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Controls on thallium uptake during hydrothermal alteration of the upper ocean crust

Controls on thallium uptake during hydrothermal alteration of the upper ocean crust
Controls on thallium uptake during hydrothermal alteration of the upper ocean crust
Hydrothermal circulation is a fundamental component of global biogeochemical cycles. However, the magnitude of the high temperature axial hydrothermal fluid flux remains disputed, and the lower temperature ridge flank fluid flux is difficult to quantify. Thallium (Tl) isotopes behave differently in axial compared to ridge flank systems, with Tl near-quantitatively stripped from the intrusive crust by high temperature hydrothermal reactions, but added to the lavas during low temperature reaction with seawater. This contrasting behavior provides a unique approach to determine the fluid fluxes associated with axial and ridge flank environments. Unfortunately, our understanding of the Tl isotopic mass balance is hindered by poor knowledge of the mineralogical, physical and chemical controls on Tl-uptake by the ocean crust.

Here we use analyses of basaltic volcanic upper crust from Integrated Ocean Drilling Program Hole U1301B on the Juan de Fuca Ridge flank, combined with published analyses of dredged seafloor basalts and upper crustal basalts from Holes 504B and 896A, to investigate the controls on Tl-uptake by mid-ocean ridge basalts and evaluate when in the evolution of the ridge flank hydrothermal system Tl-uptake occurs.

Seafloor basalts indicate an association between basaltic uptake of Tl from cold seawater and uptake of Cs and Rb, which are known to partition into K-rich phases. Although there is no clear relationship between Tl and K contents of seafloor basalts, the data do not rule out the incorporation of at least some Tl into the same minerals as the alkali elements. In contrast, we find no relationship between the Tl content and either the abundance of secondary phyllosilicate minerals, or the K, Cs or Rb contents in upper crustal basalts. We conclude that the uptake of Tl and alkali elements during hydrothermal alteration of the upper crust involves different processes and/or mineral phases compared to those that govern seafloor weathering. Furthermore, a correlation between the Tl and S concentrations of upper crustal basalts from Holes U1301B, 504B and 896A indicates that Tl is primarily incorporated into secondary sulfides. Given that some of these secondary sulfides formed as a result of microbial sulfate reduction, microbial action is at least indirectly responsible for Tl-uptake.

Thallium-enrichment of ridge flank basalts requires a Tl-bearing fluid and physical, chemical and microbial conditions that favor secondary sulfide formation. Uptake of Tl occurs in reducing environments in the background rocks away from fluid flow pathways during early ‘open’ circulation of oxidizing seawater but more pervasively throughout the system during later ‘restricted’ circulation of reducing fluids. The Tl-isotope system is therefore a useful tracer of the fluid flux through both the ‘open’ and ‘restricted’ ridge flank hydrothermal regimes.
0016-7037
25-42
Coggon, Rosalind M.
09488aad-f9e1-47b6-9c62-1da33541b4a4
Rehkämper, Mark
08591eef-a2d5-48de-9fc6-b5c0cf54f6e5
Atteck, Charlotte
8889fd62-5edb-4ec6-af53-044176807d53
Teagle, Damon A.H.
396539c5-acbe-4dfa-bb9b-94af878fe286
Alt, Jeffrey C.
d2e22a46-a2e0-4d56-abbb-37199de80dbc
Cooper, Matthew J.
54f7bff0-1f8c-4835-8358-71eef8529e7a
Coggon, Rosalind M.
09488aad-f9e1-47b6-9c62-1da33541b4a4
Rehkämper, Mark
08591eef-a2d5-48de-9fc6-b5c0cf54f6e5
Atteck, Charlotte
8889fd62-5edb-4ec6-af53-044176807d53
Teagle, Damon A.H.
396539c5-acbe-4dfa-bb9b-94af878fe286
Alt, Jeffrey C.
d2e22a46-a2e0-4d56-abbb-37199de80dbc
Cooper, Matthew J.
54f7bff0-1f8c-4835-8358-71eef8529e7a

Coggon, Rosalind M., Rehkämper, Mark, Atteck, Charlotte, Teagle, Damon A.H., Alt, Jeffrey C. and Cooper, Matthew J. (2014) Controls on thallium uptake during hydrothermal alteration of the upper ocean crust. Geochimica et Cosmochimica Acta, 144, 25-42. (doi:10.1016/j.gca.2014.09.001).

