PRECIOUS-METAL ABUNDANCES IN SELECTED IRON-METEORITES - IN-SITU AMS MEASUREMENTS OF THE 6 PLATINUM-GROUP ELEMENTS PLUS GOLD

We have measured the abundances of seven precious metals in Ni-Fe phas es (kamacite and plessite) in six iron meteorites. These in-situ analy ses, obtained by accelerator mass spectrometry (AMS) on small polished samples previously characterized by electron microprobe techniques, c onstrain the distribution of the rare siderophile elements. Within our set of irons, a small but varied suite, concentrations measured by AM S vary by factors of 9-13 for Au, Pd, Pt, Ph and Ru, and by factors of 90 and 250 for Ir and Os respectively. Data are presented for all six platinum group elements (PGE) plus gold. The AMS data suggest a varia tion in overall precious-metal abundances of a factor of 16 between th e most-enriched (Negrillos, Sigma PGE + Au = 270 ppm) and the least-en riched (Welland, 16-19 ppm). A clear illustration of the use of AMS da ta for provenance studies of meteoritic iron is presented for the Well and IIIA iron, an 1888 find from Ontario. Few published data are avail able for Welland: comparison of a type sample with a smaller piece of unknown metal, with respect to chondrite-normalized PGE patterns, majo r-element chemistry and textures of the metals, strongly support a sug gestion that the latter is a fragment of the same iron.