Core participation in mantle geochemistry: Geochemical Society Ingerson Lecture, GSA Denver, October 1999

Abstract

The siderophile element problem in mantle geochemistry has not been solved by treating the mantle as the residue of core formation in a closed system. Conventional solutions to the problem involve the addition of new material to the mantle as late accretions from outer space. However more solution options are open if the core can return a little material to the mantle from inside. The notion that such a return flux of material from the outer core is occurring is suggested by the correlated anomalies in 186Os/ 188Os and 187Os/ 188Os seen in some plume materials by R. J. Walker and his colleagues (R. J. Walker et al., 1995; R. J. Walker et al., 1997; Brandon et al., 1998; Brandon et al., 1999). These isotopic anomalies could result from 190Pt and 187Re decay in the liquid outer core following Pt-Re-Os fractionation during crystallization of the solid inner core. Partition coefficients determined for solid/liquid metal fractionation of Pt, Re, and Os at 100 kbar are consistent with those required to produce the correlated Os isotope anomalies. Oxide exsolution from cooling and crystallization of a saturated liquid outer core provides a mechanism to reimplant the Os signal back into the mantle. Other siderophile elements may have their mantle abundances boosted by this process. The details of the reimplantation process present a worthy challenge for evaluation.