The metal content of the eucrite parent body: constraints from the partitioning behavior of tungsten

Abstract

The solid metal/silicate melt partition coefficient for W has been determined experimentally to have a value of 25 ± 5 at 1190°C and an oxygen fugacity of 10 −13.4, the temperature and oxygen fugacity conditions at which eucritic basalts formed. Given this partition coefficient, scenarios for the metal content and evolution of the eucrite parent body (EPB) are constructed to explain the reduction by a factor of 30, relative to the chondrites, of the W La ratio in the eucrites. A possible model for the early geologic history of the EPB begins with accretion of a parent body, chondritic in composition with respect to nonvolatile siderophile and lithophile elements. The solid metal content was between 2% and 10%, which is within the range observed in the ordinary chondrites. Subsequent heating of the EPB caused the metal phase to separate and become isolated from the silicate phases before the degree of partial melting of the silicates reached 4% to 5%. Equilibrium partitioning of most of the W into the solid metal phase at low degrees of partial melting reduced the W La ratio in the remaining silicates. Continued partial melting of the silicates generated primary eucritic magmas which recorded the reduced W La ratio.