinopyroxene(B) at 1.8 GPa. Data from Klein et al. (2000) at 1050 °C and1.5 GPa; Jenner et al. (1993) at 3.5 GPa and 1000 °C. Errorbars (shown if bigger than symbols) are 1 �.

M.G. Barth et al. / Precambrian Research 113 (2002) 323–340 329

Sr and Yb reveal noticeable differences betweenthe two mixtures. Although Sr was not added toeither of the mixtures, the DSr of the DM 2-sam-ples vary around 0.03, whereas DSr of the DM1-samples are always lower (�0.01). DYb shows anegative anomaly (DYb�1

2(DEr+DLu)) in the DM1-samples (doped with Er). This phenomenon isnot observed in the DM 2-samples (doped withYb and Lu) and is likely to be an analyticalproblem, possibly an uncorrected interference.Therefore, we consider DYb of the DM 2-samplesto be more reliable.

The D ’s of the highly incompatible elements inthis study are similar to the D ’s published in thecompilation of Green (1994) and run 18(Py56Gr13Alm31) of van Westrenen et al. (2000)but lower than the data presented in Klein et al.(2000) (Fig. 2a). The D ’s of the compatible ele-ments, however, are mostly higher, resulting in asteeper pattern. Nicholls and Harris (1980) deter-mined partly higher DYb (10.0–44.6) in andesiticsystems (940–1420 °C, 2–3 GPa) by electron mi-croprobe. The data of Jenner et al. (1993) fortonalite, measured with LAM, are well within therange of the D ’s of this study for Sr, but are lowerfor Zr and Y and higher for Ce. Note that DTa ofJenner et al. (1993) is much higher (0.22) thanDNb in this study (0.0016–0.0045). This may indi-cate that garnet fractionates the Nb/Ta ratio aspreviously reported in basaltic systems (Green etal., 1989; Jenner et al., 1993). DZr appears to behigher than DHf, in agreement with Hauri et al.(1994) and Fujinawa and Green’s (1997) data, butlarge standard deviations mean that it is poorlyconstrained. Kelemen et al. (1993) report DNb thatare an order of magnitude higher than DLa from astudy of mantle xenoliths measured by ion mi-croprobe. In this study DNb approximately equalsDLa. Although we could not determine DNb andDLa in the same sample, this result is in accordwith Rocholl et al. (1996), Adam et al. (unpub-lished data, plotted in Green, 1994, Fig. 4), andvan Westrenen et al. (2000).

3.2. Clinopyroxene

Major element compositions are presented inTable 2. Clinopyroxenes contain significant

jadeite- and Tschermak’s-components (12–15mol% and 9–16 mol%, respectively).

The trace element concentrations and partitioncoefficients for clinopyroxene (cpx) are listed inTables 3 and 5, respectively, and partition coeffi-cients are shown in Fig. 2b. The cpx partitioncoefficients show a shallower slope than theDgarnet (Fig. 2). The samples analyzed consist oflarge garnet crystals but only small cpx crystals,necessitating the use of smaller ablation pits forcpx analyses. This results in poorer countingstatistics and detection limits, causing larger scat-ter in cpx trace element data than in the garnetdata. The negative DYb anomalies appear againonly in the DM 1-samples.

The D ’s in this study are similar to the D ’spublished in Klein et al. (2000) in tonalitic sys-tems (Fig. 2b) and are in the upper part of therange presented in the compilation of Green(1994). The partition coefficients for heavy REEin this study are greater than unity (including Gd,Dy, and Ho for most runs) and are, therefore,higher than in basaltic systems. The DSm-valuesmatch the electron microprobe data of Nichollsand Harris (1980) in an andesitic system(1000 °C, 1.5 GPa, 10 wt.% H2O), but DLa islower at all temperatures and DYb is alwayshigher, resulting in a steeper D-pattern. DHf ishigher than DZr, which is characteristic for cpx(Fujinawa and Green, 1997).

3.3. Clinopyroxene/garnet partitioning

Fig. 3 shows the relative trace element abun-dances in coexisting clinopyroxene and garnetfrom xenolithic eclogites (Harte and Kirkley,1997; Jacob and Foley, 1999) and the presentstudy. Although the two-mineral D values of thexenolithic eclogites are relevant for subsolidusconditions and cannot be considered totallyanalogous to D cpx/garnet inferred from experimen-tally determined partition coefficients (Green,1995), our high-pressure experiments resulted invalues within the range of Harte and Kirkley,(1997) values (except for Sr and Ti, Fig. 3). How-ever, our experimental data show flatter patternsthan the eclogites. Bearing in mind that the tem-perature range of Harte and Kirkley’s eclogite

M.G. Barth et al. / Precambrian Research 113 (2002) 323–340330

Table 3Trace element contents of the experimental run products

Melt (4) S.D. Garnet (10) S.D.

