The effect of Mg concentration in silicate glasses on CO 2 solubility and solution mechanism: Implication for natural magmatic systems

Abstract : Following an experimental approach conducted between 0.5 and 1.5 GPa, we investigated the change in CO2 solubility as a function of the XMg (MgO/(MgO+CaO)) for a range of silicate glasses. The synthesised CO2-bearing glasses have XMg up to 0.72, stoichiometric NBO/T (degree of polymerization) up to 2.6 corresponding to highly depolymerized compositions analogues to kimberlites. Several samples were synthesised with 17O enrichment to investigate the CO2 dissolution mechanism via the change in O species environments by NMR spectroscopy. The experimental results show that CO2 solubility increases with NBO/T in agreement with previous works. In addition, increasing XMg strongly decreases CO2 solubility: from 18 to 7 wt.% CO2 as XMg ranges from 0 to 0.6 (1.5 GPa and NBO/T ~ 2). 17O NMR results demonstrate that CO2 molecules dissolve as CO3^2- groups showing a signal at +146 ppm for which the intensity is linearly correlated to the wt.% CO2 determined by Raman. The analysis of the oxygen environments as a function of CO2 content for MgO^NBO (+62 ppm) and CaO^NBO (+103 ppm) show that CO2 dissolves preferentially in the vicinity of Ca2+ atoms. The difference in CO2 solubility is explained by the ability for Mg2+ cations to act as a weak network former and to be present in four-fold coordination or by the stronger affinity of CO2 molecules for Ca2+ rather than for Mg2+. We show that the CO2 solubility is negatively correlated to the melt ionic field strength which reflects the variation in the affinity of CO2 molecules for one cation or another. Strongly depolymerized mantle melts, such as kimberlites, melilitites, nephelinites and basanites will exhibit lower CO2 solubility than currently assumed due to their high MgO content which must imply degassing at greater depth, potentially in the sub-lithospheric mantle.
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Yann Morizet, Michael Paris, David Sifré, Ida Di Carlo, Fabrice Gaillard. The effect of Mg concentration in silicate glasses on CO 2 solubility and solution mechanism: Implication for natural magmatic systems. Geochimica et Cosmochimica Acta, Elsevier, 2017, pp.115-130. ⟨10.1016/j.gca.2016.11.006⟩. ⟨insu-01396868⟩

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