RE-OS SYSTEMATICS IN PALLASITE AND MESOSIDERITE METAL

Re-Os data on pallasite metal yield a sufficient range in Re/Os to per mit the determination of a whole rock isochron, if pallasites from dif ferent groups are considered together. The results are indistinguishab le from the data obtained on Groups IAB, IIAB, IID, IIIAB, IVA, and IV B iron meteorites. Pallasite data were obtained on samples of the Main Group, which have been considered related to Group IIIAB irons, and o n samples of the distinctive pallasites Eagle Station, Finmarken, and Marjalahti. It appears that iron meteorites and pallasites crystallize d within a time interval of 20 Ma. This result is consistent with the evidence from the short-lived chronometers Pd-107-Ag-107 and Mn-53-Cr- 53. It is also consistent with the evidence from Hf-182-W-182 that the time interval over which molten FeNi metal from different iron meteor ites equilibrated with and became segregated from silicates was simila r to 10 Ma. The well-defined Re-187-Os-187 whole-rock isochron for pal lasites and iron meteorites requires that they were formed in parent b odies that had been partially molten and segregated FeNi metal and FeS and then all crystallized within a 20 Ma time interval. This conclusi on appears to apply to members of distinct groups of iron meteorites a nd pallasites which are viewed as not being cogenetic. The short time- scale requires that the irons and pallasites were formed in small pare nt planets of less than or similar to 10 km radius or near the surface of larger bodies. In contrast to the pallasites, we find that FeNi sa mples from mesosiderites have a narrow range in Re/Os so that it is no t possible to determine a whole-rock isochron for these samples. The m esosiderite data also lie somewhat displaced from the iron meteorite a nd pallasite isochron indicating a more complex multi-stage evolution. Based on Sm-Nd data on mesosiderite silicate clasts, the Re-Os result s on mesosiderite metal are compatible with the model of melted or par tially molten FeNi cores or pods still preserved in protoplanetary bod ies when these bodies were disrupted up to 150 Ma after early formatio n and the FeNi was splashed onto the surfaces of other small, differen tiated planetesimals. Copyright (C) 1998 Elsevier Science Ltd.