14:15 〜 14:30
[MIS13-14] 新たな地質速度計をもとめて：フェーズフィールド法を用いたオンファス輝石の逆位相領域成長モデリング
★招待講演
キーワード：オンファス輝石、逆位相領域、地質速度計、フェーズフィールド法
To decipher intimate physical relationships between high-pressure metamorphism of subducted slabs and their exhumation mechanism, it is crucial to obtain short-period (< 106 yr) information on the metamorphic processes from natural samples. In particular, relatively low-temperature eclogite in orogenic belts is the most versatile metamorphosed oceanic crust. Eclogite's omphacite preserves unique disequilibrium microtexture called antiphase domains (APDs), which is formed due to cation ordering in the individual M1 and M2 sites. Because cation ordering in omphacite is basically promoted by thermal annealing, intensive analyses of growth and coarsening processes of APDs have potential to understand eclogitization kinetics.
In this study, we performed APD growth modeling with a phase-field approach, which has been widely used in metallurgy. Considering incomplete cation ordering of natural omphacite in low-temperature eclogite from Syros (Fukushima et al., 2021, Am. Mineral.), we attempted to reproduce micro-to-nanoscale textures with incipient ordering stages at 500–600℃. For numerical calculation, we adopted a simple 2-D phase-field model with a single order parameter, without considering any concentration/strain fields. Using a Landau expansion of the excess free energy by Carpenter et al. (1990, Eur. J. Mineral.), the Ginzburg–Landau equation was solved with a finite-difference method, starting from a random initial configuration.
Our simulation has demonstrated that temporal changes of both the spatial fraction and mean size of APDs depend on the input values of temperature, antiphase boundary energy, and intensity of the initial fluctuation. Notably, we have found that the initial fluctuation intensity significantly controls the incubation period of APDs after the metastable omphacite formed, and that the subsequent period of APD growth up to the fully ordered state can be shorter than 105 yr. Therefore, such transient texture due to incomplete cation ordering in natural omphacites should be a key to understanding the thermal–temporal history of short-period metamorphic events. Further combined research between natural observation of APD size/morphology and numerical modeling would open a new way to establish novel, and high-resolution geospeedometry for subduction zone geodynamics.
In this study, we performed APD growth modeling with a phase-field approach, which has been widely used in metallurgy. Considering incomplete cation ordering of natural omphacite in low-temperature eclogite from Syros (Fukushima et al., 2021, Am. Mineral.), we attempted to reproduce micro-to-nanoscale textures with incipient ordering stages at 500–600℃. For numerical calculation, we adopted a simple 2-D phase-field model with a single order parameter, without considering any concentration/strain fields. Using a Landau expansion of the excess free energy by Carpenter et al. (1990, Eur. J. Mineral.), the Ginzburg–Landau equation was solved with a finite-difference method, starting from a random initial configuration.
Our simulation has demonstrated that temporal changes of both the spatial fraction and mean size of APDs depend on the input values of temperature, antiphase boundary energy, and intensity of the initial fluctuation. Notably, we have found that the initial fluctuation intensity significantly controls the incubation period of APDs after the metastable omphacite formed, and that the subsequent period of APD growth up to the fully ordered state can be shorter than 105 yr. Therefore, such transient texture due to incomplete cation ordering in natural omphacites should be a key to understanding the thermal–temporal history of short-period metamorphic events. Further combined research between natural observation of APD size/morphology and numerical modeling would open a new way to establish novel, and high-resolution geospeedometry for subduction zone geodynamics.