The problem of excess N2, Ar and possibly CH4 is revisited. Examination of Cerro Prieto well discharges show that wells have N2 and Ar concentrations up to ten times greater than > ASW and concentrations as low as one tenth that of < ASW. Integrated average concentrations are, however, about that of ASW. Cerro Prieto fluids show a positive correlation between excess N2 and Ar, and enthalpy. The low enthalpy Steamboat Springs fluids similarly have N2 and Ar concentrations < ASW. Analysis of Mothra Black Smoker inclusions yields N2/Ar ratios similar to ratios in vent fluids, which shows that excess N2 and Ar are not due to measurement error. The distribution of Cerro Prieto gaseous species suggests a boiling-like process was responsible for the depletion of gaseous species in low N2 Ar fluids, and condensation explains fluids with N2 and Ar > ASW. Three inferences from the data are that: 1) the minerals in which we study fluid incisions are deposited by fluids with excess N2 and Ar, hence represent only part of the fluid history; 2) the mechanism responsible for fluids depleted in, and with excess N2 and Ar is a local process because both depleted and excess N2 Ar waters are observed in Cerro Prieto, and 3) the most accurate estimate of reservoir gas chemistry is obtained from geothermal fluids that have N2 and Ar concentrations near that of ASW. A mechanism proposed for enriching fluids in H2, N2, Ar, and CH4 calls on wallrock boiling and bubble trapping to sequester these species, then solution of bubbles at times of elevated pressure and temperature that occurs when fractures close.

Our analysis of black-smoker-mineral fluid inclusions indicates similar CO2/CH4 ratios as reported in the present vent fluid, thus verifying our fluid inclusion analyses. Some analyses of organic compounds in Tiwi, The Geysers, and Coso fluid inclusions show a Schultz Flory distribution characteristic of a Fisher Tropsch reaction. Experimental evidence indicates that "black smoker" methane is produced by Fisher Tropsch reactions involving olivine and magnetite at temperatures of 300 C or more. We hypothesize that deep circulating fluids subject to Fisher Tropsch relations show an inverse relationship between N2/Ar and CO2/CH4 ratios, whereas geothermal fluids that have an organic source of methane will show a positive correlation between N2/Ar and CO2/CH4 ratios. The Geysers and Broadlands fluid inclusions and Cerro Prieto fluids show a positive relationship between N2/Ar and CO2/CH4 ratios, whereas Tiwi and Coso fluid inclusions exhibit a negative correlation. Mineral alteration by methane producing reactions will in time limit the amount of methane produced. Therefore, we expect that both inorganic and organic CH4 will be mainly generated early in the life of a geothermal system.

Press the Back button in your browser, or search again.

Copyright 2001, Stanford Geothermal Program: Readers who download papers from this site should honor the copyright of the original authors and may not copy or distribute the work further without the permission of the original publisher.