EXPERIMENTAL STUDY OF THE TEXTURAL DEVELOPMENT OF IGNEOUS ROCKS IN THE LATE STAGE OF CRYSTALLIZATION: THE IMPORTANCE OF INTERFACIAL ENERGIES UNDER NON-EQUILIBRIUM CONDITIONS

Abstract

Textural development of igneous rocks in the late stage of crystallization was experimentally studied using the diopside-anorthite and diopside-forsterite-anorthite systems. Isothermal, cooling, and heating experiments were performed within the temperature range in which the melt fraction was low (less than 25 vol%). Dihedral angles at solid-melt-solid triple junctions were measured as the indicator of the interfacial energy ratio (solid-melt interfacial energy/solid-solid grain boundary energy). In isothermal experiments, the dihedral angle (i.e. interfacial energy ratio) was large at low temperatures and small at high temperatures. The dihedral angle increased during cooling and decreased during heating compared to the equilibrium value. In cooling, crystal clustering and melt segregation were observed when the dihedral angle increased. The observed clustering threshold (dihedral angle~58°) in cooling experiments almost corresponded to the theoretical threshold (60°) which causes crystal clustering and melt segregation to minimize total interfacial energy under equilibrium conditions. These experimental results suggest that the change of dihedral angle under non-equilibrium conditions is a substantial phenomenon depending on the actual change of the interfacial energy ratio. Many kinds of clustering textures are observed in natural igneous rocks. Glomeroporphyritic texture in volcanic rock and clustering texture in granite were observed as examples of clustering textures in natural igneous rocks. Owing to the analogy between the natural clustering textures and experimental textures, a textural diversification process depending on interfacial energies is suggested.