1040 °C DM 3 No. 3329.2Rb 3.2 9.13 0.90

180 0.84 –2050Ba1.75Th 0.208.11Nb 0.76

160 6.8 2.41650La14.6Ce 1.4 0.33 0.03

3.2 0.81 0.18Pb 16.118 3.0 1.1408Sr

1700Nd 180 190 551950Sm 180 1190 280

6.4 49 1573.0Zr1530Hf 130 536 133

1.2 1.99 0.32Eu 15.51.9 28.7 7.714.4Gd

1600Dy 260 8590 647300 10600 890Y 1450360 12900 13501370Er

17.5Yb 2.8 136 183.3 125 18Lu 8.84.8 52.5 8.713Sc

51.0V 7.0 168 26S.D. Garnet (4) S.D. cpx (3) S.D.Melt (5)

1040 °C DM 4 No. 31270 16.7 – 43.0 –Rb 2150

5110 7.95Ba –0.66Th 2.45 2.00 –

510 2.87 –640 10.6Nb –24.3La 0.9 �0.44 – 2.13 –

0.8Ce 0.3314.5 0.11 �1.2 –373 16 171260 50Pb 41

436Sr 11.8 13.6 3.1 34.8 10.02.0 4.8 1.6Nd 9.6424.4 2.041.7 12 421.2 17.9Sm 6.0

73.9Zr 8.5 38.2 6.6 16.9 3.73.9 9.00 1.67 10.2 2.1Hf 19.3

76 248 292120 430Eu 170150 3500 580Gd 16801950 390

2.72 87.2 10.120.2 17.7Dy 4.90.50Ho 0.19 3.7 3.6 0.73 –

4.2 168 328.2 32.9Y 9.527.8Er 6.9 207 53 44.6 11.7

Yb 1870 410 16600 4300 2460 370430 16700 47001850 2410Lu 350

Sc490 2380V 600660 4240 180

S.D. Garnet (4) S.D.Melt (4) Cpx (2) S.D.

M.G. Barth et al. / Precambrian Research 113 (2002) 323–340 331

Table 3 (Continued)

S.D. Garnet (10) S.D.Melt (4)

1000 °C DM 3 No. 3529.9 �19 –Rb 5.5

– 65.6 7.7�1.52130Ba 1401.53 0.24Th4.9 �3.4 -Nb 1.5

– 217 377.211410 160La�0.8011.4 – �2.5 –1.3Ce

– �5.1 –Pb 17.1 3.0 �4.4– 49.4 3.4�5.0446Sr 17

2701330 130 828 86160Nd450 1770Sm 3901520 150 1520

10.9 18.9 2.743.810.4Zr 67.278 639 73Hf 1400 160 5400.68 7.25 –4.35Eu 1.215.5

31.013.7 6.1 20.4 –2.1Gd1200 2720 260112001490Dy 150

2.84 0.35Ho155001510 1000 2770 270130Y

2100 2880 140209001480 180Er23518.5 25 36.4 –0.6Yb

22 22.9 2.1Lu 11.4 0.7 245Sc

14324 29 185 278VS.D. Garnet (3) S.D. Cpx (5) S.D.Melt (5)

1000 °C DM 4 No. 36–Rb �172370 –440 �11

12 0.58 – �1.2 –122Ba–Th �1.62.34 –0.21 �0.90– 3.90 –2.11340Nb 180

�0.5223.8 – 1.32 –4.2La–Ce �0.7114.2 –2.2 �0.762.3 13.1 2.510.22460 760Pb

12.9427 0.7 31.3 6.236Sr0.28 10.3 2.2Nd 23.5 3.2 3.082 11.9 5.21420.0Sm 1.3

4877.4 2 19.4 6.69.2Zr1.98 8.81Hf 1.5222.1 2.5 8.53

71 490 896892100Eu 18049801940 600 1910 29096Gd

15 27.3 7.3Dy 19.5 0.7 1210.202.870.32Ho 0.08

20323.3 9 32.5 7.24.7Y20 40.1 11.2Er 27.6 2.0 246

110 2660 240187001870 95Yb194001850 160 2420 240110Lu

Sc2970 410V 2601400

Trace element concentrations are averages, numbers in parentheses represent number of analyzed points. Elements not determinedare given as blanks. S.D.=Calculated standard deviation. If no standard deviation is listed for an element, the element was abovethe detection limit for one point analysis but below the detection limit for the other point analyses. Different detection limits fordifferent points are caused by variable spot sizes and ablation duration.

