Constraints on core formation from Pt partitioning in mafic silicate liquids at high temperatures

摘要

A new high temperature piston cylinder design has enabled the measurement of platinum solubility in mafic melts at temperatures up to 2500 degrees C, 2.2 GPa pressure, and under reducing conditions for 1-10 h. These high temperature and low f(O2) 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 D-Pt(met/sil) approximate to 10(3-4), 4-12 orders of magnitude lower than extrapolations from high f(O2) experiments at 1 bar and at temperatures no higher than 1550 degrees 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 D-Pt(met/sil) and that Pt-0 and Pt1+ may contribute significantly to the total dissolved Pt such that low f(O2) 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. (c) 2005 Elsevier Inc. All rights reserved.