Composition and evolution of crustal, geothermal and hydrothermal fluids interpreted using quantitative fluid inclusion gas analysis

Abstract

Quantitative analysis of fluid inclusion volatiles is an underutilized methodology. Fluid inclusion volatiles are analyzed by the incremental crush fast scan method using mass spectrometry. About 100mg of sample is partly and progressively crushed between pauses of 5 and 10min at room temperature while under 10−8Torr vacuum and the released gases are analyzed by dual quadrupole mass spectrometers. The gases routinely reported include H2, He, CH4, H2O, N2, O2, H2S, Ar, CO2, SO2, C2H4, C2H6, C3H6, C3H8, C4H8, C4H10, and benzene. Besides the incremental crush method, fluid inclusion gases may also be analyzed by the bulk cold crush or thermal decrepitation, Raman spectroscopy and gas chromatography.In geothermal systems and hydrothermal ore deposits, fluid inclusion gas data may be used to discriminate fluid sources (meteoric, magmatic), identify processes (boiling, condensation, mixing), constrain redox, correct fluid inclusion isochors, apply gas geothermometry, and provide valuable chemical constraints for fluid–rock equilibria modeling. CO2/CH4 versus N2/Ar plots as well as N2–Ar–He ratios are used to discriminate calc-alkaline magmatic volatiles which have N2/Ar ratios in the 100s to 1000s from meteoric (where the N2/Ar ratio is ~38) and basinal fluids (where Ar/He ratios approach 1). Boiling and condensation have negative and positive slopes respectively on CO2/N2 versus total gas plots. Organic compounds derived by Fischer–Tropsch reactions plot as straight lines on Schulz–Flory (log of concentration versus carbon number) plots whereas boiled volatiles and residual fluids create deflections. Redox is constrained by CO2–CH4–H2 ratios or alkane–alkene ratios provided the temperature is known. Gas geothermometry, similar to the approach applied in geothermal wells, is equally applicable to dissolved fluid inclusion gases with the CO2–CH4–H2 geothermometer potentially useful. Isochors can be corrected using the total partial pressures of all gas species dissolved within homogenized fluid inclusions by multiplying the gas concentration and values for Henry's Law constants. Fluid–rock equilibria modeling is enhanced by knowing the concentration of gases which control redox and a(H2S) which is vital for gold and base metal solubility constraints.The methodology is not limited to geothermal and hydrothermal systems and examples are presented from several geologic systems such as Libyan Desert glass, metamorphic systems, serpentinites, fault fluids, and artificial meteorites, thus demonstrating the versatility and flexibility of the fluid inclusion volatile analysis technique.