DIFFUSION OF KEY ISOTOPES (PB, PD, AG) IN IRON METEORITES: IMPLICATIONS FOR CLOSURE TEMPERATURES

Several isotope systems including ¹⁰⁷Pd/¹⁰⁷Ag, and Pb-Pb dating are used to measure ages iron meteorites. Limited data on diffusion in sulfides and metals precludes using accurate closure temperatures for this system, limiting their utility in constraining thermal histories. We have conducted experiments to measure Pb diffusion in FeS with direct applicability to the closure temperature of Pb in this system. Diffusion experiments were performed in evacuated silica capsules at 1 atm and 500^oC to 850^oC for a period of 24 hours to two weeks. We performed experiments using both a constant source and a thin film configuration over similar temperature ranges. The resulting concentration profiles were measured by Rutherford backscattering spectroscopy (RBS) at the University at Albany Ion Beam laboratory and at Union College. Our results suggest that the closure temperature for Pb is significantly higher than that of Os, and wide scale melting (of sulfides and metals) would likely be required to reset this isotopic clock. The activation energy for Pb is approximately 87 kJ/mol, compared to 211 kJ/mol for Os. At relatively high temperatures, the diffusivity of Pb is actually significantly less than that of Os. Using the temperatures from multiple isotopic systems can lead to further constraints on the thermal history of parent bodies during core formation and crystallization.

We will also present preliminary results on closure of the Pd/Ag system in iron meteorites inferred from the results of similar diffusion experiments.