5:15 PM - 7:15 PM
[SMP28-P01] Relationship between interface energy and grain size in the Ichino-megata peridotite xenolith: effects of melt-rock interactions
Keywords:Zener pinning, peridotite xenolith, rock-melt interaction, dihedral angle
Grain size is one of the fundamental thermodynamic parameters for considering diffusion and deformation. The previous study suggested that the grain size and volume fractions of minerals follow Zener's law, which considers the balance between grain boundary mobility and pinning force. For example, the same Zener law applied to samples of natural and experimentally synthesized peridotites within four orders of magnitude of olivine grain size (10-0.5~103.5 μm) (Tasaka et al., 2014). Based on these results, it was expected that the interfacial energy of olivine-pyroxene would be constant between natural and experimental samples, despite the different thermodynamic conditions (temperature, pressure, time, strain, stress, and chemical composition), yet studies of the measurement of interfacial energy using natural rocks are still limited. Therefore, we test the above hypothesis by determining the interfacial energy of natural peridotite from the dihedral angle of olivine-olivine-pyroxene. The natural peridotite currently exposed on the surface has undergone varying degrees of melt-rock interactions. In this study, we used eight samples from the Ichino-megata peridotite xenoliths in Akita prefecture, for which equilibrium temperature and pressure (830-1080℃, 7.2-16 Kbar) and depth (28-55 km) were determined by Sato and Ozawa (2019), and conducted microstructural analysis, and measurements of dihedral angle. We clarified the relationship between natural peridotite dihedral angles and melt-rock interactions.
Most of the samples were classified as spinel lherzolite based on microstructural observations. The relationship between the average grain size of the primary and secondary phases and the volume fraction of the secondary phase, with olivine as the primary phase and minerals other than olivine as the secondary phase, followed the same Zener's law as that of Tasaka et al. (2014). Electron backscatter diffraction (EBSD) analysis revealed that the crystallographic preferred orientation (CPO) of olivine in most samples is A and D-type, formed by deformation of peridotite without melt, while one sample in the shallowest area (28 km) is AG-type, formed by deformation of melt and olivine together. Also, more than 70 dihedral angles of olivine-olivine-orthopyroxene (ol-ol-opx) and olivine-olivine-clinopyroxene (ol-ol-cpx) were measured in the six samples. The average dihedral angles were 95-110° and 90-105° for ol-ol-opx and ol-ol-cpx, respectively. Using statistical methods (Akaike's Information Criterion, AIC), the distribution of dihedral angles was explained by a normal distribution for ol-ol-opx and a combination of two normal distributions for ol-ol-cpx.
Samples with two normal distributions of the ol-ol-cpx dihedral angle, analyzed by Sato and Ozawa (2019) as samples derived from the lithosphere-asthenosphere boundary (LAB), indicate that the two normal distributions of the ol-ol-cpx dihedral angle, the higher angle distribution represents the interface energy between olivine and pyroxene in a melt-free system. In contrast, the lower angle distribution represents the dihedral angle derived from the interfacial energy between olivine and melt, and we assume that the dihedral angle shifts to the higher angle during annealing after the melt and olivine react with a new pyroxene. The grain size and volume fraction of minerals in the samples observed in this study follow the same Zener's law as Tasaka et al. (2014), suggesting that the Ichino-megata peridotite xenoliths, which have undergone various degrees of rock-melt interaction and deformation, have similar interfacial energy and grain size to natural and experimental samples without melts if the rocks have been annealed for sufficient time to reach an equilibrium microstructure. The results of this study could be one of the important implications for understanding the grain size of melt-rock interaction of peridotite in the uppermost mantle.
Most of the samples were classified as spinel lherzolite based on microstructural observations. The relationship between the average grain size of the primary and secondary phases and the volume fraction of the secondary phase, with olivine as the primary phase and minerals other than olivine as the secondary phase, followed the same Zener's law as that of Tasaka et al. (2014). Electron backscatter diffraction (EBSD) analysis revealed that the crystallographic preferred orientation (CPO) of olivine in most samples is A and D-type, formed by deformation of peridotite without melt, while one sample in the shallowest area (28 km) is AG-type, formed by deformation of melt and olivine together. Also, more than 70 dihedral angles of olivine-olivine-orthopyroxene (ol-ol-opx) and olivine-olivine-clinopyroxene (ol-ol-cpx) were measured in the six samples. The average dihedral angles were 95-110° and 90-105° for ol-ol-opx and ol-ol-cpx, respectively. Using statistical methods (Akaike's Information Criterion, AIC), the distribution of dihedral angles was explained by a normal distribution for ol-ol-opx and a combination of two normal distributions for ol-ol-cpx.
Samples with two normal distributions of the ol-ol-cpx dihedral angle, analyzed by Sato and Ozawa (2019) as samples derived from the lithosphere-asthenosphere boundary (LAB), indicate that the two normal distributions of the ol-ol-cpx dihedral angle, the higher angle distribution represents the interface energy between olivine and pyroxene in a melt-free system. In contrast, the lower angle distribution represents the dihedral angle derived from the interfacial energy between olivine and melt, and we assume that the dihedral angle shifts to the higher angle during annealing after the melt and olivine react with a new pyroxene. The grain size and volume fraction of minerals in the samples observed in this study follow the same Zener's law as Tasaka et al. (2014), suggesting that the Ichino-megata peridotite xenoliths, which have undergone various degrees of rock-melt interaction and deformation, have similar interfacial energy and grain size to natural and experimental samples without melts if the rocks have been annealed for sufficient time to reach an equilibrium microstructure. The results of this study could be one of the important implications for understanding the grain size of melt-rock interaction of peridotite in the uppermost mantle.