Low temperature volatile production at the Lost City Hydrothermal Field, evidence from a hydrogen stable isotope geothermometer

Although commonly utilized in continental geothermal work, the water-hydrogen and methane-hydrogen isotope geothermometers have been neglected in hydrothermal studies. Here we report delta D-CH4 and delta D-H-2 values from high-temperature, black smoker-type hydrothermal vents and low-temperature carbonate-hosted samples from the recently discovered Lost City Hydrothermal Field. Methane deuterium content is uniform across the dataset at -120 +/- 12 parts per thousand. Hydrogen delta D values vary from -420 parts per thousand to -330 parts per thousand at high-temperature vents to -700 parts per thousand to -600 parts per thousand at Lost City. The application of several geothermometer equations to a suite of hydrothermal vent volatile samples reveals that predicted temperatures are similar to measured vent temperatures at high-temperature vents, and 20-60 degrees C higher than those measured at the Lost City vents. We conclude that the overestimation of temperature at Lost City reflects 1) that methane and hydrogen are produced by serpentinization at > 110 degrees C, and 2) that isotopic equilibrium at temperatures < 70 degrees C is mediated by microbial sulfate reduction. The successful application of hydrogen isotope geothermometers to low-temperature Lost City hydrothermal. samples encourages its employment with low-temperature diffuse hydrothermal fluids. (c) 2005 Elsevier B.V All rights reserved.