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Nickel isotope fractionation during adsorption on the calcite surface and coprecipitation with calcite

Abstract : The chemical and isotopic compositions of trace elements in calcite are the basis for most past temperature, atmospheric CO2, and ocean pH paleo reconstructions. The isotope compositions of divalent metals incorporated into the calcite structure also have the ability to record valuable information that reflects the geochemical conditions of formation (Galy et al., 2002; Chang et al., 2004; Beard et al., 2012; Mavromatis et al., 2013; Fantle and Tipper, 2014; Mavromatis et al., 2015; Mavromatis et al., 2019). Moreover, as the equilibrium and kinetic partition of divalent metals between calcite and fluid exhibit contrasting behaviors, it can be expected that their isotope composition in calcite exhibits distinct trends (Schott et al., 2014). Because Ni aqueous speciation strongly depends on fluid pH and alkalinity, the isotopic composition of Ni adsorbed on calcite surface and/or incorporated into the calcite lattice could provide information on the chemical environment in which this mineral originally formed. To calibrate this potentially useful tool, experiments were performed at 25°C to determine Ni isotope fractionation during 1) Ni adsorption on calcite surface as a function of solution pH (from 7.5 to 8.7) and 2) Ni coprecipitation with calcite at pH = 6.2, pCO2 = 1 atm as a function of calcite growth rates (ranging from 10-8.2 to 10-7.3 mol m-2 s-1). Results show that the equilibrium fractionation factor, Δ60Nisolid-solution, for Ni adsorption on calcite is equal to -0.49 ± 0.16‰ and is pH - independent. Light Ni isotopes are also preferentially incorporated in calcite during its coprecipitation. The extent of Ni isotope fractionation decreases from -0.3 to -1 ‰ as the calcite precipitation rate increases from 10-8.2 to 10-7.3 .mol m-2 s-1. This behavior, due to the strong hydration of the Ni2+ aqua ion, may serve to approximate calcite precipitation rates and pCO2 in past oceans.
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Maria Cristina Castillo Castillo Alvarez. Nickel isotope fractionation during adsorption on the calcite surface and coprecipitation with calcite. Mineralogy. Université Paul Sabatier - Toulouse III, 2019. English. ⟨NNT : 2019TOU30047⟩. ⟨tel-02174593v2⟩

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