Gas Measurements on Geothermal Fluids Trapped in Fracture Filling Minerals: A New Window into the History of Geothermal Systems

Abstract

The development of a sensitive mass spectrometric technique has permitted analysis of gas concentrations in geothermal fluids trapped in microscopic fluid inclusions in hydrothermal minerals. Preliminary estimates of gas concentrations and compositions may be made using small samples of veins from core or drill cuttings. Volatiles including H2O, CO2, H2S, N2, H2, O2, SO2, HC1, HF, He, Ar, and C1 through C5 hydrocarbons can be measured when they are released by crushing or heating of the minerals. Such information from actively forming vein minerals provides chemical constrains on geothermal fluids and can be used in conjunction with thermometric studies of fluid inclusions to developed a three dimensional model of an active system. Since the fluid inclusions become sealed when vein minerals are deposited, data on well constrained suites of cross cutting veins provides a window into the chemical and thermal history of hydrothermal activity in either active or fossil geothermal systems.