In the present study, we explore specific reactions that could lead to native abiotic H 2 generation by hydrothermal alteration of a biotite-rich granite using an approach based on a case study of the deep granitic basement of the Soultz-sous-Forêts geothermal site in the Upper Rhine Graben, France (Fig. 1a). The Soultz-sous-Forêts site hosts an enhanced geothermal …