CONSTRAINTS ON CORE FORMATION FROM PT PARTITIONING IN MAFIC SILICATE LIQUIDS AT HIGH TEMPERATURES

Abstract

A new high temperature piston cylinder design has enabled the measurement of platinum solubility in mafic melts at temperatures up to 2500 °C, 2.2 GPa pressure, and under reducing conditions for 1-10 h. These high temperature and low fO2 conditions may mimic a magma ocean during planetary core formation. Under these conditions, we measured tens to hundreds of ppm Pt in the quenched silicate glass corresponding to DPtmet/sil ≈ 103-4, 4-12 orders of magnitude lower than extrapolations from high fO2 experiments at 1 bar and at temperatures no higher than 1550 °C. Moreover, the new experiments provide coupled textural and compositional evidence that noble metal micro-nuggets, ubiquitous in experimental studies of the highly siderophile elements, can be produced on quench: we measure equally high Pt concentrations in the rapidly quenched nugget-free peripheral margin of the silicate as we do in the more slowly quenched nugget-bearing interior region. We find that both temperature and melt composition exercise strong control on DPtmet/sil and that Pt0 and Pt1+ may contribute significantly to the total dissolved Pt such that low fO2 does not imply low Pt solubility. Equilibration of metal alloy with liquid silicate in a hot primitive magma might not have depleted platinum to the extent previously believed. © 2005 Elsevier Inc. All rights reserved.