Low-temperature peridotite hydration in the shallow subsurface in Oman

Abstract:

The Samail Ophiolite in Oman contains over 10 trillion tons of peridotite undergoing hydration and carbonation reactions at low temperature [1]. The active coupling between subsurface hydrology, geochemistry and microbiology will soon be accessible via targeted drilling by the International Continental Scientific Drilling Program and the Sloan Foundation, among others. In advance, we collected hyperalkaline fluids with millimolar concentrations of H₂ and CH₄ from 300-meter deep groundwater monitoring wells to characterize low-temperature reaction pathways in the aquifer. We are also conducting low-temperature serpentinization experiments (40°C, 55°C, 100°C) with crushed dunite and harzburgite from Oman to identify rock-derived energy sources for microorganisms. We are measuring H₂ production (up to 400nmol/g), aqueous chemical changes and production of acetate and formate (40-80µM). The aqueous geochemical data from Oman fluids and experiments is coupled with detailed mineralogical investigation of natural samples (XRD, QEMSCAN and synchrotron-based x-ray fluorescence spectroscopy of Fe speciation) to identify the operative reaction pathways. The Oman fluids are rich in dissolved H₂, but there is an absence of magnetite formation associated with this; instead, we observe extensive serpentine. We are currently investigating the possibility that H₂ production is associated with oxidized, Fe³⁺ in low temperature serpentine.

We have enriched stable cultures of methanogenic organisms from deep fluids from site NSHQ04 (pH 10.5, ~1.44 µm/mL CH₄). This exciting discovery points to the possibility of deep subsurface life in Oman subsisting under energetic and C limitation. Quantifying the extent of in-situ methanogenesis will be critical to future investigations of microorganisms inhabiting the Oman aquifer to fully understand subsurface carbon cycling and the potential of microbes to control peridotite weathering rates, CH₄ production, and carbonate formation.

1. Kelemen & Matter PNAS 2008