Siderophile Elements in Metal from Metal-rich Angrite Nwa 2999

Introduction: The angrites are an important class of differentiated meteorites, mainly ultramafic rocks, the number of which has grown to over eleven with recent finds from Antarctica and hot deserts [1, 2]. The major chemical characteristics of the angrites include a refractory element enriched, volatile element depleted, oxidized composition. Based on alkali element abundances (e.g., K/La~10xCI [3, 4]) the angrites are the most extremely depleted meteorite or planetary samples known. Paradoxically, volatile siderophile (e.g., Ge) and chalcophile elements are notably higher in angrites than the alkalis [4, 5]. These chemical characteristics, and oxygen isotopes, distinguish angrites from Martian meteorites and HED achondrites [4]. Some chemical features of angrites were noted to be similar to those of group IVB irons, and a common asteroidal origin of both groups has been proposed [6], although a candidate asteroid is not known. The discovery of a third metamorphosed angrite, NWA 2999, with symplectite and corona features led to the proposal that the large planetary size required may imply an origin from Mercury [1, 7]. Despite their relatively oxidized nature, 10 out of the 11 angrites contain metallic Fe-Ni alloy (<2%), but the presence of about 10% coarse Fe-Ni metal in NWA 2999 reported by [7] led to an attempt to examine the proposed link between group IVB irons and angrites in this meteorite. Samples and analytical methods: NWA 2999 is a metamorphosed and formerly brecciated igneous rock and, except for its anomalously high metal content, has textural and mineralogical similarities to Angra dos Reis and LEW 86010 [7]. A polished thick section of NWA 2999 studied by electron microprobe [7] was used for siderophile element analysis by laser ablation ICP-MS. A New Wave UP213 laser ablation system coupled to a Thermo Finnigan ElementTM in the Plasma Analytical Facility of the National High Magnetic Field Laboratory was used for the analyses [8]. Metal grains were selected using the reflected light microscope on the UP213. The peaks Mg, P, S, Cr, Fe, Co, Ni, Cu, Ga, Ge, As, Mo, Ru, Pd, W, Re, Os, Ir, Pt and Au were determined in low resolution mode (R=300). A spot size of 40 or 55 μm with 7-10 s dwell time was used for NWA 2999 metal, and 100 μm tracks or area raster were used for the standards. Relative sensitivity factors were obtained from the standards NIST SRM 1263a (P, S, Cr, Fe, Co, Ni, Mo), North Chile (Filomena, IIAB: Cu, Ga, Ge, As, W, Au) and Hoba (IVB: Ru, Pd, Re, Os, Ir, Pt). Concentrations were obtained by normalizing to Fe+Co+Ni=100%. Results: Sixteen individual metal grains and 1 silicate grain were analyzed for siderophile elements. The metals grains have 4.9-10.7% Ni, and chondritic relative abundances of most highly siderophile elements (HSE: Ru, Pd, Re, Os, Ir, Pt, Au). The silicate grain (calculated as Fe+Co+Ni=100%) had 0.5% Ni and extremely low values of most HSE (e.g. <0.03 ppm Ir). The abundances of many elements (W, Ga, Cr, P, Cu, Re, Mo) in NWA 2999 metal may have been affected by metamorphic equilibration between metal and silicate. The metal was depleted in P, Cr, Cu and Ga, but mainly exhibited enrichment in W. Correlations between Re and other HSE were poor indicating Re disturbance. Correlations between Mo and other HSE were good, with about half the grains showing positive enrichments in Mo/Ir (1.5-3xCI). The correlation between Ni and HSE was weak, and most grains had Ni depletions relative to Pd, implying that the abundances of Fe and Ni may also have been subject to metamorphic disturbance. Figure 1 is a plot of the most immobile elements. The metal grains exhibit chondritic