Retention of biosignatures and mass-independent fractionations in pyrite: Self-diffusion of sulfur

Self-diffusion of sulfur in pyrite (FeS2) was characterized over the temperature range ∼500–725 °C (∼1 bar pressure) by immersing natural specimens in a bath of molten elemental 34S and characterizing the resulting diffusive-exchange profiles by Rutherford backscattering spectroscopy (RBS). The temperature dependence of the sulfur diffusivity (DS) conforms to DS = Do exp(−Ea/RT), where the pre-exponential constant (Do) and the activation energy (Ea) are constrained as follows:logDo=-13.76±0.71(Doin m2s-1)Ea=132.1±12.5(kJmol-1)In comparison with cation diffusion in silicate and oxide minerals, S diffusion in pyrite is relatively fast, with closure temperatures ranging from ∼300 °C to ∼700 °C for the full range of plausible cooling rates and grain sizes. Sulfur isotope signatures acquired at near-ambient temperatures will be preserved during heating at 200 °C or lower for indefinite periods of geologic time. Higher temperatures may lead to some open-system behavior, depending critically upon temperature, effective grain size, and time. Four experiments on natural sphalerite (ZnS) reveal that S diffusion in this mineral is very similar to that in pyrite, suggesting possible uniformity of S diffusion behavior in sulfides as a group.