A NMR and molecular dynamics study of CO2-bearing basaltic melts and glasses

Abstract

International audienceThe presence of volatile, especially carbon dioxide (CO2), in silicate liquids is considered as a key parameter to magmatic degassing and eruptive processes. Unfortunately, due to experimental difficulties, our current knowledge on the CO2 effect on silicate melt structure is weak and relies on the observation of ex-situ recovered CO2-bearing glasses.In the present work, we confront the results obtained from NMR spectroscopic observations of glass synthesised at pressure between 0.5 and 3.0 GPa and theoretical investigations from first-principles molecular dynamics (FPMD) simulations conducted at 5.0 and 8.0 GPa on high temperature melt for a simplified basaltic composition.The results obtained on the aluminosilicate framework (molar fraction of silicon species and Al average coordination number) suggest that both ex-situ and in-situ results compare adequately. The results are in agreement with our current knowledge on the change in aluminosilicate melt/glass structure with changing intensive conditions. Increasing pressure from 0.5 to 8.0 GPa induces 1) an increase in the average Al coordination number from 4.1 to almost 5.0 and 2) an increase in the degree of polymerisation with NBO/Si changing from 1.30 to 0.80.The presence of CO2 does not seem to induce a dramatic change on both the average Al coordination number and the NBO/Si. FPMD simulations performed with 0 and 20 wt.% CO2 at 8.0 GPa result in a change from 4.84 to 4.96 for the average Al coordination number and in a change from 0.87 to 0.80 for the NBO/Si value, respectively.On the contrary, there is a lack of consistency in between the CO2 speciation obtained from NMR spectroscopy and from FPMD simulations. Whereas the analysis of glasses does not reveal the presence of CO2mol species, the FPMD simulation results suggests the existence of a small proportion of CO2mol. Further work with in-situ experimental approach is therefore required to explain the observed lack of consistency between the CO2 speciation in glass and in high temperature melt with basaltic composition

The effect of Mg concentration in silicate glasses on CO2 solubility and solution mechanism: Implication for natural magmatic systems

“…This underestimation might reflect several CO 3 2environments consisting of that contributing to the peak at +146 ppm and an additional unidentified one. This hypothesis is consistent with the fact that several dissolution mechanisms for CO 2 as CO 3 2are invoked (Fine and Stolper, 1985;Kohn et al, 1991;Kubicki and Stolper, 1995;Tossell, 1995;Brooker et al, 1999Brooker et al, , 2001bMorizet et al, 2014aMorizet et al, , 2015.…”

“…In the present state we prefer to assign the +140 ppm peak to CO 3 2groups since 1) the nature of carbonate groups is probably of different kinds (i.e. free ionic carbonates or carbonates connected to non-bridging oxygens; see Morizet et al, 2015) and 2) there is no relevant literature on the present NMR signal. This assignment is corroborated by the results shown in Figure 4 and 5.…”

“…change in the degree of polymerization or not; Moussallam et al, 2016a) is not clear. Recent works (Morizet et al, 2015;Moussallam et al, 2016a) suggests that CO 2 forms free ionic carbonate clusters such as CO 3 2-M n+ . Brooker et al (2001b) and Morizet et al (2014a; suggests that CO 2 molecules might use available Non-Bridging Oxygen (NBO) present in the melt to form NBO-CO 3 2-M n+ units.…”

“…The high-pressure experiments were conducted in end-loaded piston-cylinder apparatus at pressure between 0.5 and 1. with an accuracy of ±5°C. We used a special design for the 3/4 inch pressure plate with holes drilled into the plate to ensure higher quench rate (>200°C/s; Morizet et al, 2015). The run duration was at least 30 min to ensure equilibrium (Mysen and Seitz, 1975;Jakobsson, 1997;Brooker et al, 1999), followed by an isobaric quench.…”

Magmatic carbon outgassing and uptake of CO2 by alkaline waters

“…The speciation of carbon dioxide in glasses (i.e., at surface P-T conditions) however is not necessarily that of the melt (i.e., at depth). Evidence from annealing experiments (Morizet et al 2001;Nowak et al 2003), molecular dynamics simulation studies (Guillot and Sator 2011;Morizet et al 2015;Moussallam et al 2016) and in situ spectroscopy studies (Konschak and Keppler 2014) show that the CO 2 + O 2-= CO 3 2equilibrium in the melt shifts toward CO 2 with increasing temperature and that in the melt, molecular CO 2 is present even in basaltic and carbonated melt compositions.…”