TRACE-ELEMENT PARTITIONING BETWEEN GARNET, CLINOPYROXENE AND FE-RICH PICRITIC MELTS AT 3 TO 7 GPA

Abstract

We have determined mineral-melt partition coefficients (D values) for 20 trace elements in garnet-pyroxenite run products, generated in 3 to 7 GPa, 1,425–1,750°C experiments on a high-Fe mantle melt (97SB68) from the Paraná-Etendeka continental-flood-basalt (CFB) province. D values for both garnet (~Py63Al25Gr12) and clinopyroxene (~Ca0.2Mg0.6Fe0.2Si2O6) show a large variation with temperature but are less dependent on pressure. At 3 GPa, D cpx/liq values for pyroxenes in garnet-pyroxenite run products are generally lower than those reported from Ca-rich pyroxenes generated in melting experiments on eclogites and basalts (~Ca0.3–0.5Mg0.3–0.6Fe0.07–0.2Si2O6) but higher than those for Ca-poor pyroxenes from peridotites (~Ca0.2Mg0.7Fe0.1Si2O6). D grt/liq values for light and heavy rare-earth elements are ≤0.07 and >0.8, respectively, and are similar to those for peridotitic garnets that have comparable grossular but higher pyrope contents (Py70–88All7–20Gr8–14). 97SB68 D LREE grt/liq values are higher and D HREE grt/liq values lower than those for eclogitic garnets which generally have higher grossular contents but lower pyrope contents (Py20–70Al10–50Gr10–55). D values agree with those predicted by lattice strain modelling and suggest that equilibrium was closely approached for all of our experimental runs. Correlations of D values with lattice-strain parameters and major-element contents suggest that the wollastonite component and pyrope:grossular ratio exert major controls on 97SB68 clinopyroxene and garnet partitioning, respectively. These are controlled by the prevailing pressure and temperature conditions for a given bulk-composition. The composition of co-existing melt was found to have a relatively minor effect on 97SB68 D values. The variations in D values displayed by different mantle lithologies are subtle and our study confirms previous investigations which have suggested that the modal proportions of garnet and clinopyroxene are by far the most influential factor in determining incompatible trace-element concentrations in mantle melts. The trace-element partition coefficients we have determined may be used to place high-pressure constraints on garnet-pyroxenite melting models.