M.G. Barth et al. / Precambrian Research 113 (2002) 323–340332

suite (1100�100 °C) overlaps the temperaturerange of our experiments (1000–1040 °C), thesmaller differences between partition coefficients ofhighly incompatible and moderately incompatibletrace elements in our melt-related data may becaused, at least in part, by compositional differ-ences. One of the differences between the naturaland experimental mineral pairs is the higher TiO2

content of the experimental minerals (garnet: 1.0–1.5 wt.% vs. 0.06–0.40 wt.% TiO2, cpx: 1.5–4.0wt.% vs. 0.04–0.41 wt.% TiO2), which may besignificant in terms of trace element substitutionwhere charge balancing coupled substitutions areinvolved.

In the plotted data set from Jacob and Foley(1999), one sample contains rutile, whereas allothers are rutile-free. The rutile-bearing sample(43) has the lowest DSr (86) of the set, but this isstill considerably higher than the experimentalvalues. The complete overlap between the data setsof Harte and Kirkley (1997) and Jacob and Foley(1999) also indicate that non-equilibrium partition-

ing inherited from lower temperature metamorphicconditions does not appear to be the source of thediscrepancy between natural and experimental re-sults: the Udachnaya eclogites (Jacob and Foley,1999) melted during subduction and so should havere-equilibrated trace elements, whereas manyRoberts Victor eclogites may not have experiencedmelting.

4. Discussion

4.1. Calculation of the strain-free partition coeffi-cient D0

It is well known (e.g. Onuma et al., 1968) thatplots of log Di versus the ionic radius ri for seriesof isovalent cations describe parabolas with max-ima corresponding to the size of the crystal latticesite(s) on which substitution occurs (Fig. 4 Table5). The shape and relative position of a givenparabola depends on melt chemistry, pressure, and

Table 4Partition coefficients for garnet and tonalitic melt

1000 °C DM 3 1000 °C DM 41040 °C DM 41040 °C DM 31 �No. 35 1 � No. 36No. 33 1 � No. 31 1 �

[0.0010] �0.0042Rb [0.0078]�0.0007 �0.005�0.006Ba 0.00041 0.00004

0.0016 0.0002Nb 0.0045 0.0036� 0.0210.000570.00512�0.018La 0.00410 0.00151

0.0079 �0.070 � 0.053Ce 0.0225 0.0031 0.02270.014 �0.25 0.0042Pb 0.050 0.015 0.013 0.013

0.0303 0.0030�0.0110.0070.031Sr 0.0072 0.00280.07 0.203 0.097 0.131Nd 0.11 0.03 0.0220.200.199 1.0 0.3 0.712Sm 0.610 0.155 0.582 0.116

0.6140.190.650.108 0.0790.517Zr 0.67 0.210.13 0.386 0.072 0.39Hf 0.35 0.09 0.47 0.10

0.281 0.049 0.327 0.044Eu 0.129 0.0230 0.117 0.0140.20 0.71 0.23 1.6Ti 0.79 0.10 0.93 0.2

2.57 0.330.33 0.62.31.79Gd 1.99 0.597.51 1.08 6.20 0.82Dy 5.39 0.96 4.32 0.77

8.9 2.2Ho 7.4 7.71.56 10.2 1.1 8.70Y 7.31 1.62 5.96 1.80

2.214.12.64 8.92 0.997.43Er 9.44 2.653.00 [13] [1.4] 10Yb [7.8] [1.62] 0.58.86

0.6112.421.63.39.0Lu 14.3 5.8Sc 4.12 1.69

2.8 6.0 2.3 0.906V 3.3 0.7 0.1823.6

Values in brackets are considered to be unreliable due to melt inclusions or analytical problems.

