The effect of Mg concentration in silicate glasses on CO2 solubility and solution mechanism: Implication for natural magmatic systems. (1st February 2017)

• Record Type: Journal Article
• Authors: Morizet, Yann; Paris, Michael; Sifré, David; Di Carlo, Ida; Gaillard, Fabrice
• Is Part Of: Geochimica et cosmochimica acta. Volume 198(2017:Feb. 01)
• Page Start: 115
• Page End: 130
• Publication Date: 2017-02-01
• Subjects: CO2 solubility in low silica melt -- 17O NMR spectroscopy -- Melt Ionic Field Strength; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2016.11.006
• Languages: English
• ISSNs: 0016-7037

Abstract

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 17 O 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). 17 O 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 Mg O NBO (+62 ppm) and Ca O NBO (+103 ppm) show that CO2 dissolves preferentially in the vicinity of Ca 2+ atoms. The difference in CO2 solubility is explained by the ability for Mg 2+ cations to act as a weak network former and to be present in fourfold coordination or by the stronger affinity of CO2 molecules for Ca 2+ rather than for Mg 2+ . 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.