Re-Os systematics in pallasite and mesosiderite metal

Abstract

Re-Os data on pallasite metal yield a sufficient range in Re/Os to permit the determination of a whole rock isochron, if pallasites from different groups are considered together. The results are indistinguishable from the data obtained on Groups IAB, IIAB, IID, IIIAB, IVA, and IVB iron meteorites. Pallasite data were obtained on samples of the Main Group, which have been considered related to Group IIIAB irons, and on samples of the distinctive pallasites Eagle Station, Finmarken, and Marjalahti. It appears that iron meteorites and pallasites crystallized within a time interval of 20 Ma. This result is consistent with the evidence from the short-lived chronometers ^(107)Pd-^(107)Ag and ^(53)Mn-^(53)Cr. It is also consistent with the evidence from ^(182)Hf-^(182)W that the time interval over which molten FeNi metal from different iron meteorites equilibrated with and became segregated from silicates was ∼10 Ma. The well-defined ^(187)Re-^(187)Os whole-rock isochron for pallasites and iron meteorites requires that they were formed in parent bodies that had been partially molten and segregated FeNi metal and FeS and then all crystallized within a 20 Ma time interval. This conclusion appears to apply to members of distinct groups of iron meteorites and pallasites which are viewed as not being cogenetic. The short time-scale requires that the irons and pallasites were formed in small parent planets of ≲10 km radius or near the surface of larger bodies. In contrast to the pallasites, we find that FeNi samples from mesosiderites have a narrow range in Re/Os so that it is not possible to determine a whole-rock isochron for these samples. The mesosiderite data also lie somewhat displaced from the iron meteorite and pallasite isochron indicating a more complex multi-stage evolution. Based on Sm-Nd data on mesosiderite silicate clasts, the Re-Os results on mesosiderite metal are compatible with the model of melted or partially molten FeNi cores or pods still preserved in protoplanetary bodies when these bodies were disrupted up to 150 Ma after early formation and the FeNi was splashed onto the surfaces of other small, differentiated planetesimals.

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