M.G. Barth et al. / Precambrian Research 113 (2002) 323–340 333

Table 5Partition coefficients for clinopyroxene and tonalitic melt

1040 °C DM 4 1000 °C DM 41000 °C DM 31 �Cpx 361 �Cpx 351 �Cpx 31

0.020 0.003Rb �0.00700.073Ba �0.00960.0040.03080.0030.82 0.22Th

Nb 0.017 0.013 �0.70 0.0029 0.0004La 0.0875 0.003 0.154 0.032 0.056 0.010

�0.080 �0.22Ce �0.0500.0053Pb 0.039 0.034 �0.29 0.0019

0.080 0.023 0.111Sr 0.009 0.073 0.016Nd 0.110.440.1000.6230.0900.396

0.250.060.28 0.090.060.23ZrHf 0.53 0.15 0.457 0.074 0.40 0.08

0.26Sm 0.84 0.29 1.17 0.28 0.59Eu 0.0470.2320.040.470.080.21

1.70.31.1 0.40.151.3Ti0.23 0.985Gd 0.861 0.212 0.1581.49

0.38Dy 0.88 0.27 1.83 0.25 1.40Ho 1.5 0.5

1.17 0.38 1.83Y 0.24 1.39 0.42Er 1.6 0.6 1.95 0.26 1.46 0.42

0.151.420.201.971.31 0.35Yb0.151.30Lu 0.35 1.312.02 0.23

2.1 0.5V 6.4 4.7 7.7 2.8

temperature at the time of crystallization (e.g.Jensen, 1973). By fitting the parabolas to theexperimentally determined Di we can calculate thestrain-free partition coefficient D0 for defined P, Tand X, the optimum radius r0 of the lattice siteand the apparent Young’s Modulus E of thelattice site (Blundy and Wood, 1994, equation 2).A cation with the radius ri=r0 can be incorpo-rated without straining the crystal lattice. We usedthe ionic radii of Shannon (1976) and calculatedonly the X site (8-fold coordination) for garnetbecause most trace elements, except for someHFSE, are too large for the Y site (6-fold coordi-nation), which is occupied nearly exclusively byAl3+ ions (Novak and Gibbs, 1971). For cpx, wecalculated the parameters for the M2 site.

This method can be applied particularly well tothe trivalent cations because Sc and the REEcover a wide range of ionic radii (rSc=0.870 A� ,rREE=0.977–1.16 A� in 8-fold coordination), andbecause the REE can be measured particularlywell by LAM-ICP-MS.

The data are insufficient to delineate any tem-perature dependencies. However, the higher D0 at1000 °C compared with 1040 °C (Table 6) areconsistent with the observation that D0 for garnet

Fig. 3. Clinopyroxene/garnet partition coefficients inferredfrom experimentally determined partition coefficients (sym-bols) and from eclogite xenoliths from the Roberts Victorkimberlite pipe, South Africa (light gray field; data from Harteand Kirkley, 1997) and from Udachnaya, Siberia (lines, datafrom Jacob and Foley, 1999).

M.G. Barth et al. / Precambrian Research 113 (2002) 323–340334

Fig. 4. Onuma diagrams for clinopyroxene at 1000 °C and 1.8GPa, showing partition coefficients for trivalent cations enter-ing the clinopyroxene M2 site together with non-linear least-squares fits (solid line) to equation (2) in Blundy and Wood(1994) and the predictive model (dotted line) of Wood andBlundy (1997). Error bars (shown if bigger than symbols) are1 �. Ionic radii are taken from Shannon (1976).

REE partition coefficient to predict partition co-efficients for Y, REE for both garnet and cpx andSc for garnet.

The model of Wood and Blundy (1997) predictspartition coefficients for cpx that agree within errorwith our measured partition coefficients (Fig. 4).The Wood and Blundy model, however, predicthigher values of r0 and E than our fitted values(Table 6). As a result the right limbs of both thepredicted and fitted parabolas are very similar,while the left limb of the predicted parabola showslower values. The reason for this discrepancyprobably is that fit values of r0 and E are highlycorrelated, so that small errors in D0 propagateinto large correlated errors in r0 and E (Wood andBlundy, 1997).

Similarly, the model of van Westrenen et al.,(2000) predicts higher values of D0, r0, and E thanour fitted values (Fig. 5 and Table 6, see also Fig.6 in van Westrenen et al., 2000). The power law fitfor E of van Westrenen et al. (2000) probablyoverestimates the stiffness of the garnet lattice forintermediate pyrope-grossular garnets such as thegarnets in this study. That is, the power law fit doesnot include the ‘softening’ of elastic constants forintermediate garnets due to structural heterogenei-ties caused by non-ideal solid solution (Ungarettiet al., 1995; Carpenter and Salje, 1998; BoffaBallaran and Carpenter, 1999). Thus, our fittedvalues of E are consistent with the observation ofvan Westrenen et al. (1999) that the X sites in

increases with decreasing temperature (Zack et al.,1997; Klein et al., 2000).

