Chemical reaction diversity of geofluids revealed by hydrothermal experiments under sub‐ and supercritical states

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Abstract
Abstract  Hydrothermal experiments of dissolution of granite and quartz with pure water up to 600°C and 60 MPa were carried out in order to evaluate chemical reaction under sub‐ and supercritical conditions. The supercritical region beyond the critical point for water has been inferred to be a homogeneous state, which does not correspond to either a true liquid phase, or a true vapor phase. Results of dissolution experiments of granite and quartz in a supercritical state (artificially defined as higher pressures and temperatures than the critical point), show this fluid can be subdivided into two apparent phases comprising a ‘liquid‐like’ region and a ‘vapor‐like’ region. Chemical phenomena with respect to dissolution reactions in the ‘liquid‐like’ region are more similar to those of subcritical water, whilst those reactions in the ‘vapor‐like’ region even in the supercritical state are considered to be weak. The critical point of various kinds of geofluids, composed of solutions in the H2O‐CO2‐NaCl system was experimentally determined using a visible type autoclave. Chemical reaction diversity, depending on location of the critical point for a given solution and apparent phase boundary in terms of chemical reaction within the supercritical state, was recognized by hydrothermal experiments.

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