Large metal nodules in mesosiderites

Abstract

The remarkable mixture of subequal amounts of metal and basaltic and pyroxenitic silicates in mesosiderites seems to require that a core-like metal mass produced in one asteroid accreted onto the surface of another asteroid; our INAA data on 11 large mesosiderite metal nodules show that the accreted metal mass had a composition similar to the mean IIIAB iron-meteorite core. The similarity of the siderophile compositional pattern to mean IIIAB irons, the very limited compositional range, the inferred large projectile/target ratio, and the electrical interconnectedness of mesosiderite metal indicate that the bulk of the metal was molten when accreted. Mesosiderites have previously been divided into plagioclase- and tridymite-rich subgroup A and orthopyroxene-rich subgroup B on the basis of modal silicate mineralogy. The same subgroups can be defined by metal compositions; Au, As and Ni are generally higher and W and Cr lower in subgroup A than in subgroup B. A plausible model is that subgroup A best preserves the regolithic silicate and accreted core compositions; subgroup B was formed from a mixture of these materials with a minor amount of accreted core-mantle materials consisting of olivine and refractory, Ni-poor metal. The covariation of Ga, Re, Ir and Pt may reflect fractionations produced during impact heating events. Reckling Peak A79015 is an anomalous mesosiderite consisting of extremely heterogeneous patches of silicate and metal. It is strongly depleted in refractory siderophiles and slightly enriched in volatile siderophiles. This composition could have been produced either by impact-induced heating and fractionation (as observed for large metal nodules in H chondrites) or by fractional crystallization of metal different in composition from that in the other mesosiderites.