4.2. Comparison with predicti�e models

Blundy and Wood (1994), Wood and Blundy(1997), van Westrenen et al., (2000) presentedquantitative models of trace element partitioningbetween cpx, garnet, and melt based on the Brice(1975) equation. These models require the knowl-edge of the crystal bulk composition and just one

Table 6Results of fitting partitioning data for trivalent cations entering the garnet X site and clinopyroxene M2 site

Fit Predictedr0 [A� ] � �E [GPa] � E [GPa]D0 r0 [A� ] � � D0

1040 °C14.8405380.0060.96Garnet 33 30 185870.0050.9431.5

30 180.007 538 40 10 2 0.942 0.005Garnet 31 5940.9651.40.006 225Cpx 31 300.985 1.3 0.15 1.000 0.009 284

1000 °C30 300.96Garnet 35 0.007 540 45 21.6 1.6 0.943 0.005 588

30305940.0050.9423Garnet 36 19605280.010.95534Average garnet 0.955 20.3

0.985 0.007 210 30 2.1Cpx 35 0.13 1.010 0.009 285 1.90.007 1.452850.0091.0060.150.99 1.45Cpx 36 35255

0.988 233 1.8Average cpx

Predicted parameters calculated using the models of van Westrenen et al. (2000) and Wood and Blundy (1997).

M.G. Barth et al. / Precambrian Research 113 (2002) 323–340 335

Fig. 5. Onuma diagrams for garnet at 1040 °C and 1.8 GPa,showing partition coefficients for trivalent cations entering thegarnet X site together with non-linear least-squares fits (solidline) to equation (2) in Blundy and Wood (1994) and thepredictive model (dotted line) of van Westrenen et al. (2000).Error bars (shown if bigger than symbols) are 1 �. Ionic radiiare taken from Shannon (1976).

Table 7Recommended mineral/tonalitic melt partition coefficients fortrivalent cations at 1000 °C and 1.8 GPa

CpxIonic radius [A� ] Garnet

5.880.87ScLa 1.16 0.0032 0.12Ce 1.143 0.014 0.20Pr 0.321.126 0.052

0.17 0.48Nd 1.1090.860.961.079Sm

1.066Eu 3+ 1.041.803.13Gd 1.053 1.23

Tb 1.401.04 5.03Dy 1.027 7.52 1.56Y 1.631.019 9.30

10.2 1.67Ho 1.0151.7412.9Er 1.004

0.994 1.77Tm 15.31.77Yb 0.985 17.2

Lu 0.977 18.6 1.76

Ionic radii of 8-fold coordinated cations from Shannon (1976).intermediate garnets appear softer than in pyrope-or grossular-rich garnets.

4.3. An internally consistent set of partitioncoefficients

Modeling partial melting of eclogites requiresan internally consistent set of partition coeffi-

cients. Therefore, we calculated partition coeffi-cients for trivalent cations using equation (2) ofBlundy and Wood (1994) and the average fittedparameters of the 1000 °C experiments (Table 6).The calculated D-values (Table 7) are consistentwith our measured D ’s since both data sets agreewithin error for most elements (Fig. 2) but havethe advantage of being a smooth function of theionic radius.

The partitioning data used here in combinationwith data for rutile from Foley et al. (2000a) haveimportant implications:1. Higher D ’s for heavy REE and lower D ’s for

highly incompatible elements for both garnetand cpx in tonalitic systems than in basalticsystems indicate that we cannot apply D ’sdetermined in basaltic systems to geochemicalmodeling in tonalitic systems. For example,the use of tonalitic D ’s yields melts withsteeper trace element patterns at a given degreeof partial melting than the use of basaltic D ’s.

2. The high Dgarnet for heavy REE suggests that acomparatively small amount of garnet in theresidue can cause the heavy REE depletionduring partial melting of the mafic protolith.

