A new experimental method has been developed for the investigation of kinetically-rapid devolatilization reactions at pressures up to approximately 8 kbar. Using a volumometer system within an internally-heated pressure vessel, these reactions can be detected by monitoring volume changes of encapsulated samples. Thus, for example, devolatilization increases the volume of an experimental sample, and this increase can be measured electronically. An important advantage of our procedure is that reactions are monitored during experimentation, thereby avoiding quench problems that can occur with conventional hydrothermal techniques.

We have employed our volumetric method to obtain the following equilibrium brackets for the reaction brucite ⇌ periclase + water: 690–725°C at 4.0 kbar, 720–745°C at 5.1 kbar, 740–765°C at 6.1 kbar, and 785–806°C at 8.1 kbar. These results compare favorably with those of recent studies, and they are also in accord with an equilibrium pressure-temperature curve derived from thermochemical data.

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