Investigating native H2 generation by alteration of Fe-rich olivine within Precambrian granitoid crust through HREM

International audienceNatural hydrogen generation has long been believed to only occur during serpentinization of ultramafic to mafic rocks at mid-oceanic ridges or onto ophiolitic complexes. Nevertheless, several studies have shown that H2 could also be produced in the middle of continents, within geologic formations overlying ancient cratonic basements. To determine the role of basement rocks in producing native H2 through redox reactions, we studied drill cores obtained from the H2-emitting DR1-A well, located in Kansas (USA). In this area the basement is mainly composed of Precambrian granitoids rich in ferromagnesian minerals. Petrographic observations of samples from a depth of 452 m within the basement show fractured olivine distributed in a matrix of amphibole, pyroxene, feldspar, quartz and oxides. As revealed by SEM, these Fe-rich olivines (fayalite, Fe2SiO4) are crosscut by veins filled with two types of phyllosilicates, more or less associated with iron oxides. XANES spectroscopy at the Fe L-edge showed that these phases differ in ferric iron content: the external part of the veins exhibits almost 30% of ferric iron vs only 20% for the center part. Transmission electron microscopy investigations conducted on ultrathin FIB sections extracted across these veins show that both phyllosilicates have high Fe/(Fe+Mg) values. Electronic diffraction reveals that these two phyllosilicates are a mixture of/ interstratified chlorite-serpentine. They appear to have different crystallization trend suggesting they have been formed at different moment of times. The one of the borders is sharing a clear crystallographic link with the Fe-rich olivine suggesting that it may have formed first, followed by the crystallization of the center of the vein. Those phases bear chemical and textural resemblances to clays minerals observed on various olivine-rich meteorites. Although atypical in such geological context, the presence of fayalite and chlorite and serpentine with a heterogeneous but significant ferric iron content strongly suggests water-rock interactions leading to the local generation of H2. Furthermore, Precambrian granitoids rich in ferromagnesian minerals are present on every continent. They could correspond to potential mineral sources for native hydrogen production through redox reactions