3. Rutile is the only one of the investigatedphases that can preferentially remove Nb andTa. Thus, rutile is a likely candidate to caused

Fig. 6. Primitive mantle normalized trace element diagramshowing the source rock (thick black line, average ArcheanSula Mountain basalt of Rollinson, 1999), calculated melts(light gray field, 5 to 20% batch melting), calculated residues(light gray field, 15% batch melting to 15% fractional melting),range of Koidu low MgO eclogite xenoliths (dark gray field,Barth et al., 2001), and average Archean TTG (diamonds,Martin, 1994). Element abundances are normalized to theprimitive mantle values of McDonough and Sun (1995).

M.G. Barth et al. / Precambrian Research 113 (2002) 323–340336

the negative Nb–Ta anomaly (Foley et al.,2000a). Note that low-Mgc [molar Mg/(Mg+Fe)] amphibole may also be capable offixing Nb in the residue (Foley et al., 2000b;Tiepolo et al., 2001).

5. Petrologic implications: The origin of ArcheanTTG and partial melting of eclogites

Granitoids of the TTG (tonalite– trondhjemite–granodiorite) suite and grey gneisses, their high-grade metamorphic equivalent, are the dominantconstituent of the Earth’s early continental crust.These Na-rich rocks consist of plagioclase, quartz,and biotite with minor K-feldspar and horn-blende. Their distinct trace element compositionsreveal depletions in Nb, Ta, Ti, and P relative toother incompatible elements such as La. The rareearth element (REE) patterns are strongly frac-tionated (La/Yb)N=38, with low heavy REEcontents and, on average, no significant Euanomaly (Martin, 1994). Taken together, thesetrace element characteristics indicate that TTGsformed in equilibrium with garnet and probablyrutile or ilmenite. The presence of garnet requiresa depth of melting �40 km, but the precise depthand the tectonic setting remain uncertain. Meltingmay have occurred in the deeper, mafic portion ofa thickened continental crust (Atherton and Pet-ford, 1993) or within oceanic crust that was sub-ducted into the upper mantle (Martin, 1986).Possible residues are garnet-bearing amphibolite,granulite, or eclogite. Here we performed partialmelting calculations of liquids in equilibrium withrutile-bearing eclogitic residues because somexenolithic eclogites have complementary majorand trace element compositions to ArcheanTTGs, indicating that both were derived from acommon basaltic parent rock (Ireland et al., 1994;Rollinson 1997; Jacob and Foley, 1999; Barth etal., 2001). Moreover, rutile-bearing eclogites maybe an important reservoir of Nb and Ta in thecrust–mantle system (McDonough, 1991; Rud-nick et al., 2000).

Following the experimental results of Rapp etal. (1991) at 2.2 GPa and 1050 °C, the residue hasa garnet/cpx ratio of 40:60 and contains 0.5%

rutile. As a protolith we assumed the medianArchean Sula Mountains basalt (Rollinson, 1999),which has a slightly enriched trace element pat-tern typical of greenstone belt basalts (cf. Arndt etal., 1997). No corrections for element fractiona-tion during dehydration at low pressures weremade. For garnet and cpx we used the calculatedpartition coefficients at 1000 °C and 1.8 GPa(Table 7) for trivalent cations and measured parti-tion coefficients for other elements (Tables 4 and5). Rutile partition coefficients at 1100 °C and 2.5GPa are taken from Foley et al. (2000a).

The degree of melting appropriate for TTGformation is poorly constrained. Although SiO2-rich melts are relatively viscous and are believedto be difficult to segregate from their source rocksat low degrees of melting (Wickham, 1987; PatinoDouce et al., 1990), recent studies indicate thatviscosities of silicic melts may be close to those ofbasalts when sufficiently wet (Richet et al., 1996).Consequently, the segregation of tonalitic melts isnot as sluggish as previously anticipated (Laporte,1994; Lupulescu and Watson, 1999). Melt connec-tivity in tonalitic melts may be realized at meltfractions lower than 5% (Wolf and Wyllie, 1991;Lupulescu and Watson, 1999). Although theselow degrees of melting may not produce largeamounts of magma, it may be important in deter-mining the trace element signatures of both thesegregated liquid and the residuum. We calculatedmelts for 5–20% partial melting.

