The Trace Element Chemistry of the Metal in IAB Iron Meteorites

Abstract

Abstract Metal grains from a silicate inclusion in Landes, a group IAB iron-meteorite, were separated and analyzed via IN A A for 14 siderophile elements. The same 14 elements were determined in matrix metal from Landes, 5 other IAB meteorites and three anomalous iron meteorites. Compared to the other samples and to chondritic metal, the inclusion metal is enriched in siderophile elements. In chondritic metal, the siderophile element contents are higher when Fe0 is oxidized to FeO. Silicates in IAB inclusions contain little FeO, yet the metal is as rich in siderophile elements as the most oxidized chondrites. Evidently the IAB body contained its full cosmic complement of S resulting in an FeS content ~5 x that of ordinary chondrites. This explains the composition of the metal since sulfurization, like oxidation, reduces the available Fe0. IAB meteorites contain their full cosmic complement of other moderately volatile elements, which implies an equally high S content. A high S content significantly affects the melting and freezing of FeNi-metal. About 85% of the metal melts at the Fe-FeS eutectic (~960 °C), and all of the metal is molten at ~1050°C. During cooling, ~85% of the metal crystallizes with a Ni content of 6±1% between 1050°c and 960°C. Besides a small depletion in elements with low melting points or chalcophile tendencies this metal is little fractionated. Bellow 960 °C, the Ni content of the metal increases until the melt is exhausted. This Ni-rich metal also becomes progressively depleted in refractory elements and enriched in volatile elements. The crystallization process thus yields products that mimic the abundance and composition of group IAB meteorites.