The water-carbon dioxide miscibility surface to 450 °C and 7 GPa

Abstract

Fluid-fluid immiscibility in the water-carbon dioxide binary system has been measured to a pressure of 7 GPa and temperature of 450 °C, providing the first such data under conditions overlapping those found in subduction zones and the icy worlds of the outer solar system. Water and carbon dioxide were loaded as inhomogeneous mixtures in externally-heated diamond-anvil cells. Mole fractions were determined by isotopic doping of the initial constituents followed by measurement of the equilibrated isotopic content of the decanted samples. Homogenization and de-homogenization of fluids were observed visually for mole fractions of carbon dioxide from 20 to 90 percent. The recorded pressure-temperature points of homogenization were inverted to give a miscibility surface. The path of the critical curve continues to the highest pressures measured, with no indication of an upper critical endpoint. For mole fractions of carbon dioxide less than 40 percent, the minimum temperature necessary to form a homogeneous solution decreases sharply above a compositionally-dependent pressure. Based on measured intensities of Raman lines, this behavior is linked to a change in speciation as the equilibrium shifts from dissolved carbon dioxide to, presumably, either bicarbonate ion or carbonic acid. Published equations of state give poor representations of the miscibility surface, neither the abrupt behavior due to speciation nor the critical curves being correctly predicted. The experimental constraints provided by the current data should allow useful refinements of equations of state over a wide pressure-temperature-compositional space where only extrapolations and molecular dynamic simulations were previously available. Improved equations of state are a prerequisite for better geochemical models for Earth and extraterrestrial ocean worlds.