Why is hydrogen formation in serpentinisation an exothermic reaction? In the process of serpentinisation, protons act as oxidising agents for ferrous ions, which results in the production of dihydrogen gas. How is that possible?
Dihydrogen being a strong reductor, I woud expect this reaction spontaneously going the opposite way.
Under normal circumstances your suggestion might be correct, but bear in mind that the serpentinization reaction takes place under very negative Eh, and very high pH conditions (pH 12 or more). This abnormal combination is right on the boundary of water-hydrogen stability conditions. In hyperalkaline springs, which result from the reaction water + peridotite = serpentine, there is hardly a free molecule of oxygen left in solution.