Chemical compositions of siderophile element-rich opaque assemblages in an Allende inclusion

Ten opaque assemblages, or Fremdlinge, weighing 0.17-19 μg, from an Allende Type B, Ca-, Al-rich, coarse-grained inclusion were analyzed by INAA for Ca, Na, Mn, Sc, Ta, Ti, V, Cr, Fe, Ni, Co, W, Re, Os, Ir, Mo, Ru, Pt, Rh, Au, As, Sb and Se. Refractory siderophiles are usually enriched by factors of 10³-10⁴ relative to Cl chondrites, but not uniformly. From sample to sample, C1 chondrite-normalized Re Os, Ir Pt, Os Ru and Pt Rh ratios vary from 0.81-1.03, 0.70->2.25, 0.67-3.32 and 0.45-1.43, respectively. Attempts to model the abundances of refractory siderophiles by condensation from the solar nebula into a single phase, assuming ideal solution, fail to match the observed combination of subchondritic Re Os and Ir Pt ratios, even allowing for fractionation of the highest-temperature condensates. A different model, in which all metals whose high-temperature crystal structures are hcp condense into one alloy, bcc into another and fcc into a third, is capable of matching virtually all refractory siderophile fractionations in these Fremdlinge. In this model, all isostructural metals are assumed to form ideal solutions with one another and metals of different structure are assumed to be insoluble in one another. After fractionation of high-temperature hcp and fcc alloys, individual proto-Fremdlinge formed by mixing of high- and low-temperature varieties of each of the three alloys, either in the nebula before and during CAI accretion, or during later partial melting of the silicate, oxide and metal components of CAIs.