Compositions of large metal nodules in mesosiderites: Links to iron meteorite group IIIAB and the origin of mesosiderite subgroups

Abstract

Large mesosiderite metal nodules were studied petrographically and by neutron activation; their large sizes minimized the effects of silicate contamination and dilution by Fe reduced from FeO. The remarkable mixture of subequal amounts of metal and basaltic and pyroxenitic silicates in mesosiderites can be explained by the accretion of a core-like metal mass from one asteroid onto the surface of another asteroid; our data show that the accreted mass had a composition roughly similar to that of the IIIAB iron-meteorite core, consistent with other evidence of close genetic links between these and other related groups. The siderophile compositional pattern, 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 been divided into plagioclase- and tridymiterich subgroup A and orthopyroxene-rich subgroup B on the basis of modal silicate mineralogy. Roughly the same subgroups are defined by metal compositions; Au, As, Ni, and Cu are generally higher, and W lower in subgroup A than in subgroup B. In our sample set, a Widmanstätten pattern is only observed in nodules from subgroup A. A plausible model is that subgroup A best preserves the compositions of the target regolithic silicates and accreted core; subgroup B is comprised of these materials together with a minor amount of coremantle materials consisting of olivine and refractory, Ni-poor metal. The independent covariation of Ga, Re, and Ir may in part reflect fractionations during impact heating events. The anomalous mesosiderite Reckling Peak A79015 is strongly depleted in refractory siderophiles and slightly enriched in volatile siderophiles; such evolved metal requires a formation distinct from common mesosiderite metal.