PARTIAL MELTING IN ARCHEAN SUBDUCTION ZONES: CONSTRAINTS FROM EXPERIMENTALLY DETERMINED TRACE ELEMENT PARTITION COEFFICIENTS BETWEEN ECLOGITIC MINERALS AND TONALITIC MELTS UNDER UPPER MANTLE CONDITIONS

Abstract

The partitioning of an extensive suite of trace elements between garnet, clinopyroxene, and hydrous tonalitic melts has been studied experimentally at 1.8 GPa and 1000–1040°C. The knowledge of trace element partitioning between minerals and coexisting tonalitic melts is essential for geochemical models in order to explain the genesis of Archean tonalite-trondhjemite-granodiorite suites (TTGs). Our experiments were performed on a natural tonalite doped with two different trace element mixtures and with TiO2 to stabilize titanates. Trace element concentrations of crystals and coexisting melts were measured by laser ablation microprobe. The partition coefficients (D's) are independent of the trace element concentration over the concentration ranges used. The partition coefficients reported in this paper are a considerable extension of the few available data sets in intermediate to silicic systems. The D-values mostly agree well with previously published data and with predictive models. The garnet partition coefficients show steeper patterns with higher D's for heavy REE, lower D's for highly incompatible elements and lower La/Yb-ratios than previously published data for basaltic systems, whereas clinopyroxene partition coefficients show similar patterns but higher values than for basaltic systems. A set of calculated partition coefficients based on best-fit parabolas was used to model trace element fractionation in melts in equilibrium with rutile-bearing eclogitic residues. The calculated melts reproduce the overall trace element patterns of Archean TTGs, i.e. the melts show strongly fractionated REE patterns, 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 earth element patterns similar to rutile-bearing xenolithic eclogites reported from Siberia and West Africa. Restites from eclogite melting are complementary to Archean TTGs and may be an important reservoir of Nb and Ta.