Fluid mixing and thermal regimes beneath the PACMANUS hydrothermal field, Papua New Guinea: Helium and oxygen isotope data

Fluid mixing processes and thermal regimes within the Snowcap and Roman Ruins vent sites of the PACMANUS hydrothermal system, Papua New Guinea, were investigated using 3He/4He ratios from fluid inclusions within pyrite and anhydrite and the δ18O signature of anhydrite. Depressed 3He/4He ratios of 0.2–6.91RA appear to be caused by significant atmospheric diffusive exchange, whilst He–Ne diffusive fractionation precludes correction using 20Ne. 40Ar/36Ar ratios of 295–310 are elevated above seawater, indicating the majority of argon is seawater derived but with a magmatic component. δ18O anhydrite ratios are 6.5‰ to 11‰ for Snowcap and 6.4‰ to 11.9‰ for Roman Ruins. Using oxygen isotope fractionation factors for the anhydrite–water system, the temperatures calculated assuming isotopic equilibrium at depth are up to 100 °C cooler than fluid inclusion trapping temperatures. It is likely that anhydrite is precipitated rapidly, preventing δ18O equilibration. By comparing new δ18O values for anhydrite with corresponding published 87Sr/86Sr ratios, seawater is inferred to penetrate deep into the Snowcap system with little conductive heating. A simple fluid mixing model has been constructed whereby the differing venting styles can be explained by a plumbing system at depth which favors delivery of end-member hydrothermal fluid to the high temperature sites.