Multitracer determination 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 (3 H), noble gases (He, 4 He, Ne, Ar, Kr, Xe), stable isotopes (δ 18 O, δ 2 H), 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 years 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 δ 18 O), and can have NGTs 6-7 o 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 39 Ar and 81 Kr.

Measurement of Volume Change and Mass Transfer During Serpentinization: Insights From the Oman Drilling Project

“…Fluids collected at surface seeps or in wells in the Oman ophiolite generally display high Na, Cl, Sr, and Ca concentrations (Chavagnac et al, ; Canovas et al, ; Paukert et al, ; Paukert Vankeuren et al, ), the same as the elements that display a progressive decrease from the main vein in the sample studied here. Modern water sources and seawater both contain all these components at relatively high concentration (Millero et al, ; Paukert Vankeuren et al, ). The time of serpentinization can thus not be determined with the data presented here.…”

An Overview of the Status and Challenges of CO2 Storage in Minerals and Geological Formations

“…Experiments with olivine as the alkalinity source used NaHCO 3 as a catalyst that buffers the pH (O'Connor et al, 2005;Chizmeshya et al, 2007;Gadikota et al, 2014). In the natural reactive flow of water through ultramafic rocks, a pH-swing occurs spontaneously making both dissolution of the alkalinity source and precipitation of carbonate possible (Bruni et al, 2002;Paukert et al, 2012;Canovas et al, 2017;Paukert Vankeuren et al, 2019;Leong and Shock, under review). Carbon mineralization rates of serpentine may be increased by heat treatment, which removes hydroxyl groups from the mineral and increases the rate of dissolution and carbon mineralization (e.g., O'Connor et al, 2005;Li et al, 2009;Larachi et al, 2012).…”

Groundwater flow characterization of an ophiolitic hard-rock aquifer from cross-borehole multi-level hydraulic experiments

“…Prior sampling and analysis of groundwater in a nearby government monitoring borehole , which is situated in the same catchment as the observatory, reveal the occurrence of hyperalkaline (pH 11-12), Ca(OH) 2 -rich waters, which are the product of extensive water-rock interaction (e.g., Paukert et al, 2012;Miller et al, 2016;Rempfert et al, 2017;Paukert-Vankeuren et al, 2019). -the lower pressure port (P 1 ) that measures the pressure below the lower packer;…”

“…Oman (Paukert-Vankeuren et al, 2019)) because of the high reactivity of the rock-forming minerals which makes some water sources unsuitable for human use. Ophiolites are composed of rocks such as basalt, dolerite, gabbro and peridotite, implying that they are characterized by the coexistence of different lithological units, which can be fractured at many scales due to cooling and (multiple) chemical alteration mechanisms.…”