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Composition of hydrothermal fluids and mineralogy of associated chimney material on the East Scotia Ridge back-arc spreading centre

Composition of hydrothermal fluids and mineralogy of associated chimney material on the East Scotia Ridge back-arc spreading centre
Composition of hydrothermal fluids and mineralogy of associated chimney material on the East Scotia Ridge back-arc spreading centre
The East Scotia Ridge is an active back-arc spreading centre located to the west of the South Sandwich island arc in the Southern Ocean. Initial exploration of the ridge by deep-tow surveys provided the first evidence for hydrothermal activity in a back-arc setting outside of the western Pacific, and we returned in 2010 with a remotely operated vehicle to precisely locate and sample hydrothermal sites along ridge segments E2 and E9. Here we report the chemical and isotopic composition of high- and low-temperature vent fluids, and the mineralogy of associated high-temperature chimney material, for two sites at E2 (Dog’s Head and Sepia), and four sites at E9 (Black & White, Ivory Tower, Pagoda and Launch Pad). The chemistry of the fluids is highly variable between the ridge segments. Fluid temperatures were ~350 °C at all vent sites except Black & White, which was significantly hotter (383 °C). End-member chloride concentrations in E2 fluids (532 - 536 mM) were close to background seawater (540 mM), whereas Cl in E9 fluids was much lower (98 - 220 mM) indicating that these fluids are affected by phase separation. Concentrations of the alkali elements (Na, Li, K and Cs) and the alkaline earth elements (Ca, Sr and Ba) co-vary with Cl, due to charge balance constraints. Similarly, concentrations of Mn and Zn are highest in the high Cl fluids but, by contrast, Fe/Cl ratios are higher in E9 fluids (3.8 – 8.1 × 10-3) than they are in E2 fluids (1.5 - 2.4 × 10-3) and fluids with lowest Cl have highest Cu. Although both ridge segments are magmatically inflated, there is no compelling evidence for input of magmatic gases to the vent fluids. Fluid δD values range from 0.2 to 1.5 ‰, pH values (3.02 - 3.42) are not especially low, and F concentrations (34.6 - 54.4 µM) are lower than bottom seawater (62.8 µM). The uppermost sections of conjugate chimney material from E2, and from Ivory Tower and Pagoda at E9, typically exhibit inner zones of massive chalcopyrite enclosed within an outer zone of disseminated sulphide, principally sphalerite and pyrite, in an anhydrite matrix. By contrast, the innermost part of the chimneys that currently vent fluids with lowest Cl (Black & White and Launch Pad), is dominated by anhydrite. By defining and assessing the controls on the chemical composition of these vent fluids, and associated mineralisation, this study provides new information for evaluating the significance of hydrothermal processes at back-arc basins for ocean chemistry and the formation of seafloor mineral deposits.
0016-7037
47-71
James, Rachael H.
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Green, Darryl R.H.
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Stock, Michael J.
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Alker, Belinda J.
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Banerjee, Neil R.
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Cole, Catherine
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German, Christopher R.
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Huvenne, Veerle A.I.
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Powell, Alexandra M.
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Connelly, Douglas P.
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James, Rachael H.
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Green, Darryl R.H.
0d7d86c4-d674-4514-8aa6-259134dc7b57
Stock, Michael J.
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Alker, Belinda J.
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Banerjee, Neil R.
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Cole, Catherine
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German, Christopher R.
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Huvenne, Veerle A.I.
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Powell, Alexandra M.
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Connelly, Douglas P.
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James, Rachael H., Green, Darryl R.H., Stock, Michael J., Alker, Belinda J., Banerjee, Neil R., Cole, Catherine, German, Christopher R., Huvenne, Veerle A.I., Powell, Alexandra M. and Connelly, Douglas P. (2014) Composition of hydrothermal fluids and mineralogy of associated chimney material on the East Scotia Ridge back-arc spreading centre. Geochimica et Cosmochimica Acta, 139, 47-71. (doi:10.1016/j.gca.2014.04.024).

Record type: Article

Abstract

The East Scotia Ridge is an active back-arc spreading centre located to the west of the South Sandwich island arc in the Southern Ocean. Initial exploration of the ridge by deep-tow surveys provided the first evidence for hydrothermal activity in a back-arc setting outside of the western Pacific, and we returned in 2010 with a remotely operated vehicle to precisely locate and sample hydrothermal sites along ridge segments E2 and E9. Here we report the chemical and isotopic composition of high- and low-temperature vent fluids, and the mineralogy of associated high-temperature chimney material, for two sites at E2 (Dog’s Head and Sepia), and four sites at E9 (Black & White, Ivory Tower, Pagoda and Launch Pad). The chemistry of the fluids is highly variable between the ridge segments. Fluid temperatures were ~350 °C at all vent sites except Black & White, which was significantly hotter (383 °C). End-member chloride concentrations in E2 fluids (532 - 536 mM) were close to background seawater (540 mM), whereas Cl in E9 fluids was much lower (98 - 220 mM) indicating that these fluids are affected by phase separation. Concentrations of the alkali elements (Na, Li, K and Cs) and the alkaline earth elements (Ca, Sr and Ba) co-vary with Cl, due to charge balance constraints. Similarly, concentrations of Mn and Zn are highest in the high Cl fluids but, by contrast, Fe/Cl ratios are higher in E9 fluids (3.8 – 8.1 × 10-3) than they are in E2 fluids (1.5 - 2.4 × 10-3) and fluids with lowest Cl have highest Cu. Although both ridge segments are magmatically inflated, there is no compelling evidence for input of magmatic gases to the vent fluids. Fluid δD values range from 0.2 to 1.5 ‰, pH values (3.02 - 3.42) are not especially low, and F concentrations (34.6 - 54.4 µM) are lower than bottom seawater (62.8 µM). The uppermost sections of conjugate chimney material from E2, and from Ivory Tower and Pagoda at E9, typically exhibit inner zones of massive chalcopyrite enclosed within an outer zone of disseminated sulphide, principally sphalerite and pyrite, in an anhydrite matrix. By contrast, the innermost part of the chimneys that currently vent fluids with lowest Cl (Black & White and Launch Pad), is dominated by anhydrite. By defining and assessing the controls on the chemical composition of these vent fluids, and associated mineralisation, this study provides new information for evaluating the significance of hydrothermal processes at back-arc basins for ocean chemistry and the formation of seafloor mineral deposits.

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Accepted/In Press date: May 2014
Published date: 15 August 2014
Organisations: Geochemistry, Marine Geoscience

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Local EPrints ID: 364742
URI: http://eprints.soton.ac.uk/id/eprint/364742
ISSN: 0016-7037
PURE UUID: 83baf3d9-6c0b-4d05-8ca2-bde7d23b3f20
ORCID for Rachael H. James: ORCID iD orcid.org/0000-0001-7402-2315
ORCID for Veerle A.I. Huvenne: ORCID iD orcid.org/0000-0001-7135-6360

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Date deposited: 07 May 2014 16:06
Last modified: 28 Oct 2023 01:59

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Contributors

Author: Darryl R.H. Green
Author: Michael J. Stock
Author: Belinda J. Alker
Author: Neil R. Banerjee
Author: Catherine Cole
Author: Christopher R. German
Author: Veerle A.I. Huvenne ORCID iD
Author: Alexandra M. Powell
Author: Douglas P. Connelly

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