Sulfur stable isotope ratios of the Mt. Apo geothermal fluid, Philippines

摘要

Sulfur isotope ratios in the Mt. Apo geothermal system effectively define neutral and acid zones in the field. Near the upflow region, S isotopes are in equilibrium, as predicted temperatures from sulfur pairs agree with measured downhole temperatures. Here, δ~(34)S_(SO_4) ratios are typically +17 to +20 per thousand, and δ~(34)S_(H_2S) is around —3 per thousand. In the neutral Marbel corridor, δ~(34)S_(SO_4) ratios are characteristically between +10 and +15 per thousand while δ~(34)S_(H_2S) remains at about —3 per thousand. Sulfur isotopes from this sector onward along the outflow path, S-isotope temperatures do not coincide with measured well temperatures. Anhydrite derives its sulfur from the deep, δ~(34)S-enriched SO_4 dissolved in fluid. Pyrite may have a sulfur source apart from H_2S, as isotope ratios in pyrite are higher ( > 0 per thousand) than those in H_2S (—3 per thousand). Magmatic SO_2 may be this other sulfur source. Sulfur isotope mineral geothermometry gives temperatures much higher than present day fluid temperatures.