Formation of metal in the CH chondrites ALH 85085 and PCA 91467

Abstract

Siderophile element distributions within individual metal grains in two CH chondrites, Allan Hills 85085 and Pecora Escarpment 91467, were measured by laser ablation inductively coupled plasma mass spectrometry. Those metal grains that are zoned in Ni were also found to be zoned in other refractory siderophile elements, such as Ru, but not in Pd, which is not refractory but is highly siderophile. This pattern is consistent with an origin by condensation from a gas of approximately solar composition, but not with an origin by redox processes or fractional crystallization. The unzoned metal grains in CH chondrites were found to be frequently depleted in Ru but not in Pd, consistent with later stage condensation from a solar gas after removal of the zoned metal. Gold is inversely correlated with Ni in the unzoned metal grains, and mean Au abundances in zoned metal are always low. Both zoned and unzoned metal in CH chondrites could plausibly be produced from a thermostatically regulated nebula, followed by rapid removal of the zoned metal, and slower removal of the unzoned metal, both at temperatures near or above the condensation temperature of Au (∼1250 K). This is also consistent with the isolation temperatures inferred from silicate grains in CH chondrites by previous workers based on their volatile element inventories. The volatile siderophile Cu is enriched in the rims relative to the interiors of both zoned and unzoned grains, and is interpreted as the product of diffusion during low-grade thermal processing. The similarity of Cu distributions, and degree of kamacite/taenite exsolution, between zoned and unzoned metal in CH chondrites suggests that the two populations of metal experienced modest thermal metamorphism after they were brought together in the same environment, probably on the CH parent body. Fragmentation and size-sorting of the metal must have post-dated the Cu zoning, and may have occurred in a regolith on the CH parent body. The compositions of CH metal, like that of metal from QUE 94411 and HH 237, are consistent with a nebular origin, and may be the most primitive nebular materials (as distinct from presolar grains) sampled by chondrites.