The fine-structure of the dehydration boundary of brucite (Mg(OH)2) up to 3.5 GPa - indication of anomalous PVT behaviour of supercritical H2O

Abstract

Liquid and supercritical H 2 O, the most abundant and important non-crystalline component in rock forming processes, is assumed to exhibit continuous PVT behaviour at high pressures and temperatures. However, a number of experimental observations at high pressure and temperatures challenges this concept. Direct measurements of PVT properties of H 2 O at supercritical conditions are very difficult. Information on the PVT behaviour at these P-T conditions may be gained by certain dehydration reactions. An experimental re-examination of the reaction Mg(OH) 2 (brucite) = MgO (periclase) + H 2 O has been conducted between 0.4 to 3.5 GPa and 650 to 1200 °C. The study was performed with a piston cylinder apparatus using the in-situ differential pressure analysis (DPA)-technique. The data reveal that the P-T slope of the equilibrium boundary is not continuous but exhibit a fine-structure which is characterised by three inflections at 1.1 GPa/830 °C, 1.9GPa/945 °C and 2.7 GPa/1040 °C. Because the solid phases, Mg(OH) 2 and MgO, exhibit no structural changes at these conditions, these effects are thought to be due to anomalous PVT behaviour of supercritical H 2 O.