Dissolved carbon dioxide in basaltic glasses: concentrations and speciation

Abstract

Carbon dioxide dissolved in both synthetic Ca±Mg-bearing silicate glasses and natural basaltic glasses has been characterized using infrared spectroscopy. CO_2 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). This speciation contrasts with the case of CO_2-bearing sodium aluminosilicate glasses, which contain both dissolved molecular CO_2 and dissolved Na-carbonate ionic-complexes. The difference in speciation in Ca±Mg-bearing melts may result in part from a higher activity of oxygens that react with CO_2 molecules to produce carbonate. Dissolved CO_2 contents of natural basaltic glasses can 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 CO_2 contents in natural basaltic glasses since it is non-destructive, can be aimed at regions of glass a few tens of microns in size, and can discriminate 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 CO_2, measured using the infrared technique. These concentrations are lower than most previous reports for similar basaltic glasses. No general relationship is observed between dissolved CO_2 content and depth of magmatic eruption, although some correlation might be present in restricted geographic locales.