An experimental investigation of the behavior of carbon dioxide in rhyolitic melt

Citation

Blank, Jennifer Glee (1993) An experimental investigation of the behavior of carbon dioxide in rhyolitic melt. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/tq3x-2059. https://resolver.caltech.edu/CaltechETD:etd-05302007-075656

Abstract

The nature and behavior of CO2 in rhyolitic melt at low to moderate pressures was examined using a variety of experimental and analytical techniques. The results are applicable to and critical for understanding the inventory of carbon in the mantle, the growth and evolution of atmospheres, and the processes governing magma degassing. In this study, Fourier transform infrared (FTIR) spectroscopy was calibrated against vacuum extraction/manometry and used to measure solubilities and diffusivities of CO2 in rhyolitic glasses. The difference in 13C/12C ratios between coexisting CO2 vapor and CO2 dissolved as CO2 molecules in rhyolitic melt was also determined experimentally and measured using vacuum extraction and mass spectrometry. Together, these data establish a basis with which to interpret the interaction of carbon dioxide with silicic systems.

The solubility behavior of CO2 in rhyolitic melt, studied at 750-850°C and pressures up to ~ 1500 bars, obeys Henry's Law. Only molecular CO2 was observed in the glasses under these conditions, but both CO2 molecules and CO3(2-) ions have been detected in a single CO2-bearing rhyolitic glass quenched from much higher pressure (25 kbars) by other workers, suggesting that the reaction CO2 (melt) + O2- (melt) = CO3(2-) (melt) is favored in rhyolitic melts as pressure increases. Using the CO2 solubility data, a thermodynamic model was developed to describe CO2 solubility in rhyolitic melt as a function of temperature and pressure. The solubility of CO2 in rhyolite is approximately 30% lower than that in basalt, in which all dissolved CO2 has been detected as CO3(2-). CO2 and water solubility data can be used to interpret volatile contents of silicic eruption products. Modeled CO2-H2O vapor saturation curves permit estimation of minimum entrapment pressures of rhyolitic melt inclusions.

The diffusive behavior of CO2 in rhyolitic melt was examined through a set of experiments conducted at 450-1050°C and 550-1050 bars in which the silicate glasses were not allowed to fully equilibrate with CO2 vapor. CO2 concentration profiles, determined by FTIR spectroscopy, were fit to an analytical solution of the diffusion equation assuming constant DCO2. DCO2 has a strong, direct correlation with temperature but is only weakly responsive to changes in pressure. The observed Arrhenius relation between diffusivities and temperature over a 600°C interval was used to calculate the activation energy for CO2 diffusion. The relation between the size of a CO2 molecule and its activation energy derived from the experimental results mimics the positive correlation reported for noble gases, suggesting that the diffusive behavior of CO2 in the melt is similar to that of other neutral species. CO2 diffusivity in rhyolitic melt is lower than bulk water diffusivity (at 0.2 wt% total water) for temperatures below 1050°C but is higher at higher temperatures. CO2 diffusivity in rhyolitic melt is similar to that in basaltic melt, but the activation energy for CO2 diffusion in rhyolite is somewhat lower. These diffusion results can be applied to an evaluation of diffusive transport of CO2 in magmas, diffusional fractionation of CO2 and H2O, and growth rates of CO2-rich bubbles in magmas.

The effect of increasing water content on CO2 solubility in rhyolite was examined through another set of experiments involving variable proportions of CO2 and H2O in the vapor phase. Under the conditions of the experiments, which are relevant to degassing of common silicic magmas near the earth's surface, Henry's law is obeyed for both water and carbon dioxide in rhyolitic melts. Thus, the amount of CO2 dissolved in a rhyolitic melt saturated with H2O-CO2 vapor at a given pressure and temperature will be lower than if the vapor were pure CO2 by a factor equal to the ratio of the fugacity of CO2 in the mixed vapor to the fugacity of CO2 in pure CO2 vapor at the same conditions. This is essentially a dilution effect. The same is true for the amount of water dissolved in the melt, except that it is the amount of molecular water that is lowered proportionately to the fugacity of water by this dilution effect. There is no evidence of an enhancement of CO2 solubility in mixed H2O-CO2 systems over systems in which only CO2 is present, as has been reported previously for significantly higher pressures. Further work will be needed to confirm reported non-Henrian behavior at elevated pressures and the implied significance for solubility mechanisms under these conditions, but in the meantime, it appears that efforts to model high level crustal and volcanic phenomena are considerably simplified by the validity of Henry's law for the major volatile species.

The isotopic partitioning of 13C between CO2 vapor and coexisting dissolved CO2 was measured through rhyolite-CO2 experiments at 800-1200°C and 250-1444 bars. The abundance and isotopic composition of CO2 dissolved in the glass were determined by stepped heating and the yields were checked against IR spectroscopic analysis. The 13C/12C of coexisting vapor was determined by direct sampling. No detectable isotopic fractionation between CO2 vapor and CO2 molecules dissolved in rhyolitic melt was observed. These results are very different from those for the system basalt-CO2, in which degassing leaves behind CO3(2-) depleted in 13C dissolved in the melt. The 13C/12C ratios of natural rhyolitic samples offer potential for a direct measurement of the carbon composition of the source region of this magma type.

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