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

Multitracer determinatin of apparent groundwater ages in peridotite aquifers within the Samail ophiolite, Sultanate of Oman
Multitracer determinatin of apparent groundwater ages in peridotite aquifers within the Samail ophiolite, Sultanate of Oman
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.
CO sequestration, environmental tracer, hydrogeology, noble gas, peridotite, water-rock interaction
0012-821X
37-48
Paukert Vankeuren, Amelia N.
302db08a-6d35-4e87-969b-a03e90096866
Matter, Jürg M.
abb60c24-b6cb-4d1a-a108-6fc51ee20395
Stute, Martin
1d6a7971-cf3f-48d4-8962-da892b51b25b
Kelemen, Peter B.
1e6e5819-666e-4011-9c11-3e07279bee7c
Paukert Vankeuren, Amelia N.
302db08a-6d35-4e87-969b-a03e90096866
Matter, Jürg M.
abb60c24-b6cb-4d1a-a108-6fc51ee20395
Stute, Martin
1d6a7971-cf3f-48d4-8962-da892b51b25b
Kelemen, Peter B.
1e6e5819-666e-4011-9c11-3e07279bee7c

Paukert Vankeuren, Amelia N., Matter, Jürg M., Stute, Martin and Kelemen, Peter B. (2019) Multitracer determinatin of apparent groundwater ages in peridotite aquifers within the Samail ophiolite, Sultanate of Oman. Earth and Planetary Science Letters, 516, 37-48. (doi:10.1016/j.epsl.2019.03.007).

Record type: Article

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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Paukert Vankeuren et al EPSL manuscript_final - Accepted Manuscript
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Accepted/In Press date: 5 March 2019
e-pub ahead of print date: 9 April 2019
Published date: June 2019
Keywords: CO sequestration, environmental tracer, hydrogeology, noble gas, peridotite, water-rock interaction

Identifiers

Local EPrints ID: 429664
URI: http://eprints.soton.ac.uk/id/eprint/429664
ISSN: 0012-821X
PURE UUID: 3ba6fee2-6caf-42e8-bd1c-d451db03381e
ORCID for Jürg M. Matter: ORCID iD orcid.org/0000-0002-1070-7371

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Date deposited: 03 Apr 2019 16:30
Last modified: 26 Nov 2021 07:00

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Contributors

Author: Amelia N. Paukert Vankeuren
Author: Jürg M. Matter ORCID iD
Author: Martin Stute
Author: Peter B. Kelemen

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