10:45 〜 12:15
[SCG58-P01] Petrologic and geochemical investigation of the Kanasaki ophicarbonate, Kanto Mountains, Japan.
キーワード：蛇紋岩、蛇灰岩、炭酸塩化作用、炭素循環
Ophicarbonate (carbonated serpentinite) would be an important carbon reservoir influencing the Earth's global carbon cycle. In this study, we investigated ophicarbonate from Kanasaki (Kanasaki ophicarbonate), Kanto Mountain, Japan, to understand the carbonation of ultramafic rocks. The Kanasaki ophicarbonate is exposed over an area of ~ 50 m as blocks embedded in pelitic schist along the Arakawa River. The Kanasaki area belongs to the Mikabu unit of the Sanbagawa belt and is located at the chlorite zones (Hashimoto et al. 1992), whose peak metamorphic condition is ~360 °C and 0.5 - 0.6 GPa based on mineral equilibria (Enami 1983). The spinel compositions of the Kanasaki ophicarbonate are Cr/(Cr+Al) = 0.35 - 0.55, Mg/(Mg+Fe2+) = 0.53 - 0.67, and TiO2 < 0.3 wt% (Hisada et al. 1993).
Samples of ophicarbonate and pelitic schist were collected from the Kanasaki area. The pelitic schist mainly consists of quartz, plagioclase, muscovite, and chlorite. The ophicarbonate is composed mostly of serpentine, carbonate (calcite and dolomite), chlorite, spinel, and minor amounts of ilmenite and sulfides. Two types of serpentine microtextures were observed: massive serpentine, which is made up solely of antigorite, and mesh texture serpentine, which is composed of lizardite + chrysotile at the mesh core and antigorite at the mesh rim. The occurrence of carbonate differs between the western and eastern sides of the Kanasaki area. In the western side, carbonation is extensive, and the carbonate occurs as a cement material of serpentine breccia. In contrast, on the eastern side, the carbonate occurs as a vein. The isotopic composition of carbonate from the ophicarbonate samples shows that δ13C ranges from +2 to -7‰, and δ18O ranges from +13 to -19‰. The δ13C values tend to decrease from the western to the eastern side of the Kanasaki area. The range of carbonate δ13C and δ18O are similar to those found in the pelitic schist and metabasalt of the Kanto mountains (Morohashi et al. 2008).
The Raman spectroscopy analysis of the carbonaceous material thermometer indicated a peak temperature of approximately 360 °C, which agrees with the peak temperature obtained from the mineral equilibria. During this temperature range, lizardite is gradually replaced by antigorite at the grain boundaries through dissolution–precipitation processes, as described by previous studies (Evans, 2004; Schwartz et al., 2013). The composite mesh texture from the Kanasaki ophicarbonate implies that the serpentine underwent prograde metamorphism. Further petrological/mineralogical observations will be conducted to investigate the timing of carbonation and serpentinization, and to assess the influence on global carbon cycling at the subduction zone.
References
Enami, M., Wallis, S. R., & Banno, Y. (1994). Paragenesis of sodic pyroxene-bearing quartz schists: implications for the PT history of the Sanbagawa belt. Contributions to Mineralogy and Petrology, 116, 182-198.
Evans, B. W. (2004). The serpentinite multisystem revisited: chrysotile is metastable. International Geology Review, 46(6), 479-506.
Hisada, K., Nakazawa, E. and Arai, S. (1993) Sedimentary origin of ophicalcite in the Sambagawa metamorphic rocks, Kanto Mountains, central Japan. Annual Report of the Institute Geosciences, the University of Tsukuba, no. 19, 43–47.
Morohashi, K., Okamoto, A., Satish-Kumar, M., & Tsuchiya, N. (2008). Variations in stable isotope compositions (δ13C, δ18O) of calcite within exhumation-related veins from the Sanbagawa metamorphic belt. Journal of Mineralogical and Petrological Sciences, 103(5), 361-364.
Schwartz, S., Guillot, S., Reynard, B., Lafay, R., Debret, B., Nicollet, C., ... & Auzende, A. L. (2013). Pressure–temperature estimates of the lizardite/antigorite transition in high pressure serpentinites. Lithos, 178, 197-210.
Samples of ophicarbonate and pelitic schist were collected from the Kanasaki area. The pelitic schist mainly consists of quartz, plagioclase, muscovite, and chlorite. The ophicarbonate is composed mostly of serpentine, carbonate (calcite and dolomite), chlorite, spinel, and minor amounts of ilmenite and sulfides. Two types of serpentine microtextures were observed: massive serpentine, which is made up solely of antigorite, and mesh texture serpentine, which is composed of lizardite + chrysotile at the mesh core and antigorite at the mesh rim. The occurrence of carbonate differs between the western and eastern sides of the Kanasaki area. In the western side, carbonation is extensive, and the carbonate occurs as a cement material of serpentine breccia. In contrast, on the eastern side, the carbonate occurs as a vein. The isotopic composition of carbonate from the ophicarbonate samples shows that δ13C ranges from +2 to -7‰, and δ18O ranges from +13 to -19‰. The δ13C values tend to decrease from the western to the eastern side of the Kanasaki area. The range of carbonate δ13C and δ18O are similar to those found in the pelitic schist and metabasalt of the Kanto mountains (Morohashi et al. 2008).
The Raman spectroscopy analysis of the carbonaceous material thermometer indicated a peak temperature of approximately 360 °C, which agrees with the peak temperature obtained from the mineral equilibria. During this temperature range, lizardite is gradually replaced by antigorite at the grain boundaries through dissolution–precipitation processes, as described by previous studies (Evans, 2004; Schwartz et al., 2013). The composite mesh texture from the Kanasaki ophicarbonate implies that the serpentine underwent prograde metamorphism. Further petrological/mineralogical observations will be conducted to investigate the timing of carbonation and serpentinization, and to assess the influence on global carbon cycling at the subduction zone.
References
Enami, M., Wallis, S. R., & Banno, Y. (1994). Paragenesis of sodic pyroxene-bearing quartz schists: implications for the PT history of the Sanbagawa belt. Contributions to Mineralogy and Petrology, 116, 182-198.
Evans, B. W. (2004). The serpentinite multisystem revisited: chrysotile is metastable. International Geology Review, 46(6), 479-506.
Hisada, K., Nakazawa, E. and Arai, S. (1993) Sedimentary origin of ophicalcite in the Sambagawa metamorphic rocks, Kanto Mountains, central Japan. Annual Report of the Institute Geosciences, the University of Tsukuba, no. 19, 43–47.
Morohashi, K., Okamoto, A., Satish-Kumar, M., & Tsuchiya, N. (2008). Variations in stable isotope compositions (δ13C, δ18O) of calcite within exhumation-related veins from the Sanbagawa metamorphic belt. Journal of Mineralogical and Petrological Sciences, 103(5), 361-364.
Schwartz, S., Guillot, S., Reynard, B., Lafay, R., Debret, B., Nicollet, C., ... & Auzende, A. L. (2013). Pressure–temperature estimates of the lizardite/antigorite transition in high pressure serpentinites. Lithos, 178, 197-210.