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Chemical weathering processes in the Great Artesian Basin: Evidence from lithium and silicon isotopes

Chemical weathering processes in the Great Artesian Basin: Evidence from lithium and silicon isotopes
Chemical weathering processes in the Great Artesian Basin: Evidence from lithium and silicon isotopes
Variations in lithium and silicon isotope ratios in groundwaters of the Great Artesian Basin in Australia, and the causes of these variations, have been explored. The chemistries of Li and Si in groundwater are influenced by the dissolution of primary phases, the formation of secondary minerals, and the reaction of solid phases with dissolved constituents, while isotopic variations are generated by uptake into clays, which preferentially incorporate the light isotopes. The lithium isotopic composition (expressed as View the MathML source?Li7) of the groundwaters ranges from +9 to +16‰+16‰, and clearly reflects changes in aquifer conditions. Reaction-transport modelling indicates that changes in Li concentrations are principally controlled by the ratio of the weathering rate of primary minerals to the precipitation rate of secondary minerals, whereas View the MathML source?Li7 is affected by the extent of isotope fractionation during secondary mineral formation (which is dependent on mineralogy). The patterns of groundwater Si concentrations and View the MathML source?Si30 values versus flow distance suggest that Si is at steady state in the aquifer. The View the MathML source?Si30 value of most of the groundwater samples is close to ?1‰?1‰, which is significantly lower than the View the MathML source?Si30 value of the reservoir rocks (?0‰?0‰). Since precipitation of clays preferentially removes the light Si isotopes from solution, the most plausible explanation for these low groundwater View the MathML source?Si30 values is addition of Si by dissolution of isotopically light secondary minerals. These data, together with model calculations, show that Li isotopes are extremely sensitive to changes in the chemical and physical conditions in the aquifer, whereas Si is not. Importantly, the model suggests that even in large aquifers with long fluid residence times, where steady-state would be expected to be reached, the concentrations and isotopic fractionation of trace elements are not controlled by Li adsorption. The model developed here provides a basis for using Li isotopes measured in groundwaters and surface waters to constrain weathering processes.
groundwater, weathering, lithium isotopes, silicon isotopes, Great Artesian Basin, Australia
0012-821X
24-36
Pogge von Strandmann, Philip A.E.
2de2f498-427b-4dcd-a435-ae9253ef20a3
Porcelli, Don
25796e44-83b9-47cf-a59d-47479be20af9
James, Rachael H.
79aa1d5c-675d-4ba3-85be-fb20798c02f4
van Calsteren, Peter
3b220d46-ad2c-4a70-91b1-ebae2ee67bb9
Schaefer, Bruce
99e09ebe-4046-478e-8f83-b98cd930e2cd
Cartwright, Ian
5bc40ed5-fc78-4621-bbc4-08e29b42ca97
Reynolds, Ben C.
22eff074-87a5-4ee7-a9e6-c95bd573655a
Burton, Kevin W.
b17a2651-0697-4369-bfa7-ece9a9f0a3f1
Pogge von Strandmann, Philip A.E.
2de2f498-427b-4dcd-a435-ae9253ef20a3
Porcelli, Don
25796e44-83b9-47cf-a59d-47479be20af9
James, Rachael H.
79aa1d5c-675d-4ba3-85be-fb20798c02f4
van Calsteren, Peter
3b220d46-ad2c-4a70-91b1-ebae2ee67bb9
Schaefer, Bruce
99e09ebe-4046-478e-8f83-b98cd930e2cd
Cartwright, Ian
5bc40ed5-fc78-4621-bbc4-08e29b42ca97
Reynolds, Ben C.
22eff074-87a5-4ee7-a9e6-c95bd573655a
Burton, Kevin W.
b17a2651-0697-4369-bfa7-ece9a9f0a3f1

Pogge von Strandmann, Philip A.E., Porcelli, Don, James, Rachael H., van Calsteren, Peter, Schaefer, Bruce, Cartwright, Ian, Reynolds, Ben C. and Burton, Kevin W. (2014) Chemical weathering processes in the Great Artesian Basin: Evidence from lithium and silicon isotopes. Earth and Planetary Science Letters, 406, 24-36. (doi:10.1016/j.epsl.2014.09.014).

Record type: Article

Abstract

Variations in lithium and silicon isotope ratios in groundwaters of the Great Artesian Basin in Australia, and the causes of these variations, have been explored. The chemistries of Li and Si in groundwater are influenced by the dissolution of primary phases, the formation of secondary minerals, and the reaction of solid phases with dissolved constituents, while isotopic variations are generated by uptake into clays, which preferentially incorporate the light isotopes. The lithium isotopic composition (expressed as View the MathML source?Li7) of the groundwaters ranges from +9 to +16‰+16‰, and clearly reflects changes in aquifer conditions. Reaction-transport modelling indicates that changes in Li concentrations are principally controlled by the ratio of the weathering rate of primary minerals to the precipitation rate of secondary minerals, whereas View the MathML source?Li7 is affected by the extent of isotope fractionation during secondary mineral formation (which is dependent on mineralogy). The patterns of groundwater Si concentrations and View the MathML source?Si30 values versus flow distance suggest that Si is at steady state in the aquifer. The View the MathML source?Si30 value of most of the groundwater samples is close to ?1‰?1‰, which is significantly lower than the View the MathML source?Si30 value of the reservoir rocks (?0‰?0‰). Since precipitation of clays preferentially removes the light Si isotopes from solution, the most plausible explanation for these low groundwater View the MathML source?Si30 values is addition of Si by dissolution of isotopically light secondary minerals. These data, together with model calculations, show that Li isotopes are extremely sensitive to changes in the chemical and physical conditions in the aquifer, whereas Si is not. Importantly, the model suggests that even in large aquifers with long fluid residence times, where steady-state would be expected to be reached, the concentrations and isotopic fractionation of trace elements are not controlled by Li adsorption. The model developed here provides a basis for using Li isotopes measured in groundwaters and surface waters to constrain weathering processes.

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More information

Published date: 15 November 2014
Keywords: groundwater, weathering, lithium isotopes, silicon isotopes, Great Artesian Basin, Australia
Organisations: Geochemistry, Marine Geoscience

Identifiers

Local EPrints ID: 370571
URI: http://eprints.soton.ac.uk/id/eprint/370571
ISSN: 0012-821X
PURE UUID: 2d00096a-5b11-47ef-b80f-881620600aae
ORCID for Rachael H. James: ORCID iD orcid.org/0000-0001-7402-2315

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Date deposited: 29 Oct 2014 16:23
Last modified: 09 Jan 2022 03:28

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Contributors

Author: Philip A.E. Pogge von Strandmann
Author: Don Porcelli
Author: Peter van Calsteren
Author: Bruce Schaefer
Author: Ian Cartwright
Author: Ben C. Reynolds
Author: Kevin W. Burton

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