The temporal evolution of magnesium isotope fractionation during hydromagnesite dissolution, precipitation, and at equilibrium

Eric H. Oelkers, Ulf Niklas Berninger, Andrea Pérez-Fernàndez, Jérôme Chmeleff, Vasileios Mavromatis

Publikation: Beitrag in einer FachzeitschriftArtikelForschungBegutachtung

Abstract

This study provides experimental evidence of the resetting of the magnesium (Mg) isotope signatures of hydromagnesite in the presence of an aqueous fluid during its congruent dissolution, precipitation, and at equilibrium at ambient temperatures over month-long timescales. All experiments were performed in batch reactors in aqueous sodium carbonate buffer solutions having a pH from 7.8 to 9.2. The fluid phase in all experiments attained bulk chemical equilibrium within analytical uncertainty with hydromagnesite within several days, but the experiments were allowed to continue for up to 575 days. During congruent hydromagnesite dissolution, the fluid first became enriched in isotopically light Mg compared to the dissolving hydromagnesite, but this Mg isotope composition became heavier after the fluid attained chemical equilibrium with the mineral. The δ26Mg composition of the fluid was up to ∼0.35‰ heavier than the initial dissolving hydromagnesite at the end of the dissolution experiments. Hydromagnesite precipitation was provoked during one experiment by increasing the reaction temperature from 4 to 50 °C. The δ26Mg composition of the fluid increased as hydromagnesite precipitated and continued to increase after the fluid attained bulk equilibrium with this phase. These observations are consistent with the hypothesis that mineral-fluid equilibrium is dynamic (i.e. dissolution and precipitation occur at equal, non-zero rates at equilibrium). Moreover the results presented in this study confirm (1) that the transfer of material from the solid to the fluid phase may not be conservative during stoichiometric dissolution, and (2) that the isotopic compositions of carbonate minerals can evolve even when the mineral is in bulk chemical equilibrium with its coexisting fluid. This latter observation suggests that the preservation of isotopic signatures of carbonate minerals in the geological record may require a combination of the isolation of fluid-mineral system from external chemical input and/or the existence of a yet to be defined dissolution/precipitation inhibition mechanism.

Originalspracheenglisch
Seiten (von - bis)36-49
Seitenumfang14
FachzeitschriftGeochimica et Cosmochimica Acta
Jahrgang226
DOIs
PublikationsstatusVeröffentlicht - 1 Apr 2018

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Fractionation
temporal evolution
Isotopes
Magnesium
magnesium
Dissolution
fractionation
dissolution
isotope
Fluids
fluid
Minerals
mineral
Carbonate minerals
Chemical analysis
carbonate
experiment
Experiments
resetting
Batch reactors

Schlagwörter

    ASJC Scopus subject areas

    • !!Geochemistry and Petrology

    Dies zitieren

    The temporal evolution of magnesium isotope fractionation during hydromagnesite dissolution, precipitation, and at equilibrium. / Oelkers, Eric H.; Berninger, Ulf Niklas; Pérez-Fernàndez, Andrea; Chmeleff, Jérôme; Mavromatis, Vasileios.

    in: Geochimica et Cosmochimica Acta, Jahrgang 226, 01.04.2018, S. 36-49.

    Publikation: Beitrag in einer FachzeitschriftArtikelForschungBegutachtung

    Oelkers, Eric H. ; Berninger, Ulf Niklas ; Pérez-Fernàndez, Andrea ; Chmeleff, Jérôme ; Mavromatis, Vasileios. / The temporal evolution of magnesium isotope fractionation during hydromagnesite dissolution, precipitation, and at equilibrium. in: Geochimica et Cosmochimica Acta. 2018 ; Jahrgang 226. S. 36-49.
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    abstract = "This study provides experimental evidence of the resetting of the magnesium (Mg) isotope signatures of hydromagnesite in the presence of an aqueous fluid during its congruent dissolution, precipitation, and at equilibrium at ambient temperatures over month-long timescales. All experiments were performed in batch reactors in aqueous sodium carbonate buffer solutions having a pH from 7.8 to 9.2. The fluid phase in all experiments attained bulk chemical equilibrium within analytical uncertainty with hydromagnesite within several days, but the experiments were allowed to continue for up to 575 days. During congruent hydromagnesite dissolution, the fluid first became enriched in isotopically light Mg compared to the dissolving hydromagnesite, but this Mg isotope composition became heavier after the fluid attained chemical equilibrium with the mineral. The δ26Mg composition of the fluid was up to ∼0.35‰ heavier than the initial dissolving hydromagnesite at the end of the dissolution experiments. Hydromagnesite precipitation was provoked during one experiment by increasing the reaction temperature from 4 to 50 °C. The δ26Mg composition of the fluid increased as hydromagnesite precipitated and continued to increase after the fluid attained bulk equilibrium with this phase. These observations are consistent with the hypothesis that mineral-fluid equilibrium is dynamic (i.e. dissolution and precipitation occur at equal, non-zero rates at equilibrium). Moreover the results presented in this study confirm (1) that the transfer of material from the solid to the fluid phase may not be conservative during stoichiometric dissolution, and (2) that the isotopic compositions of carbonate minerals can evolve even when the mineral is in bulk chemical equilibrium with its coexisting fluid. This latter observation suggests that the preservation of isotopic signatures of carbonate minerals in the geological record may require a combination of the isolation of fluid-mineral system from external chemical input and/or the existence of a yet to be defined dissolution/precipitation inhibition mechanism.",
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    T1 - The temporal evolution of magnesium isotope fractionation during hydromagnesite dissolution, precipitation, and at equilibrium

    AU - Oelkers, Eric H.

