Development and use of in situ laser sulfur isotope analyses for pyrite-anhydrite geothermometry: An example from the pyrite deposits of the Cameros Basin, NE Spain

Abstract

We describe a system for the in situ sulfur isotope analysis of small (>100 μm) anhydrite crystals and investigate its application to anhydrite-pyrite geothermometry. Anhydrite inclusions (<1 mm) have been analyzed in spectacular, museum quality pyrite crystals from the Mesozoic Cameros Basin in NE Spain. Some of the data yield isotopic equilibrium temperatures (367 ± 6°C) consistent with other geothermometric estimates of metamorphic temperature. This suggests that isotopic equilibrium was established between the host pyrite and anhydrite inclusions and was not affected by re-equilibration. However, other data points yield anhydrite compositions consistently too 34S-depleted, resulting in a geologically unrealistic temperature (610 ± 20°C). Experiments show that where pyrite becomes overheated by the laser during anhydrite decomposition, solid phase reaction can incorporate pyrite-sulfur into the sampled gas in a stoichiometric fashion, therefore, the consistency of the erroneous temperature estimates from this group. Successful analyses are only obtained when overheating of the pyrite is avoided during laser decomposition of anhydrite inclusions. The laser system allows isotopic measurement of anhydrite inclusions too small to be analyzed conventionally.