11:00 AM - 11:15 AM
[SGC32-02] Systematic depth variation of water in the lithosphere-asthenosphere boundary beneath Ichinomegata, NE Japan
Keywords:Water in Nominally Anhydrous Minerals, Subduction zone, Lithosphere-Asthenosphere Boundary, Fourier-transform infrared spectroscopy, Geothermobarometry
Mantle xenoliths are fragments of mantle materials rapidly brought by alkali-basalt and kimberlite magmas. They are source of information for better understanding dynamics operated in the lithosphere, asthenosphere, and their boundary. Water is one of the most important components because it significantly affects the solidus temperature (e.g., Kushiro et al, 1968) and rheological properties such as viscosity (e.g., Mei & Kohlstedt, 2000) and deformation mechanism (e.g., Jun et al., 2006). For instance, the chemical stratification of water has been considered as one of the key factors for defining lithosphere-asthenosphere boundary (LAB) (e.g., Hirth and Kohlstedt, 1996; Lee et al., 2005). In this study, we examined the role of water in defining LAB in the arc settings by analysis of water contents of minerals in mantle xenoliths from Ichinomegata maar in NE Japan, where thermal and petrologic structures of the upper mantle are constrained by our geothermobarometry (Sato & Ozawa, 2019). The mantle beneath Ichinomegata is granular, amphibole- and plagioclase-bearing, and subsolidus at the depth of 28-32 km and porphyroclastic, amphibole- and plagioclase-free, and partially molten in the depth range of 41-55 km. They correlated the former to the lithospheric mantle and the latter to the LAB-zone.
We analyzed water contents of 57 olivine, 71 orthopyroxene, and 33 clinopyroxene grains hand-picked from 14 peridotite xenoliths (2-7 grains for each mineral in each xenolith). IR spectra of three random but orthogonal sections of each grain were measured using polarized light to determine total absorbance (Libowitzky & Rossman, 1996) with a Jasco FTIR-6100 spectrometer with a Jasco IRT-5000 microscope in GIGCAS. The calibration factors determined by Bell et al., (1995; 2003) were used. The obtained water contents of olivine, orthopyroxene, and clinopyroxene range 16.8-37.0, 198-547, 477-886 wt. ppm, respectively. The error (1SD) for each grain is ~3% and that for each mineral in each xenolith is ~8%. Satsukawa et al. (2017) measured water contents of olivine and pyroxenes in peridotite xenoliths from the shallow depths. Their water contents are consistently lower than our results probably due to the selection of samples or underestimation caused by the methods. The water contents of olivine are higher than those reported from spinel peridotites (Demouchy & Bolfan-Casanova, 2016). The range of clinopyroxene is comparable to the reported range, but the highest water content of orthopyroxene exceeds the reported highest values from the world xenolith localities. These water contents in the Ichinomegata xenoliths reflect high water contents in the wedge mantle above subducting slab.
We conducted chemical analysis of olivine using EPMA (JEOL JXA-8230 in GIGCAS) to estimate the derivation depths and temperatures by combining pyroxene geothermobarometry. The CaO contents in olivine range 297-937 wt. ppm, which split into two clusters corresponding to the 8 granular samples (297-398 ppm) and 6 porphyroclastic samples (634-937 ppm). Their estimated depths range 25-48 km and temperature of 820-1055 °C. There are depth variations in the water contents. The water contents of olivine and pyroxenes in the granular samples are quite variable and high in the maximum and mean water contents. By contrast, those in the shallow samples are less variable and low in the maximum and mean water contents. The xenolith registering the highest water contents underwent partial melting and the deformation featuring porphyroclastic texture when it stayed at 45 km and 1024 °C. The estimated water contents in minerals are consistent with experimentally determined water solubility of minerals coexisting with basaltic melts containing ~5 wt.% of H2O at 1.2 GPa and 1200 °C (Hauri et al., 2006). These suggest that water was supplied into the deeper levels corresponding to the LAB zone beneath Ichinomegata from underlying asthenospheric mantle through heterogenous paths.
We analyzed water contents of 57 olivine, 71 orthopyroxene, and 33 clinopyroxene grains hand-picked from 14 peridotite xenoliths (2-7 grains for each mineral in each xenolith). IR spectra of three random but orthogonal sections of each grain were measured using polarized light to determine total absorbance (Libowitzky & Rossman, 1996) with a Jasco FTIR-6100 spectrometer with a Jasco IRT-5000 microscope in GIGCAS. The calibration factors determined by Bell et al., (1995; 2003) were used. The obtained water contents of olivine, orthopyroxene, and clinopyroxene range 16.8-37.0, 198-547, 477-886 wt. ppm, respectively. The error (1SD) for each grain is ~3% and that for each mineral in each xenolith is ~8%. Satsukawa et al. (2017) measured water contents of olivine and pyroxenes in peridotite xenoliths from the shallow depths. Their water contents are consistently lower than our results probably due to the selection of samples or underestimation caused by the methods. The water contents of olivine are higher than those reported from spinel peridotites (Demouchy & Bolfan-Casanova, 2016). The range of clinopyroxene is comparable to the reported range, but the highest water content of orthopyroxene exceeds the reported highest values from the world xenolith localities. These water contents in the Ichinomegata xenoliths reflect high water contents in the wedge mantle above subducting slab.
We conducted chemical analysis of olivine using EPMA (JEOL JXA-8230 in GIGCAS) to estimate the derivation depths and temperatures by combining pyroxene geothermobarometry. The CaO contents in olivine range 297-937 wt. ppm, which split into two clusters corresponding to the 8 granular samples (297-398 ppm) and 6 porphyroclastic samples (634-937 ppm). Their estimated depths range 25-48 km and temperature of 820-1055 °C. There are depth variations in the water contents. The water contents of olivine and pyroxenes in the granular samples are quite variable and high in the maximum and mean water contents. By contrast, those in the shallow samples are less variable and low in the maximum and mean water contents. The xenolith registering the highest water contents underwent partial melting and the deformation featuring porphyroclastic texture when it stayed at 45 km and 1024 °C. The estimated water contents in minerals are consistent with experimentally determined water solubility of minerals coexisting with basaltic melts containing ~5 wt.% of H2O at 1.2 GPa and 1200 °C (Hauri et al., 2006). These suggest that water was supplied into the deeper levels corresponding to the LAB zone beneath Ichinomegata from underlying asthenospheric mantle through heterogenous paths.