Multitracer determinatin of apparent groundwater ages in peridotite aquifers within the Samail ophiolite, Sultanate of Oman

Abstract

CO₂ sequestration in the form of carbonate minerals via alteration of oceanic crust and upper mantle is an important part of the global carbon cycle, but the annual rate of CO₂ mineralization is not well quantified. This study aimed to constrain groundwater ages within the Samail ophiolite, Sultanate of Oman. Such ages could provide upper bounds on the time required for ongoing low temperature CO₂ mineralization. While we were able to estimate apparent groundwater ages for modern waters, results from hyperalkaline boreholes and springs were disappointing. Waters from boreholes and hyperalkaline springs within the ophiolite were characterized using multiple environmental tracers including tritium (³H), noble gases (³He, ⁴He, Ne, Ar, Kr, Xe), stable isotopes (δ¹⁸O, δ²H), and chemical parameters (pH, Ca, Mg, DIC, etc.). Shallow peridotite groundwater and samples from boreholes near the mantle transition zone have a pH < 9.3, are 4-40 yrs old, have little to no non-atmospheric He accumulation, NGTs (noble gas temperatures) equivalent to the modern mean annual ground temperature, and stable isotopes within the range of current local precipitation. In contrast, hyperalkaline springs and deeper samples from peridotite boreholes have pH > 10, are pre-H-bomb (older than 1952), have significant non-atmospheric helium accumulation (30-70% of dissolved helium), often are isotopically heavier (enriched in δ¹⁸O), and can have NGTs 6-7 °C lower than the modern ground temperature. These differences suggest that groundwater in deep (>50 m) peridotite aquifers is considerably older than shallow groundwater in peridotite and water in deeper aquifers near the mantle transition zone. Unfortunately, how much older remains an open question. The low NGT of groundwater from one deep (300 m) peridotite borehole indicates it is probably glacial in origin. If so, it must date back to at least the late Pleistocene, the most recent glacial period; He accumulation suggests it could be from 20-220 ka. The inefficacy of this suite of environmental tracers to quantitatively estimate apparent groundwater age for hyperalkaline fluids necessitates the use of different techniques. Future work to constrain groundwater ages should utilize a packer system to isolate discrete depth intervals within boreholes and less common environmental tracers such as ³⁹Ar and ⁸¹Kr.

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ORCID for Jürg M. Matter: orcid.org/0000-0002-1070-7371

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