The University of Southampton
University of Southampton Institutional Repository

The behaviour of Li and Mg isotopes during primary phase dissolution and secondary mineral formation in basalt

The behaviour of Li and Mg isotopes during primary phase dissolution and secondary mineral formation in basalt
The behaviour of Li and Mg isotopes during primary phase dissolution and secondary mineral formation in basalt
This study presents lithium (Li) and magnesium (Mg) isotope data from experiments designed to assess the effects of dissolution of primary phases and the formation of secondary minerals during the weathering of basalt. Basalt glass and olivine dissolution experiments were performed in mixed through-flow reactors under controlled equilibrium conditions, at low pH (2–4) in order to keep solutions undersaturated (i.e. far-from equilibrium) and inhibit the formation of secondary minerals. Combined dissolution–precipitation experiments were performed at high pH (10 and 11) increasing the saturation state of the solutions (moving the system closer to equilibrium) and thereby promoting the formation of secondary minerals.

At conditions far from equilibrium saturation state modelling and solution stoichiometry suggest that little secondary mineral formation has occurred. This is supported by the similarity of the dissolution rates of basalt glass and olivine obtained here compared to those of previous experiments. The ?7Li isotope composition of the experimental solution is indistinguishable from that of the initial basalt glass or olivine indicating that little fractionation has occurred. In contrast, the same experimental solutions have light Mg isotope compositions relative to the primary phases, and the solution becomes progressively lighter with time. In the absence of any evidence for secondary mineral formation the most likely explanation for these light Mg isotope compositions is that there has been preferential loss of light Mg during primary phase dissolution.

For the experiments undertaken at close to equilibrium conditions the results of saturation state modelling and changes in solution chemistry suggest that secondary mineral formation has occurred. X-ray diffraction (XRD) measurements of the reacted mineral products from these experiments confirm that the principal secondary phase that has formed is chrysotile. Lithium isotope ratios of the experimental fluid become increasingly heavy with time, consistent with previous experimental work and natural data indicating that 6Li is preferentially incorporated into secondary minerals, leaving the solution enriched in 7Li. The behaviour of Mg isotopes is different from that anticipated or observed in natural systems. Similar to the far from equilibrium experiments initially light Mg is lost during olivine dissolution, but with time the ?26Mg value of the solution becomes increasingly heavy. This suggests either preferential loss of light, and then heavy Mg from olivine, or that the secondary phase preferentially incorporates light Mg from solution. Assuming that the secondary phase is chrysotile, a Mg-silicate, the sense of Mg fractionation is opposite to that previously associated with silicate soils and implies that the fractionation of Mg isotopes during silicate precipitation may be mineral specific. If secondary silicates do preferentially remove light Mg from solution then this could be a possible mechanism for the relatively heavy ?26Mg value of seawater. This study highlights the utility of experimental studies to quantify the effects of natural weathering reactions on the Li and Mg geochemical cycles.
0016-7037
5259-5279
Wimpenny, Josh
ba1df67c-5530-4c5f-ae32-dba3de0fcaa9
Gíslason, Sigurður R.
1fba4984-5465-43cc-b3c1-f73cbaa829e0
James, Rachael H.
79aa1d5c-675d-4ba3-85be-fb20798c02f4
Gannoun, Abdelmouhcine
5e98e48d-cfd0-4500-b399-630bdd2fe277
Pogge Von Strandmann, Philip A.E.
2de2f498-427b-4dcd-a435-ae9253ef20a3
Burton, Kevin W.
b17a2651-0697-4369-bfa7-ece9a9f0a3f1
Wimpenny, Josh
ba1df67c-5530-4c5f-ae32-dba3de0fcaa9
Gíslason, Sigurður R.
1fba4984-5465-43cc-b3c1-f73cbaa829e0
James, Rachael H.
79aa1d5c-675d-4ba3-85be-fb20798c02f4
Gannoun, Abdelmouhcine
5e98e48d-cfd0-4500-b399-630bdd2fe277
Pogge Von Strandmann, Philip A.E.
2de2f498-427b-4dcd-a435-ae9253ef20a3
Burton, Kevin W.
b17a2651-0697-4369-bfa7-ece9a9f0a3f1

Wimpenny, Josh, Gíslason, Sigurður R., James, Rachael H., Gannoun, Abdelmouhcine, Pogge Von Strandmann, Philip A.E. and Burton, Kevin W. (2010) The behaviour of Li and Mg isotopes during primary phase dissolution and secondary mineral formation in basalt. Geochimica et Cosmochimica Acta, 74 (18), 5259-5279. (doi:10.1016/j.gca.2010.06.028).

