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

• Published in: Geochimica et cosmochimica acta Vol. 73; no. 16; pp. 4792 - 4802
• Main Authors: Watson, E. Bruce, Cherniak, Daniele J., Frank, Elizabeth A.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 15.08.2009
• Subjects: Cations; Diffusion; Diffusion rate; Grain size; Mathematical analysis; Minerals; Pyrite; Signatures; Sulfur
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2009.05.060

Self-diffusion of sulfur in pyrite (FeS 2) 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 ( D S) conforms to D S = D o exp(− E a/ RT), where the pre-exponential constant ( D o) and the activation energy ( E a) are constrained as follows: log D o = - 13.76 ± 0.71 ( D o in m 2 s - 1 ) E a = 132.1 ± 12.5 ( kJ mol - 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.