Trace element partitioning between garnet and chondritic melt from 5 to 9 GPa: implications for the onset of the majorite transition in the martian mantle

New garnet–melt element partitioning data for Sm, Nd, Tb, Yb, Lu, Y, Sc, Zr, and Hf from 5 to 9 GPa in an ordinary chondrite bulk composition yield insight into the effect of the garnet–majorite transition on trace element partitioning. A significant majorite component appears in garnet at pressures as low as ∼5 GPa, substantially lower than reported in previous studies, as evidenced by decreasing Al and increasing octahedral Si and Fe per formula unit with increasing pressure. The majorite component in our garnets exerts a clear influence on trace element partitioning, most strongly for the heavy rare earth elements and yttrium, whose D values decrease markedly with increasing pressure and majorite component. The effect on the lighter REE (e.g. Nd) is less pronounced, with D values slightly higher than are those for lower-pressure garnets; a similarly small effect on Sc is shown by its slightly lower D values than at lower pressure. D values for Zr and Hf show essentially no change compared both to lower- and higher-pressure garnets. ation of the lattice-strain model for element partitioning shows that there are mismatches between recently proposed predictive relationships using this model for partitioning of trivalent cations in garnet and garnets that exhibit the transformation to majorite. These mismatches are largely due to the structural changes attendant upon the majorite transformation with increasing pressure, and possibly on coexisting melt composition, because other parameters (temperature, bulk composition) have very narrow ranges in our experiments. sults do provide, however, very good fits to the lattice-strain model that allow the calculation of D values for any trivalent cation entering the X site of the majoritic garnet crystal structure, at pressures from 5 to 9 GPa and in the presence of ultramafic silicate melt. Using these D values for fractionation of an assemblage containing majoritic garnet from a chondritic liquid, we show the very different signatures imparted by lower-pressure vs. higher-pressure garnets on important geochemical features such as REE patterns. These new data support the hypothesis that superchondritic CaO/Al2O3 in martian meteorites is the result of garnet–majorite fractionation or partial melting in the garnet–majorite stability field in the martian mantle.