Lead diffusion in iron sulfide: Implications for closure temperatures and dating iron meteorites

Many iron meteorites have enough radiogenic lead in their sulfide phases to allow for absolute age dating using the Pb-Pb system (Chen and Wasserbuerg, 1983). Absolute ages have been measured by 207Pb/206Pb dating of troilite nodules in the IVA iron meteorites (e.g. Blichert-Toft et al. 2010). This, in conjunction with established cooling rates and the age of core segregation from 182Hf/182W isotopes, (Kleine et al. 2005, 2009 ) can provide constraints on the timing from accretion and differentiation through crystallization and final cooling. Scarce data on Pb diffusion in sulfides precludes using an accurate closure temperature for this system, limiting its utility in constraining thermal histories. Proxies such as the closure of Os in pyrrhotite have been used but may not be realistic. We have conducted a series of 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 500oC to 850oC for a period of 24 hours to two weeks. The resulting concentration profiles were measured by Rutherford backscattering spectroscopy (RBS) at the University at Albany Ion Beam laboratory. Our preliminary 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. In fact, the diffusivity is close to six orders of magnitude slower than Os at temperatures close to FeS crystallization ( 1000oC), resulting in a much higher capability of retaining an isotopic signature.