Abstract
Carbonic acid (H2CO3) forms in small amounts when CO2 dissolves in H2O, yet decomposes rapidly under ambient conditions of temperature and pressure. Despite its fleeting existence, H2CO3 plays an important role in the global carbon cycle and in biological carbonate-containing systems. The short lifetime in water and presumed low concentration under all terrestrial conditions has stifled study of this fundamental species. Here, we have examined CO2/H2O mixtures under conditions of high pressure and high temperature to explore the potential for reaction to H2CO3 inside celestial bodies. We present a novel method to prepare solid H2CO3 by heating CO2/H2O mixtures at high pressure with a CO2 laser. Furthermore, we found that, contrary to present understanding, neutral H2CO3 is a significant component in aqueous CO2 solutions above 2.4 GPa and 110 °C as identified by IR-absorption and Raman spectroscopy. This is highly significant for speciation of deep C–O–H fluids with potential consequences for fluid-carbonate-bearing rock interactions. As conditions inside subduction zones on Earth appear to be most favorable for production of aqueous H2CO3, a role in subduction related phenomena is inferred.
Highlights
The first set of experiments, H2O/CO2 mixtures were compressed to the desired pressure and heated by irradiation with a CO2 laser
We have shown that molecular H2CO3 appears to be an integral component of CO2/H2O mixtures at and above pressures of 2.4 GPa at elevated temperature
The relative abundance of H2CO3 and its dissociation products HCO3− and CO32− in an aqueous solution of CO2 is largely determined by the dielectric constant (ε) and the autodissociation constant Kw of its host solvent H2O, and both are complex functions of pressure (p) and temperature (T)[23,24]
Summary
The first set of experiments, H2O/CO2 mixtures were compressed to the desired pressure and heated by irradiation with a CO2 laser. The laser was focused to irradiate only a fraction of the sample In this way, the behavior of the system at high T can be investigated in the center of the heated area while phenomena of the transition down to room temperature can be investigated on the hot spot periphery. The stability of the solid product with respect to pressure was investigated by compression to 25 GPa after synthesis at 4.3 GPa. In the second set of experiments, the sample was heated resistively to up to 280 °C after compression to the desired pressure. The evolution of the system was monitored in situ by IR absorption spectroscopy. In both sets of heating experiments, the pressure range from 1.5 to 4.6 GPa was covered
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