Laser ablation ICP-MS study of IIIAB irons and pallasites: constraints on the behaviour of highly siderophile elements during and after planetesimal core formation

We report the concentration of the platinum group elements (PGE) in addition to the highly siderophile elements (HSE) Re and Au, obtained using LA-ICP-MS, in spatially resolved FeNi-metal (kamacite and taenite) from a suite of IIIAB irons and Brenham main group pallasite (MGP). These samples are products of fractional crystallization and define a coherent magmatic trend, from the least evolved (Henbury and Wabar) to the most evolved (Mount Edith). Fractional crystallization continued after segregation of the pallasite parent melt, and incorporation of olivine into this metal melt did not affect the highly siderophile element systematics. Solid metal-liquid metal partition coefficients indicate the following compatibility: (1) highly compatible (Re, Os and It), (2) moderately compatible (Pt, Ru and Rh), and (3) incompatible (Pd and Au). This compatibility sequence is broadly consistent with the melting point systematics of the HSE. The highly fractionated HSE pattern of the evolved IIIAB and Brenham pallasite indicates that nonmetallic elements such as S play a fundamental role in the evolution of planetisimal core. Taenite-kamacite partition coefficients (D-T/K) illustrate that all of the HSE enter preferentially taenite during subsolidus partitioning and that subsolidus partitioning is comparable between magmatic and nonmagmatic irons. D-T/K appears to be independent of the physical conditions prevailing on the parent bodies and the light element molar fraction in the bulk composition, but instead the controlling factors on D-T/K behaviour seems to be a combination of the degree of atomic symmetry coupled with atomic radius. (C) 2004 Elsevier B.V. All rights reserved.