Timescales and rates for peridotite-groundwater reactions in the Samail Ophiolite, Sultanate of Oman

Abstract:

The peridotite section of the Samail Ophiolite, Sultanate of Oman is a site of ongoing low temperature serpentinization and carbonation. We present geochemical data for waters collected from boreholes in the peridotite to help describe subsurface water-rock reactions and rates. We constrained groundwater age using ³H-³He dating, He accumulation, dissolved noble gas thermometry, and stable isotopes (δ²H, δ¹⁸O). Dissolved gas samples were collected from boreholes and used to estimate ongoing serpentinization rates.

Boreholes in peridotite contain two water types: Type I Mg²⁺-HCO₃^- water and Type II Ca²⁺-OH^- water. All of our groundwater dating techniques suggest boreholes with Type I water contain modern recharge. Type I boreholes have ³H-³He dates of 0-40 years, ³He/⁴He ratios similar to the atmosphere, and low levels of He accumulation. Noble gas thermometry shows recharge temperatures of 32^oC, the modern mean annual ground temperature. Stable isotopes in Type I waters fall between the northern and southern local meteoric water lines, similar to other modern northern Oman groundwaters (Weyhenmeyer et al., Wat. Resour. Res., 2002). In contrast, Type II hyperalkaline groundwaters from boreholes in peridotite appear to be significantly older. Waters have pre-bomb ³H levels, and significant He accumulation from sources such as crustal production from radioactive decay or mantle He. Noble gas temperatures range from 30^oC to 25^oC, with the cooler samples potentially having recharged during a glacial period. Stable isotopes for Type II waters plot either between the northern and southern meteoric water lines or are enriched in δ¹⁸O. The enrichment may be due to evaporation or a change in the source of local water vapor during glacial periods.

Dissolved gases from boreholes with Type I water are primarily N₂, while gases from boreholes with Type II water include millimolar concentrations of H₂, CH₄, or both. These dissolved gases suggest the Type II boreholes are in or near areas of active low temperature serpentinization. Serpentinization rates calculated from dissolved gas concentrations and groundwater flow vary from 4x10^-8 to 2x10^-6 volume fraction peridotite serpentinized per year.