Phase Relations in the Carbon Dioxide/Water System at Pressures and Temperatures Relevant to Subducting Slabs

Abstract:

Aqueous solutions of carbon dioxide, produced in situ, may be presumed to be important vehicles for both carbon transport and metasomatism in subducting slabs. Due to the many difficulties of working with these solutions, data to date are scarce; previously reported observations of fluid-fluid phase behavior have been limited to pressures lower than 3 kbar. We report measurements of this system, including fluid-fluid and fluid-solid equilibria, to pressures of 70 kbar and temperatures of 400°C. Carbon dioxide-to-water ratios, as loaded into the diamond-anvil cell, are quantified through isotopic exchange.

As pressures are raised above 3 kbar, carbon dioxide and water at first become increasingly immiscible, but with further increase of pressure this process reverses to produce a homogeneous fluid. This reversal may be indicative of a significant change in chemical character of the final fluid. A surface of solubility isotherms, deriving from currently available data, is shown in the accompanying figure. The black and white dashed line indicates the critical curve. Data below 3 kbar are from Todheide and Franck [Z. physick Chem. Neue Folge, 37, 387 (1963)].

At lower pressures (<3 kbar) addition of salts (e.g., NaCl) is known to stabilize the existence of the two-fluid regime. The effects of salts, such as would be pertinent to metasomatism, are as yet unknown at the higher pressures of the current experiments; the observed miscibility reversal in pure carbon dioxide/water argues that such effects are unlikely to be well estimated on the basis of lower pressure trends.