    AU - Berninger, Ulf Niklas

    AU - Pérez-Fernàndez, Andrea

    AU - Chmeleff, Jérôme

    AU - Mavromatis, Vasileios

    PY - 2018/4/1

    Y1 - 2018/4/1

    N2 - This study provides experimental evidence of the resetting of the magnesium (Mg) isotope signatures of hydromagnesite in the presence of an aqueous fluid during its congruent dissolution, precipitation, and at equilibrium at ambient temperatures over month-long timescales. All experiments were performed in batch reactors in aqueous sodium carbonate buffer solutions having a pH from 7.8 to 9.2. The fluid phase in all experiments attained bulk chemical equilibrium within analytical uncertainty with hydromagnesite within several days, but the experiments were allowed to continue for up to 575 days. During congruent hydromagnesite dissolution, the fluid first became enriched in isotopically light Mg compared to the dissolving hydromagnesite, but this Mg isotope composition became heavier after the fluid attained chemical equilibrium with the mineral. The δ26Mg composition of the fluid was up to ∼0.35‰ heavier than the initial dissolving hydromagnesite at the end of the dissolution experiments. Hydromagnesite precipitation was provoked during one experiment by increasing the reaction temperature from 4 to 50 °C. The δ26Mg composition of the fluid increased as hydromagnesite precipitated and continued to increase after the fluid attained bulk equilibrium with this phase. These observations are consistent with the hypothesis that mineral-fluid equilibrium is dynamic (i.e. dissolution and precipitation occur at equal, non-zero rates at equilibrium). Moreover the results presented in this study confirm (1) that the transfer of material from the solid to the fluid phase may not be conservative during stoichiometric dissolution, and (2) that the isotopic compositions of carbonate minerals can evolve even when the mineral is in bulk chemical equilibrium with its coexisting fluid. This latter observation suggests that the preservation of isotopic signatures of carbonate minerals in the geological record may require a combination of the isolation of fluid-mineral system from external chemical input and/or the existence of a yet to be defined dissolution/precipitation inhibition mechanism.

    AB - This study provides experimental evidence of the resetting of the magnesium (Mg) isotope signatures of hydromagnesite in the presence of an aqueous fluid during its congruent dissolution, precipitation, and at equilibrium at ambient temperatures over month-long timescales. All experiments were performed in batch reactors in aqueous sodium carbonate buffer solutions having a pH from 7.8 to 9.2. The fluid phase in all experiments attained bulk chemical equilibrium within analytical uncertainty with hydromagnesite within several days, but the experiments were allowed to continue for up to 575 days. During congruent hydromagnesite dissolution, the fluid first became enriched in isotopically light Mg compared to the dissolving hydromagnesite, but this Mg isotope composition became heavier after the fluid attained chemical equilibrium with the mineral. The δ26Mg composition of the fluid was up to ∼0.35‰ heavier than the initial dissolving hydromagnesite at the end of the dissolution experiments. Hydromagnesite precipitation was provoked during one experiment by increasing the reaction temperature from 4 to 50 °C. The δ26Mg composition of the fluid increased as hydromagnesite precipitated and continued to increase after the fluid attained bulk equilibrium with this phase. These observations are consistent with the hypothesis that mineral-fluid equilibrium is dynamic (i.e. dissolution and precipitation occur at equal, non-zero rates at equilibrium). Moreover the results presented in this study confirm (1) that the transfer of material from the solid to the fluid phase may not be conservative during stoichiometric dissolution, and (2) that the isotopic compositions of carbonate minerals can evolve even when the mineral is in bulk chemical equilibrium with its coexisting fluid. This latter observation suggests that the preservation of isotopic signatures of carbonate minerals in the geological record may require a combination of the isolation of fluid-mineral system from external chemical input and/or the existence of a yet to be defined dissolution/precipitation inhibition mechanism.

    KW - Dynamic equilibrium

    KW - Hydromagnesite

    KW - Isotope exchange

    KW - Mg isotopes

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    U2 - 10.1016/j.gca.2017.11.004

    DO - 10.1016/j.gca.2017.11.004

    M3 - Article

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    JO - Geochimica et Cosmochimica Acta

    JF - Geochimica et Cosmochimica Acta

    SN - 0016-7037

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