SELF DIFFUSION OF SULFUR IN PYRITE

Bio-mineralization of sulfides can produce an ‘anomalous' S isotope ratio in the mineral (a ‘biosignature') that is useful as an indicator of biological activity in the geologic past. The reliance upon S isotopes for this purpose is predicated upon the assumption that the isotopic ratio does not change over geologic time through recrystallization or diffusive exchange of S with the mineral surroundings. The validity of the ‘no diffusion' assumption can be evaluated using knowledge of the S diffusivity in relevant minerals. In the absence of existing data, we determined the self-diffusion coefficient of S in pyrite by annealing natural pyrite crystals in ³⁴S-enriched sulfur vapor and measuring ³⁴S-³²S-diffusive exchange profiles in the near-surface of the pyrite using Rutherford backscattering spectroscopy. The experiments were conducted at 600°-725°C, a sufficient range to obtain a preliminary description of the T dependence of the self diffusion coefficient (D_S) of S in pyrite. This has the form D_S = D_oexp(-E_a/RT), where D_o and E_a are constants found to have values of ~ 2x10^-10 m²s^-1 and 200 (±90) kJ mol^-1, respectively. At the time of this writing the uncertainties are significant, but the data indicate that S isotope biosignatures in pyrite are resistant to diffusive alteration in geological time frames. This conclusion depends upon pyrite grain size, but a typical crystal of 1 mm radius would experience ~60%, ~10%, and ~1% exchange of S with its surroundings if held for 10 million years at 600°, 500° and 400°C, respectively.