Record type: Article

Abstract

Hydrothermal circulation is a fundamental component of global biogeochemical cycles. However, the magnitude of the high temperature axial hydrothermal fluid flux remains disputed, and the lower temperature ridge flank fluid flux is difficult to quantify. Thallium (Tl) isotopes behave differently in axial compared to ridge flank systems, with Tl near-quantitatively stripped from the intrusive crust by high temperature hydrothermal reactions, but added to the lavas during low temperature reaction with seawater. This contrasting behavior provides a unique approach to determine the fluid fluxes associated with axial and ridge flank environments. Unfortunately, our understanding of the Tl isotopic mass balance is hindered by poor knowledge of the mineralogical, physical and chemical controls on Tl-uptake by the ocean crust.

Here we use analyses of basaltic volcanic upper crust from Integrated Ocean Drilling Program Hole U1301B on the Juan de Fuca Ridge flank, combined with published analyses of dredged seafloor basalts and upper crustal basalts from Holes 504B and 896A, to investigate the controls on Tl-uptake by mid-ocean ridge basalts and evaluate when in the evolution of the ridge flank hydrothermal system Tl-uptake occurs.

Seafloor basalts indicate an association between basaltic uptake of Tl from cold seawater and uptake of Cs and Rb, which are known to partition into K-rich phases. Although there is no clear relationship between Tl and K contents of seafloor basalts, the data do not rule out the incorporation of at least some Tl into the same minerals as the alkali elements. In contrast, we find no relationship between the Tl content and either the abundance of secondary phyllosilicate minerals, or the K, Cs or Rb contents in upper crustal basalts. We conclude that the uptake of Tl and alkali elements during hydrothermal alteration of the upper crust involves different processes and/or mineral phases compared to those that govern seafloor weathering. Furthermore, a correlation between the Tl and S concentrations of upper crustal basalts from Holes U1301B, 504B and 896A indicates that Tl is primarily incorporated into secondary sulfides. Given that some of these secondary sulfides formed as a result of microbial sulfate reduction, microbial action is at least indirectly responsible for Tl-uptake.

Thallium-enrichment of ridge flank basalts requires a Tl-bearing fluid and physical, chemical and microbial conditions that favor secondary sulfide formation. Uptake of Tl occurs in reducing environments in the background rocks away from fluid flow pathways during early ‘open’ circulation of oxidizing seawater but more pervasively throughout the system during later ‘restricted’ circulation of reducing fluids. The Tl-isotope system is therefore a useful tracer of the fluid flux through both the ‘open’ and ‘restricted’ ridge flank hydrothermal regimes.

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More information

Accepted/In Press date: 1 September 2014
e-pub ahead of print date: 16 September 2014
Published date: 1 November 2014
Organisations: Geochemistry

Identifiers

Local EPrints ID: 371737
URI: http://eprints.soton.ac.uk/id/eprint/371737
ISSN: 0016-7037
PURE UUID: 2690157c-e22c-4a8a-81c6-24fdff3ec4ae
ORCID for Rosalind M. Coggon: ORCID iD orcid.org/0000-0002-9228-9707
ORCID for Damon A.H. Teagle: ORCID iD orcid.org/0000-0002-4416-8409
ORCID for Matthew J. Cooper: ORCID iD orcid.org/0000-0002-2130-2759

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Date deposited: 13 Nov 2014 09:57
Last modified: 15 Mar 2024 03:45

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Contributors

Author: Mark Rehkämper
Author: Charlotte Atteck
Author: Jeffrey C. Alt

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