15:30 〜 17:00
[SGC37-P08] Absence of water content contrast across the lithosphere-asthenosphere boundary beneath Ichinomegata, NE Japan
キーワード：水の分布、LAB、拡散、マントルカンラン岩捕獲岩、沈み込み帯
The distribution of water between the rigid lithosphere and the ductile asthenosphere is crucial for understanding the Earth’s dynamics. This is mainly because it has been proposed that the origin of the lithosphere–asthenosphere boundary (LAB; i.e., the bottom of the plate) is related to possible elevated water content in the asthenosphere than the lithosphere. Therefore, obtaining the depth profile of water content using mantle xenoliths is an essential problem. However, it is still difficult due to (1) the rapid diffusive loss of hydrogen during magma ascent and eruption and (2) the difficulty of geothermobarometry for spinel peridotite facies. We challenge this problem in the subduction zone settings using the spinel peridotite xenoliths from Ichinomegata maar in Northeast Japan. In our previous study, Sato and Ozawa (2019, Amer. Min.) conducted accurate geothermobarometry for Ichinomegata xenoliths and reconstructed petrological structures of the mantle beneath Ichinomegata.
In this study, we present diffusion profiles of water determined by FTIR mapping and SIMS line analysis for olivine and orthopyroxene of Ichinomegata xenoliths and demonstrate the depth profile of water content beneath Ichinomegata, which were published as Sato et al. (2023, EPSL). Extensive analysis of water contents in 17 xenolith samples revealed that Ichinomegata xenoliths exhibit a variety of zoning patterns, from which the timescale of diffusive loss was estimated. Ichinomegata xenoliths underwent only limited diffusive water loss (<1h duration) because of rapid quench in the maar deposit. The timescale of water loss is much shorter than the duration of the xenolith transportation by the host magma, which is 1-68 days estimated from the Ca zoning in the outermost rim of olivine in contact with clinopyroxene. This suggests that the water loss started only after the opening of grain boundaries at a shallower depth.
Water contents of the mantle are well-preserved in the homogeneous core parts of large olivine and pyroxene grains. The water contents of olivine suggest clear correlations with petrogenetic factors (modal % of CPX) except those marking anomalous high values, implying hydrous metasomatism by a water-rich fluid. Combining the estimated water contents of the minerals and depth estimation of the xenolith samples, we obtained the depth profile of the water content of the mantle, including the LAB, down to ~55 km. Our results show that olivine, orthopyroxene, and clinopyroxene in the lithospheric mantle (depth range in 28–38 km) contain 21 ± 2, 302 ± 64, and 616 ± 99 wt. ppm H2O, respectively, which are similar to those in the top of the asthenosphere (depth range in 39–52 km) that contain 20 ± 2, 258 ± 38, and 561 ± 80 wt. ppm H2O. In the region which has experienced local metasomatism, higher water contents are recorded (30 ± 4, 414 ± 48, and 741 ± 43 wt. ppm H2O). This study verifies that there is no water content contrast across the LAB in our studied area. Therefore, we support the ‘partial-melting model’ for the origin of the LAB rather than the ‘olivine-water model’ in the western Pacific Plate subduction zone.
In this study, we present diffusion profiles of water determined by FTIR mapping and SIMS line analysis for olivine and orthopyroxene of Ichinomegata xenoliths and demonstrate the depth profile of water content beneath Ichinomegata, which were published as Sato et al. (2023, EPSL). Extensive analysis of water contents in 17 xenolith samples revealed that Ichinomegata xenoliths exhibit a variety of zoning patterns, from which the timescale of diffusive loss was estimated. Ichinomegata xenoliths underwent only limited diffusive water loss (<1h duration) because of rapid quench in the maar deposit. The timescale of water loss is much shorter than the duration of the xenolith transportation by the host magma, which is 1-68 days estimated from the Ca zoning in the outermost rim of olivine in contact with clinopyroxene. This suggests that the water loss started only after the opening of grain boundaries at a shallower depth.
Water contents of the mantle are well-preserved in the homogeneous core parts of large olivine and pyroxene grains. The water contents of olivine suggest clear correlations with petrogenetic factors (modal % of CPX) except those marking anomalous high values, implying hydrous metasomatism by a water-rich fluid. Combining the estimated water contents of the minerals and depth estimation of the xenolith samples, we obtained the depth profile of the water content of the mantle, including the LAB, down to ~55 km. Our results show that olivine, orthopyroxene, and clinopyroxene in the lithospheric mantle (depth range in 28–38 km) contain 21 ± 2, 302 ± 64, and 616 ± 99 wt. ppm H2O, respectively, which are similar to those in the top of the asthenosphere (depth range in 39–52 km) that contain 20 ± 2, 258 ± 38, and 561 ± 80 wt. ppm H2O. In the region which has experienced local metasomatism, higher water contents are recorded (30 ± 4, 414 ± 48, and 741 ± 43 wt. ppm H2O). This study verifies that there is no water content contrast across the LAB in our studied area. Therefore, we support the ‘partial-melting model’ for the origin of the LAB rather than the ‘olivine-water model’ in the western Pacific Plate subduction zone.