Carbon Dioxide in Synthetic and Natural Silicate Glasses

Citation

Fine, Gerald Jonathan (1986) Carbon Dioxide in Synthetic and Natural Silicate Glasses. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/xmqc-xv31. https://resolver.caltech.edu/CaltechTHESIS:09032019-144830666

Abstract

Infrared spectroscopy has been used to study the speciation of CO₂ in both synthetic silicate glasses quenched from melts held at high temperatures and pressures and in natural basaltic glasses. In glasses near the NaAlO₂-SiO₂ join, absorption bands resulting from the antisymmetric stretches of both molecular CO₂ (2352 cm^-1) and CO^2-₃ (1610 cm^-1 and 1375 cm^-1) are observed. The latter are attributed to distorted Na-carbonate ionic-complexes. Molar absorptivities for each of the absorption bands have been determined; these molar absorptivities allow the quantitative determination of species concentrations in sodium aluminosilicate glasses with a precision on the order of several percent of the amount present. The accuracy of the method is estimated to be ±15-20% at present.

The ratio of molecular CO₂ to CO^2-₃ in sodium aluminosilicate glasses varies little for each silicate composition over the range of total dissolved CO₂ contents (0-1.5%), pressures (15-33 kbar) and temperatures (1400-1560°C) studied. This ratio is, however, a strong function of silicate composition, increasing both with decreasing Na₂O content along the NaAlO₂-SiO₂ join and with decreasing Na₂O content in peraluminous compositions off the join.

The molar absorptivities determined for sodium aluminosilicate glasses have also been used to measure the concentrations of CO₂ in albitic (NaAlSi₃O₈) glasses quenched from melts equilibrated with CO₂ vapor at high pressures (15-30 kbar) and temperatures (1450-1625°C). The results show that total CO₂ solubility increases with increasing pressure at constant temperature. Both molecular CO₂ and CO^2-₃ concentrations increase with pressure. At constant pressure, the solubility of molecular CO₂ decreases with temperature while the concentration of CO^2-₃ increases. The net effect is that total CO₂ solubility is not significantly dependent on temperature, decreasing slightly with increasing temperature at constant pressure.

The speciation of CO₂ in both synthetic Ca ± Mg-composition glasses and natural basaltic glasses contrasts with the case of CO₂-bearing sodium aluminosilicate glasses. CO₂ is inferred to be dissolved in these glasses as distorted Ca- or Mg-carbonate ionic-complexes that result in unique infrared absorption bands at 1515 cm^-1 and 1435 cm^-1. The molar absorptivities for each of these absorption bands were also determined. No detectable molecular CO₂ is dissolved in these glasses.

Infrared spectroscopic measurements of species concentrations in glasses provide insights into the molecular level processes accompanying CO₂ solution in melts and can be used to test and constrain thermodynamic models of CO₂-bearing melts. CO₂ speciation in silicate melts can be modelled by equilibria between molecular CO₂, CO^2-₃, and oxygen species in the melts. Consideration of the thermodynamics of such equilibria can account for the observed linear relationship between molecular CO₂ and carbonate concentrations in sodium aluminosilicate glasses, the absence of molecular CO₂ in Ca ± Mg silicate glasses, the proposed linear relationship between total dissolved CO₂ content and the activity of CO₂ in melts, and observed variations in CO₂ solubility in melts.

Dissolved CO₂ contents of natural basaltic glasses can also be determined from the intensities of the carbonate absorption bands at 1515 cm^-1 and 1435 cm^-1. The uncertainty of the method is estimated to be ±15% of the amount present. The infrared technique is a powerful tool for the measurement of dissolved CO₂ contents in natural basaltic glasses since it is nondestructive, can be aimed at regions of glass a few tens of micrometers in size, and discriminates between dissolved carbonate and carbon present as carbonate alteration, contained in fluid inclusions or adsorbed on the glass.

A set of submarine basaltic glasses dredged from a variety of locations contain 0-400 ppm dissolved CO₂, measured using the infrared technique. These concentrations are lower than most previous reports for similar basaltic glasses. No general relationship is observed between dissolved CO₂ content and depth of magmatic eruption.

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