Trace element partitioning between majoritic garnet and silicate melt at 25 GPa

We have determined the partitioning of 39 trace elements between majorite-rich garnet and silicate melt at 25 GPa and ∼2300 °C. Concentrations in crystalline and quenched melt phases were determined by both SIMS and laser ablation-ICPMS techniques with, in general, very good agreement in partition coefficients. As anticipated from size and charge considerations, DLu (where DLu=[Lu]Majorite/[Lu]Melt=0.77) is much greater than DLa (=0.02) and K, U and Th are strongly incompatible. Partition coefficients for isovalent cations entering either the X- or Y-site exhibit a near-parabolic dependence on the radius of the incorporated cations as predicted from the lattice strain model. This underlines the important contribution made by the crystal structure to the control of trace element partitioning. Our data indicate that anhydrous melting at high pressures in the presence of residual garnet could lead to the generation of Al-depleted komatiites. The partitioning data have also been used to assess the potential effects of majoritic garnet fractionation into the deep mantle during a putative magma ocean event in early earth history. Crystallisation of a maximum of 14% majoritic garnet would have slightly depleted the primitive upper mantle in Si and Al without disturbing the chondritic abundance patterns of other refractory elements. The resultant reservoir would have a high Lu/Hf ratio, but a chondritic Sm/Nd ratio and would not be enriched in heat producing elements.