The calculated partial melts are insensitive tothe melting model (batch melting, continuousmelting, or accumulated fractional melting). Themelts show strongly fractionated REE patterns((La/Yb)N=32–82 for 5–20% partial melting),depletions in Nb and heavy REE, and high Zrand Hf concentrations (low Sm/Zr) similar toArchean TTG (Fig. 6). In contrast, melts in equi-librium with rutile-bearing eclogite have higher Tiand Sr concentrations than Archean TTG andhave a shallower slope for highly incompatibleelements such as Ba, Th, and U. Martin (1999)suggested that the low Sr concentrations ofArchean TTG could indicate residual plagioclaseduring partial melting, implying TTG genesis atrelatively shallow depths (�1.5 GPa). However,the depth of melting is not as precisely con-

M.G. Barth et al. / Precambrian Research 113 (2002) 323–340 337

strained as Martin (1999) suggests, since plagio-clase is present in the 1000 °C run at 1.8 GPaand may be stable up to 2.7 GPa at low watercontents (Winther and Newton, 1991; Rapp andWatson, 1995). Alternatively, Sr may be con-trolled by additional phases not considered here.

The calculated residues are very sensitive tothe melting model, with residues of fractionalmelting being much more depleted than residuesof batch melting or continuous melting. The ru-tile-bearing residues are depleted in incompatibleelements with a positive Nb (and probably alsoTa) anomaly and a flat heavy REE pattern (Fig.6) similar to rutile-bearing xenolithic eclogitesfrom Siberia and West Africa (Rudnick et al.,2000; Barth et al., 2001). The rutile-bearingxenolithic eclogites, however, have higher Sr andlower Zr contents than the calculated residues.Rutile-free eclogite xenoliths frequently show de-pleted trace element patterns without positiveNb and Ta anomalies (Snyder et al., 1997; Ja-cob and Foley, 1999), which demonstrates thatrutile dominates the Nb and Ta budget duringpartial melting. The calculated residues are com-plementary to Archean TTGs and have highNb/La ratios. Although the uncertainties ofDNb/DTa preclude us from predicting the Nb/Taratio of the residues, the subchondritic Nb/Taratios of Archean TTGs (Martin, 1994) suggestthat refractory eclogites may have superchon-dritic Nb/Ta ratios. Thus, rutile-bearing residualeclogites may be an important reservoir of Nband Ta in the Silicate Earth and a link betweencontinental crust and mantle (McDonough,1991; Rudnick et al., 2000). However, partialmelting of subducted oceanic crust in the eclog-ite stability field may not be a unique processfor the formation of Archean TTG. Recent ex-perimental partitioning data show that amphibo-lite melting may produce similar trace elementpatterns to rutile-bearing eclogite (Foley et al.,2000b).

6. Summary and conclusions

In this study we determined partition coeffi-cients for up to 22 trace elements between gar-

net, clinopyroxene, and hydrous tonalitic meltsat 1000–1040 °C and 1.8 GPa. Garnet partitioncoefficients in this study have higher values forheavy REE and lower values for highly incom-patible elements than in basaltic systems, result-ing in a steeper pattern. Clinopyroxene partitioncoefficients have similar patterns but higher val-ues than in basaltic systems. These resultsdemonstrate that partition coefficients deter-mined in basaltic systems are inappropriate fortrace element modeling in silicic melts such asthe generation of Archean TTGs.

Our experimentally determined partition co-efficients agree well with predictive models. Us-ing best-fit parabolas we calculated an internallyconsistent set of partition coefficients for triva-lent cations for garnet and cpx. Trace elementmodeling of partial melting of rutile-bearingeclogites shows that the average trace elementpatterns of Archean TTGs can be reproducedby low to moderate degrees of melting. The cal-culated eclogitic residues are depleted in incom-patible trace elements with positive Nb (and Ta)anomalies similar to the trace element patternsof rutile-bearing eclogite xenoliths reported fromSiberia and West Africa. These rutile-bearingeclogites are complementary to Archean TTGsand may be an important Nb reservoir in theSilicate Earth.

Acknowledgements

This work is part of Matthias G. Barth’sdiploma thesis. M.G.B. wishes to thank S.Klemme, S. Melzer, T. Zack and other membersof the Mineralogisch-Petrologisches Institut,Universitat Gottingen for helpful assistance andlong discussions. The EMP analyses could nothave been performed without the generous helpof S.R. van der Laan. Discussions with C.-T.Lee, W. F. McDonough, and R.L. Rudnick andreviews by G.A. Gaetani and T.H. Green helpedto improve the paper. Financial support wasprovided by the Deutsche Forschungsgemein-schaft grant Fo 181/2-3 to S. F. F.

M.G. Barth et al. / Precambrian Research 113 (2002) 323–340338

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