Record type: Article

Abstract

This study presents lithium (Li) and magnesium (Mg) isotope data from experiments designed to assess the effects of dissolution of primary phases and the formation of secondary minerals during the weathering of basalt. Basalt glass and olivine dissolution experiments were performed in mixed through-flow reactors under controlled equilibrium conditions, at low pH (2–4) in order to keep solutions undersaturated (i.e. far-from equilibrium) and inhibit the formation of secondary minerals. Combined dissolution–precipitation experiments were performed at high pH (10 and 11) increasing the saturation state of the solutions (moving the system closer to equilibrium) and thereby promoting the formation of secondary minerals.

At conditions far from equilibrium saturation state modelling and solution stoichiometry suggest that little secondary mineral formation has occurred. This is supported by the similarity of the dissolution rates of basalt glass and olivine obtained here compared to those of previous experiments. The ?7Li isotope composition of the experimental solution is indistinguishable from that of the initial basalt glass or olivine indicating that little fractionation has occurred. In contrast, the same experimental solutions have light Mg isotope compositions relative to the primary phases, and the solution becomes progressively lighter with time. In the absence of any evidence for secondary mineral formation the most likely explanation for these light Mg isotope compositions is that there has been preferential loss of light Mg during primary phase dissolution.

For the experiments undertaken at close to equilibrium conditions the results of saturation state modelling and changes in solution chemistry suggest that secondary mineral formation has occurred. X-ray diffraction (XRD) measurements of the reacted mineral products from these experiments confirm that the principal secondary phase that has formed is chrysotile. Lithium isotope ratios of the experimental fluid become increasingly heavy with time, consistent with previous experimental work and natural data indicating that 6Li is preferentially incorporated into secondary minerals, leaving the solution enriched in 7Li. The behaviour of Mg isotopes is different from that anticipated or observed in natural systems. Similar to the far from equilibrium experiments initially light Mg is lost during olivine dissolution, but with time the ?26Mg value of the solution becomes increasingly heavy. This suggests either preferential loss of light, and then heavy Mg from olivine, or that the secondary phase preferentially incorporates light Mg from solution. Assuming that the secondary phase is chrysotile, a Mg-silicate, the sense of Mg fractionation is opposite to that previously associated with silicate soils and implies that the fractionation of Mg isotopes during silicate precipitation may be mineral specific. If secondary silicates do preferentially remove light Mg from solution then this could be a possible mechanism for the relatively heavy ?26Mg value of seawater. This study highlights the utility of experimental studies to quantify the effects of natural weathering reactions on the Li and Mg geochemical cycles.

This record has no associated files available for download.

More information

Published date: 15 September 2010
Organisations: Marine Geoscience

Identifiers

Local EPrints ID: 172243
URI: http://eprints.soton.ac.uk/id/eprint/172243
ISSN: 0016-7037
PURE UUID: 911d63e6-1df2-4331-a14b-73077b8ccd64
ORCID for Rachael H. James: ORCID iD orcid.org/0000-0001-7402-2315

Catalogue record

Date deposited: 24 Jan 2011 15:27
Last modified: 14 Mar 2024 02:53

Export record

Altmetrics

Contributors

Author: Josh Wimpenny
Author: Sigurður R. Gíslason
Author: Abdelmouhcine Gannoun
Author: Philip A.E. Pogge Von Strandmann
Author: Kevin W. Burton

Download statistics

Downloads from ePrints over the past year. Other digital versions may also be available to download e.g. from the publisher's website.

View more statistics

Atom RSS 1.0 RSS 2.0

Contact ePrints Soton: eprints@soton.ac.uk

ePrints Soton supports OAI 2.0 with a base URL of http://eprints.soton.ac.uk/cgi/oai2

This repository has been built using EPrints software, developed at the University of Southampton, but available to everyone to use.

We use cookies to ensure that we give you the best experience on our website. If you continue without changing your settings, we will assume that you are happy to receive cookies on the University of Southampton website.

×