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Magnesium isotope fractionation during hydrothermal seawater-basalt interaction

Magnesium isotope fractionation during hydrothermal seawater-basalt interaction
Magnesium isotope fractionation during hydrothermal seawater-basalt interaction
Fluid-rock interactions in hydrothermal systems at or near mid-oceanic ridges (MOR) play a major role in determining the composition of the oceanic crust and seawater. To quantify the processes that govern cation exchange in these environments we have experimentally studied the isotopic evolution of δ26/24Mg in the fluid phase during seawater-basalt interaction at 250 and 290 °C. Mass balance constraints indicate that isotopically heavy Mg was preferentially incorporated into non-exchangeable (octahedral) sites in secondary clay minerals such as saponite (Mg-rich smectite), leaving residual fluids enriched in light Mg isotopes. The magnitude of fractionation observed during smectite precipitation in our experiments () ranged from

to

. This observation, which contrasts with the preferential uptake of light Mg isotopes into biogenic and inorganic marine carbonates, highlights the potential utility of Mg isotopes as tracers of the precipitation dynamics of authigenic Mg-silicate and Mg-carbonate phases. Furthermore, although Mg isotopic fractionation is often masked by the almost complete removal of Mg in high temperature marine hydrothermal systems, our experiments demonstrate that it does become significant at lower temperatures where Mg removal by clay formation is incomplete. Under such conditions, this fractionation will create isotopically light fluids due to smectite precipitation, thus potentially represents an important component of the marine Mg isotope inventory.
Geochemical cycles, Mid-ocean ridges, Ocean chemistry, Stable isotopes
0016-7037
21-35
Voigt, Martin
adb0895a-d4f6-49bc-9461-ef390ef9595c
Pearce, Christopher R.
c83b6228-0b64-4f5a-a8ad-e5cd33a11de3
Fries, David M.
4de380eb-2770-43b6-a9f7-5bbb742a5b5b
Baldermann, Andre
adfadac2-a657-4f1e-9dc8-66d2e8589f63
Oelkers, Eric H.
3cf51d71-be44-4bed-803e-3b240bdb147b
Voigt, Martin
adb0895a-d4f6-49bc-9461-ef390ef9595c
Pearce, Christopher R.
c83b6228-0b64-4f5a-a8ad-e5cd33a11de3
Fries, David M.
4de380eb-2770-43b6-a9f7-5bbb742a5b5b
Baldermann, Andre
adfadac2-a657-4f1e-9dc8-66d2e8589f63
Oelkers, Eric H.
3cf51d71-be44-4bed-803e-3b240bdb147b

Voigt, Martin, Pearce, Christopher R., Fries, David M., Baldermann, Andre and Oelkers, Eric H. (2020) Magnesium isotope fractionation during hydrothermal seawater-basalt interaction. Geochimica et Cosmochimica Acta, 272, 21-35. (doi:10.1016/j.gca.2019.12.026).

Record type: Article

Abstract

Fluid-rock interactions in hydrothermal systems at or near mid-oceanic ridges (MOR) play a major role in determining the composition of the oceanic crust and seawater. To quantify the processes that govern cation exchange in these environments we have experimentally studied the isotopic evolution of δ26/24Mg in the fluid phase during seawater-basalt interaction at 250 and 290 °C. Mass balance constraints indicate that isotopically heavy Mg was preferentially incorporated into non-exchangeable (octahedral) sites in secondary clay minerals such as saponite (Mg-rich smectite), leaving residual fluids enriched in light Mg isotopes. The magnitude of fractionation observed during smectite precipitation in our experiments () ranged from

to

. This observation, which contrasts with the preferential uptake of light Mg isotopes into biogenic and inorganic marine carbonates, highlights the potential utility of Mg isotopes as tracers of the precipitation dynamics of authigenic Mg-silicate and Mg-carbonate phases. Furthermore, although Mg isotopic fractionation is often masked by the almost complete removal of Mg in high temperature marine hydrothermal systems, our experiments demonstrate that it does become significant at lower temperatures where Mg removal by clay formation is incomplete. Under such conditions, this fractionation will create isotopically light fluids due to smectite precipitation, thus potentially represents an important component of the marine Mg isotope inventory.

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More information

Accepted/In Press date: 21 December 2019
e-pub ahead of print date: 31 December 2019
Published date: 1 March 2020
Keywords: Geochemical cycles, Mid-ocean ridges, Ocean chemistry, Stable isotopes

Identifiers

Local EPrints ID: 439772
URI: http://eprints.soton.ac.uk/id/eprint/439772
ISSN: 0016-7037
PURE UUID: 822ffcfb-ae69-46e0-82be-db9cbfccf571

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Date deposited: 01 May 2020 16:40
Last modified: 25 Nov 2021 18:32

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Contributors

Author: Martin Voigt
Author: Christopher R. Pearce
Author: David M. Fries
Author: Andre Baldermann
Author: Eric H. Oelkers

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