Wetting properties of anorthite aggregates: Implications for fluid connectivity in continental lower crust

Abstract

The connectivity of C‐H‐O fluids in anorthite aggregate was assessed by measurements of the dihedral angle, θ, at conditions of 800–1000°C and 0.8–1.2 GPa for duration of 1–21 days. The longer‐duration experiments show reduction of the effect of crystal anisotropy on pore geometry. Especially, fluid‐filled small pores (<5 μm) in anorthite aggregate quickly approach to textural equilibrium controlled by minimization of surface energies in <3 days. Anorthite aggregates with CO2 fluid show large dihedral angle over 120°. In the anorthite aggregates with aqueous fluid the dihedral angles are over 60° at pressure of 1.0 GPa and decrease across critical angle of 60° with increasing pressure. Over the range of experimental conditions, isopleths of dihedral angle show nearly zero to negative dP/dT slope with increasing temperature. The observed reduction in dihedral angle with increasing pressure and temperature may be attributed to increase in plagioclase solubility in aqueous fluid. Natural anorthite (with small amounts of albite component) shows smaller dihedral angle than pure anorthite. At higher temperature conditions the anorthite content of the run products deviates progressively from that of starting material. This suggests that preferential solubility of albite component relative to anorthite affects the equilibrium fluid distributions of plagioclase by aqueous fluid. Experimental results point to the likelihood that plagioclase‐rich lower continental crust will not allow intergranular fluid flow of C‐H‐O fluids even in the presence of high geothermal gradient.