Huge Variation in H₂ Generation During Seawater Alteration of Ultramafic Rocks

Water-rock reactions liberate bioavailable energy, a necessary condition for chemotrophic habitability and origins of life. The major minerals of ultramafic (UM) rocks: olivine (ol), orthopyroxene (opx), and clinopyroxene (cpx), are of particular astrobiological interest as they are widespread in the solar system and known to produce significant quantities of H₂ during aqueous alteration (serpentinization). H₂ yields energy to life when oxidized by chemical species common to planetary fluids and is the most deeply rooted metabolite in Earth's phylogenetic tree. However, while field observations and calculations have corroborated the H₂-generating potential of UM rocks, specific formation pathways remain elusive, as the alteration assemblages in natural samples contain substantial heterogeneity. Here we show that variable UM compositions, temperature (T), and the mass ratio of Earth's seawater to rock (w/r) conspire to segregate alteration systems into those that produce supra-mmol H₂, and those that produce sub-µmol H₂. The oxidation of UM Fe(II) to Fe(III) provides the electrons necessary to reduce H₂O to H₂. However, the Fe(III)-bearing phases that facilitate this process form together with more abundant phases competing for the same elements, including Ca, Si, and Fe(II). As a result, H₂ abundance is determined by non-redox-active elements. Maximum H₂ at high T requires low Si and is generally supported by Fe(III)-serpentine and magnetite formation. Andradite formation, which stores Fe(III) but requires elevated Ca abundance, throttles up H₂ production as T decreases in aging systems. These conditions are achieved in ol- and cpx-rich rocks while opx-rich rocks sequester less Fe(III), producing less H₂.