An isotopic study of the chemical evolution of groundwater in the peridotite aquifers of the Oman-United Arab Emirates ophiolite

Abstract

The low-temperature alteration of peridotite in ophiolites produces two types of fluids, Mg-HCO3- rich, pH <10 water (Type I) in the shallow subsurface, and Ca-OH- rich, pH >10 water (Type II) deeper in the aquifer, through a combination of serpentinisation and carbonation reactions. Upon discharge, the Type II waters reacts with environmental CO2 and precipitate massive carbonate terraces, potentially providing a powerful means to mitigate the climate change resulting from human-induced high levels of atmospheric CO2. The impact of such carbonate precipitation on the global carbon cycle depends on the rock source of the calcium. Carbonate precipitation with Ca2+ derived from the dissolution of carbonate minerals has no net effect on atmospheric CO2, whereas Ca2+ from the dissolution of silicate phases will contribute to a net drawdown. However, the water-rock reactions responsible for the evolution of the groundwater in peridotite aquifers and the mineral phases involved are still poorly understood. In this study, the changes in ion concentrations, 87Sr/86Sr, 44/40Ca and 88Sr in water along the flow paths and the isotopic composition of a selection of rocks and secondary minerals from the Oman-United Arab Emirates ophiolite are used to investigate the water-rock reactions occurring in the ophiolite subsurface. The data reveal that multiple processes are responsible for the observed evolution of Type I and II waters: (1) Dissolution of secondary minerals, such as serpentine and brucite, which have extensively replaced peridotite primary minerals during previous alteration episodes, play an important role in the modern water-rock interactions and in the water chemistry evolution in the peridotite aquifer; (2) Dolomite precipitation affects the dissolved Ca and Mg concentrations in Type I water; and (3) Ca2+ remobilisation associated with Mg2+ exchange during dolomitisation is the main source of Ca2+ in Type II water, preventing any CO2 release from the carbonates by binding the CO32- ions with Mg2+. The original source of Mg2+ from serpentine alteration and the subsurface exchange during dolomitisation ensures that the precipitation of carbonates in hyperalkaline springs is a sink in the global carbon cycle, and indicates that engineered peridotite alteration could be useful as a climate change mitigation strategy.

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

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