SIMULATED HYDROGEN CONCENTRATIONS IN LOW-TEMPERATURE TYPE I AND TYPE II SERPENTINIZED FLUIDS PREDICTED FROM MEASURED SOURCE WATERS AT THE OMANI OPHIOLITE

Fluids resulting from serpentinization reactions have unique chemistries. Serpentinization, the aqueous alteration of olivine-rich rocks (e.g. peridotites) results in fluids rich in hydrogen, a fundamental electron-donor for subsurface biology. Even in low-temperature settings (T < 100C), hydrogen is measured in-situ. Fluids derived from serpentinization reactions have a rich investigative history. Two types of fluids are known to result from serpentinization, Type I and Type II fluids. Type I fluids result from near-surface open systems and are characterized by circumneutral waters rich in Mg⁺⁺ and HCO₃^-. In contrast, Type II fluids are thought to result from carbon-poor closed systems and are characterized by highly basic waters rich in Ca⁺⁺ and OH^-. Predictions of H₂ generation in either fluid type are lacking. In this study, we simulate the reaction of Omani ophiolite source waters with peridotites under broad range of reaction temperatures and water-to-rock ratios in both open and closed systems and seek to replicate Type I and Type II fluid chemistries. To our knowledge, this large-scale investigative parameter-space investigation has not been attempted before. We focus on hydrogen generation and carbon transformation due to their links with microbiology. We perform our investigation using EQ3/6, a software package designed for geochemical modeling. Our simulations replicate as closely as possible the “chemical path” of fluids expected in the Omani system. At first, the source waters are reacted with rock in an open system (with respect to CO₂). This simulates near-surface geochemistry and the results represent a close analogy to Type I fluids. In the second step, the fluids resulting from the near-surface simulation are reacted with rock in a system closed to atmospheric CO₂, representing Type II fluids. Finally, Type II fluids are cooled and exposed to the atmosphere. The results of these simulations yield insight into the parameter space most optimal to support subsurface biology in a low-temperature serpentinizing system.