Origin of Methane and Other Hydrocarbons in Magmatic Systems
Mentor:David Hilton, Professor of Geochemistry, UC San Diego
The study of magmatic systems provide scientists an ideal vantage point to peer inside the Earth and help advance the scientific understanding of our home planet. The origin of C-bearing volatile gases, such as methane, can be determined through their isotopic and relative abundance characteristics. Methane production mechanisms are divided into two main classes: biogenic and abiogenic. Biogenic methane can be produced by microbial processes or by thermogenic degradation of organic matter in sedimentary rocks. Abiogenic methane can be formed by gas-water-rock interactions or by magmatic processes. To gain insight into these mechanisms and processes, geothermal samples were collected from various locations reflecting contrasting geologic settings: these are the Salton Sea Geothermal System in California, the Tengchong Geothermal Province in China, the East African Rift in Ethiopia, and the Central American Volcanic Arc in Costa Rica. We aim to investigate and characterize the carbon isotopic composition of methane at different plate boundaries utilizing geothermal systems as our means to capture deep-seated gases transferred to the Earth’s surface. Primarily, we distinguish between biogenic, thermogenic, and abiogenic methane as the controlling source, or origin, of the gas. In order to study the 13C/12C ratio in methane, a network of glass lines under vacuum pressure are needed to separate, combust, and capture the carbon from geothermal samples prior to analysis using mass spectrometry. Samples collected from the SSGS indicate a strong thermogenic imprint to the methane. Analysis of the TGP revealed enriched abiogenic signatures, ranging from -14‰ to -23‰. Samples obtained from the EAR illustrates abiogenic properties associated with a lower mantle plume. Gases from Costa Rica demonstrates biogenic/thermogenic methane and small amounts of abiogenic methane due to subducting marine carbonates. Further work includes analysis (utilizing different diagnostic techniques) of Long Valley Caldera where methane analysis has not been carried out to date.