ORIGIN OF ZONED METAL GRAINS IN THE QUE94411 CHONDRITE

Abstract

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used to measure distributions of the siderophile elements P, V, Cr, Fe, Co, Ni, Mo, Ru, Rh, Pd, W, Re, Os, Ir, and Pt in metal grains in the metal-rich chondrite QUE94411 with a spatial resolution of ~30 μm. The platinum group elements (PGEs), except Pd, exhibit radial zoning in these grains that mimics that previously observed in Ni and Co; the concentrations of these elements decreases from the cores to the rims of the grains. The PGE distributions support a condensation origin for the enhanced refractory element abundances in the zoned grains; the lack of zoning in Pd refutes an origin by a redox-controlled process, and none of the PGE-Ni relationships support an origin by fractional crystallization from a metallic melt. Several models of grain formation were explored, including equilibrium fractional condensation, which failed to yield the correct radial zoning. The zoning may be the product of a nonequilibrium fractional condensation process, in which the refractory siderophiles remained supersaturated in the cooling solar nebula, or of diffusion between refractory-enhanced Fe-Ni cores and other Fe-Ni metal that may have been deposited later from the solar nebula.