The contribution of mechanoradical reactions to crustal hydrogen generation

Abstract

Hydrogen (H₂) generation within the Earth's crust contributes significantly to abiotic organic synthesis and the sustenance of deep microbial ecosystems. While natural H₂ is recognized as a potential primary energy resource, current exploration models focus solely on two sources, serpentinization and water radiolysis, leaving other H₂-generating processes largely unexplored. This study investigates mechanoradical reactions specifically, H₂ production resulting from mineral grinding in the presence of water, a process analogous to rock abrasion along fault planes during earthquakes. We performed laboratory experiments simulating these conditions by grinding quartz under controlled conditions of fluid pH, ionic strength, water/rock (W/R) ratio, and grinding energy. Our results show that H₂ production is significantly affected by these parameters. Notably, grinding in acidic environments (6< pH < 4) tripled H₂ production compared to alkaline conditions (pH >8). Increasing the W/R ratio from 0.1 to 1 resulted in an 18-fold enhancement of H₂ production. Additionally, a linear relationship was observed between H₂ production and the grinding energy applied. Extrapolating our findings to natural fault movements, we estimate that mechanoradical reactions during earthquakes of magnitude greater than 4 generate approximately 1.45 $\times$ 10¹³ mol yr^-1 of H₂. Although this production rate involved significant incertainties related to the model assumptions and may not be directly compared to serpentinization and radiolysis - since these H₂-producing processes operate on different time and space scales - our study underscores the importance of including mechanoradical processes in models of crustal H₂ fluxes. Recognizing these reactions expands our understanding of subsurface H₂ generation and its contributions to geochemical and microbial processes in the Earth's crust.