Aluminum partitioning between olivine and ultrabasic silicate liquid to 6 GPa

Abstract

Trace element analyses of 1-atm and high-pressure experiments show that in komatiite and peridotite, the olivine (OL)/liquid (L) distribution coefficient for Al2O3 (\(D_{{\text{Al}}_2 {\text{O}}_3 }^{{\text{OL}} - {\text{L}}} \)) increases with pressure and temperature. Olivine in equilibrium with liquid accepts as much as 0.2 wt% Al2O3 in solution at 6 GPa. Convergence to equilibrium compositions at this high level is shown by cation diffusion of Al into synthetic forsterite crystals of low-Al contents in the presence of melt. Convergence to low-Al equilibrium compositions at lower P and T is shown by diffusion of Al out of synthetic forsterite with high initial Al content. Isobaric and isothermal experimental data subsets reveal that temperature and pressure variations both have real effects on \(D_{{\text{Al}}_2 {\text{O}}_3 }^{{\text{OL}} - {\text{L}}} \). Variation in silicate melt composition has no detectable effect on \(D_{{\text{Al}}_2 {\text{O}}_3 }^{{\text{OL}} - {\text{L}}} \) within the limited range of experimentally investigated mixtures. Least-squares regression for 24 experiments, using komatiite and peridotite, performed at 1 atm to 6 GPa and 1300 to 1960°C, gives the best fit equation: \({\text{ln D}}_{{\text{Al}}_{\text{2}} {\text{O}}_{\text{3}} }^{{\text{OL}} - {\text{L}}} = {\text{0}}{\text{.082 }}P\left( {{\text{GPa}}} \right) - {\text{8290 1/}}T{\text{(K), }}\left[ {{\text{R = 0}}{\text{.98}}} \right]\) Increase in \(D_{{\text{Al}}_2 {\text{O}}_3 }^{{\text{OL}} - {\text{L}}} \) with increasingly higher-pressure melting is consistent with incorporation of a spinel-like component of low molar volume into olivine, although other substitutions possibly involving more complex coupling cannot be ruled out. High P-T ultrabasic melting residues, if pristine, may be recognized by the high \(D_{{\text{Al}}_2 {\text{O}}_3 }^{{\text{OL}} - {\text{L}}} \) calculated from microprobe analyses of Al2O3 concentrations in residual olivines and estimated Al2O3 concentration in the last liquid removed. In general the low levels of Al in natural olivine from mantle xenoliths suggest that pristine residues are rarely recovered.