Cosmochemistry of iron meteorites; trace element composition of metal and sulfide phases

Abstract

Trace elements composition of the metal and sulfide phases in iron meteorites have been studied in 42 iron meteorites and 2 pallasites. 19 iron meteorites thick sections containing sulfide inclusions have been mapped and their mineralogy, tex- ture and major and minor element compositions analyzed by SEM and EPMA and their trace elements composition measured using LA-ICP-MS. A highly homoge- neous fragment of Cape York troilite has been found suitable to be used as a sulfide standard for moderately volatile elements in LA-ICP-MS analysis. A further 34 sulfide samples were digested in ultrapure acids and analyzed by Solution-ICP-MS. In thick sections, the iron meteorites Dronino, Nantan and Patos de Minas showed a rim of Ni and Co-rich sulfides around troilite (FeS) nodules, they are terrestrial weathering products of the troilite. X-RD analysis produced the char- acteristic pattern for pentlandite ((Fe,Ni)9S8) and heazlewoodite (Ni3S2). Analy- sis of powder from a sulfide nodule of Uruac ̧u also showed presence of magnetite (Fe3O4) and goethite (FeO(OH)), infilling cracks within the troilite nodule. Terres- trial weathering and shock-melting are found not to affect the moderately volatile trace elements content of the sulfide, except for Ni and Co that are enriched by terrestrial weathering. Bulk iron meteorite compositions have been calculated from LA-ICP-MS and Solution-ICP-MS measurements. They show a depletion of moderately volatile ele- ments (condensing between 1350 and 650 degrees K) and the strongly volatiles Cd and In. The depletion is greater for IVA irons. IABs, IIABs, IIIABs and pallasites have similar levels of depletion which are greater than what has been observed for ordinary chondrites. The selenium content of the sulfide is inversely correlated with bulk Ni content and the bulk selenium vs. bulk sulfur content show loss of a sulfide liquid from the IIIAB and IVA parent-body but not from the IAB group which lies on the CI line.