Siderophile trace elements in the Earth's oceanic crust and upper mantle

Abstract

Abundances of Re, Ir, and Au have been determined in a suite of oceanic basalts using radiochemical neutron activation analysis. Mean abundances of highly siderophile elements in the oceanic crust have been estimated. Siderophile elements are fractionated during basalt genesis. Iridium, Os, Pd, Pt, Ru, and Rh are depleted in basalts relative to peridotites. Gold is only slightly enriched in basalts. Rhenium is favorably partitioned into the liquid during partial melting of peridotites. Concentrations of siderophile elements in the earth's upper mantle are higher than that expected from core‐mantle equilibrium. Six platinum‐group elements, Re, and Au have chondritic ratios in the upper mantle, indicating the presence of a meteoritic component. The abundances of eight highly siderophile elements can be accounted for by 0.74% of CI‐chondritelike material in the upper mantle. The low‐Ti mare basalts have lower Re contents and lower Re/Ir ratios than oceanic basalts, which may be interpreted by a lower partition ratio of Re under extremely dry and reducing conditions in the lunar interior.