Pore pressure during metamorphism of carbonate rock: effect of volumetric properties of H2O–CO2 mixtures

The molar volume of mixtures of CO₂ and H₂O is a strong function of the fluid composition. Both CO₂ and H₂O participate in the metamorphism of carbonate rocks, resulting in a change in the fluid composition during reaction. One of the effects of the change in composition is the increase in pore-fluid pressure with only small increases in extent of reaction, ;. Pressure calculated from the volumetric properties of CO₂-H₂O mixtures at 400 °C increases greatly with small increases of ; but drops at greater values because of the increase in pore volume as a result of (V_solid. The pore pressure increase at small values of ;, though, readily exceeds the reported tensile strength of carbonate rocks, and the rock cannot sustain significant reaction without fracturing. The result of a small amount of reaction is a fractured rock with increased permeability, which promotes fluid transport.