Presentation Information
[R6P-01]The Dihedral Transition from a Partial Meliting State to Plutonic Rock State
*Ryoko HIBARA1, Takehiko Hiraga1 (1. University of Tokyo Earthquake Research Institute)
Keywords:
partial melting,Forsterite,Diopside,Plutonic Rock
The diverse textures observed in plutonic rocks may result from a transitional process ranging from textures reflecting solid-liquid equilibrium structures during partial melting to solid-solid equilibrium structures following complete crystallization. In particular, the shapes of the solid-liquid interfaces that develop between the partially melted melt and mineral grains differ significantly from those of the solid-solid interfaces that develop between mineral grains after crystallization; the extent of this transition may be reflected in the dihedral angles between mineral grains and the surrounding interface shapes. Elucidating the details of these transitional structures will lead to the establishment of methods for deriving geological information—such as the cooling history of rocks—from their textural features.
In Hibara (2025, Master’s thesis), rock experiments using high-density polycrystalline samples composed of forsterite (92 vol.%) and diopside (8 vol.%) suggested that the transition from a partial melting rock texture to a plutonic texture was caused by grain-boundary diffusion driven by interfacial energy. On the other hand, the histogram of dihedral angles obtained using a proprietary dihedral angle analysis method showed a distribution that differed from a normal distribution, raising questions about the accuracy of the measurement method. For dihedral angle analysis, we developed a method using MATLAB to accurately measure the local dihedral angle θ from microstructural image data. Using the radii of curvature (r1, r2) and the center coordinates (c1x, c1y) and (c2x, c2y) of each grain boundary, we obtain a triangle with side lengths of √ {(c2x - c1x)2 + (c2y - c1y)2}, r1, and r2, respectively. By applying the law of cosines to this triangle, we calculate θ1 and θ2, and obtain the dihedral angle θ = θ1 + θ2.
In Hibara (2025, Master’s thesis), the coordinates of three points on the grain boundary were obtained manually to measure the curvature of the grain boundary, which resulted in errors depending on the location where the coordinates were obtained. Therefore, in this presentation, we numerically redefined the term “local” based on the grain size distribution and performed dihedral angle analysis. While the dihedral angle immediately after crystallization from partial melting was acute, the dihedral angle of samples held at the subsolidus temperature for an extended period after partial melting reached 105°, which is nearly equivalent to the pre-experimental value of 101°. In this presentation, we will present additional data, compare it with samples that did not undergo partial melting, and discuss the results.
In Hibara (2025, Master’s thesis), rock experiments using high-density polycrystalline samples composed of forsterite (92 vol.%) and diopside (8 vol.%) suggested that the transition from a partial melting rock texture to a plutonic texture was caused by grain-boundary diffusion driven by interfacial energy. On the other hand, the histogram of dihedral angles obtained using a proprietary dihedral angle analysis method showed a distribution that differed from a normal distribution, raising questions about the accuracy of the measurement method. For dihedral angle analysis, we developed a method using MATLAB to accurately measure the local dihedral angle θ from microstructural image data. Using the radii of curvature (r1, r2) and the center coordinates (c1x, c1y) and (c2x, c2y) of each grain boundary, we obtain a triangle with side lengths of √ {(c2x - c1x)2 + (c2y - c1y)2}, r1, and r2, respectively. By applying the law of cosines to this triangle, we calculate θ1 and θ2, and obtain the dihedral angle θ = θ1 + θ2.
In Hibara (2025, Master’s thesis), the coordinates of three points on the grain boundary were obtained manually to measure the curvature of the grain boundary, which resulted in errors depending on the location where the coordinates were obtained. Therefore, in this presentation, we numerically redefined the term “local” based on the grain size distribution and performed dihedral angle analysis. While the dihedral angle immediately after crystallization from partial melting was acute, the dihedral angle of samples held at the subsolidus temperature for an extended period after partial melting reached 105°, which is nearly equivalent to the pre-experimental value of 101°. In this presentation, we will present additional data, compare it with samples that did not undergo partial melting, and discuss the results.