Subduction and Hydrogen Release: The Case of Bolivian Altiplano

Abstract

Natural hydrogen is known to be generated in the crust by water/rock interactions, especially the oxidation of iron-rich rock or radiolysis. However, other sources, especially deeper ones, exist. In the context of subduction, the dehydration of the slab, the destabilization of the NH4, and the hydration of the mantle wedge above the subducting lithosphere may generate H2. We present here a compilation of the known gases in the central part of the Pacific subduction and the results of a first field acquisition dedicated to H2 measurements in Bolivia between La Paz and South Lipez. Various zones have been studied: the emerging thrust faults of the western borders of the Eastern Cordillera, the Sajama area that corresponds to the western volcanic zone near the Chile border northward from the Uyuni Salar, and finally, the Altiplano-Puna Volcanic Complex in South Lipez. Soil gas measurement within and around the Salar itself was not fully conclusive. North of the Uyuni Salar, the gases are very rich in CO2, enriched in N2 and poor in H2. On the opposite, southward, all the samples contain some H2; the major gas is nitrogen, which may overpass 90% after air correction, and the CO2 content is very limited. On the western border of the Cordillera, the δC13 isotope varies between −5 and −13‰, and it is not surprisingly compatible with volcanic gas, as well as with asthenospheric CO2. The methane content is close to 0, and only a few points reach 1%. The isotopes (−1‰) indicate an abiotic origin, and it is thus related to deep H2 presence. The high steam flow in the geothermal area of South Lipez combined with the H2 content in the water results in at least 1 ton of H2 currently released per day from each well and may deserve an evaluation of its economic value. The nitrogen content, as in other subduction or paleo-subduction areas, questions the slab alteration.