09:15 〜 09:30
[SMP24-02] 三波川帯産泥質片岩に含まれる炭質物の炭素同位体地球化学
キーワード：炭質物、炭素同位体、三波川帯
Carbon in metasediments exists in the form of carbonate minerals and/or carbonaceous materials (CMs), and during prograde metamorphism, carbon is potentially released from carbonates through dissolution and decarbonation reactions induced by temperature increase. To date, extensive natural observations, thermodynamic modeling, and laboratory experiments have been undertaken to assess the potential occurrence of these processes in the subduction zone conditions. However, the behavior of CMs, which are the primary source of organic carbon in the subduction zone, remains poorly understood. Although CMs have been traditionally considered to be immobile during subduction-zone metamorphism, recent natural observations have provided evidence of their degassing and fluid-mediated dissolution, as well as graphite formation via the reduction of carbonate minerals1–3. These observations suggest that the behavior of CMs is more complex than previously thought. To elucidate the behavior of organic carbon in sediments during subduction, we are systematically conducting carbon isotope analysis on up to 200 samples of CMs in pelitic schists, covering a range from the low-temperature chlorite zone to the high-temperature biotite zone of the Sambagawa Belt in central Shikoku, Japan. In this contribution, we present the carbon isotope composition of CMs extracted from pelitic schist samples collected along the north-south transect of the Asemigawa area, where we observe regional variations in phengite K–Ar ages and O–H isotopes.
The carbon isotope composition of a total of 51 samples showed a wide range (δ13C = –28.7 to –21.4‰). A slight increase in δ13C values was observed with increasing metamorphic grade (chlorite zone: δ13C = –28.7 to –23.4‰, garnet zone: δ13C = –27.6 to –22.5‰, albite-biotite zone: δ13C = –23.0 to –21.9‰, oligoclase-biotite zone: δ13C = –25.8 to –21.4‰). We interpret the δ13C trend as being influenced by two processes that modify the CMs carbon isotope composition: (1) carbon isotope exchange between carbonate minerals and CMs and (2) devolatilization. Based on the carbon isotope fractionation between carbonate minerals and graphite4, carbon isotope exchange between them can result in an increase in δ13C values of graphite derived from organic carbon. Considering the common occurrence of minor calcite in Sambagawa pelitic schists5, the isotope exchange could impact the δ13C values. However, significant changes in δ13C of carbonaceous material require large amounts of CMs. The effect of devolatilization on δ13C of CMs depends on whether carbon is released as CO2 or CH4; note that CO2 release can decrease the δ13C of CMs, while CH4 release can cause an increase6. The observed high δ13C values imply the release of CH4-rich fluid during prograde metamorphism. This CH4 release aligns with the presence of CH4-bearing fluid inclusions in the Sambagawa pelitic schists7. However, a significant modification in δ13C requires the degassing of a substantial amount of carbon as CH4. Attempting to attribute the obtained δ13C trend to each process individually is unrealistic. Therefore, a combination of both processes may offer an explanation for the carbon isotope compositions of CMs in pelitic schists of the Sambagawa Belt.
Reference
1 Galvez et al., 2013, Nat. Geosci. 6, 473–477. https://doi.org/10.1038/ngeo1827
2 Vitale Brovarone et al., 2020, Chem. Geol. 549, 119682. https://doi.org/10.1016/j.chemgeo.2020.119682
3 Zhang et al., 2018, Chem. Geol. 490, 30–44. https://doi.org/10.1016/j.chemgeo.2018.05.003
4 Dunn & Valley, 1992, J. Metamorph. Geol. 10, 487–501. https://doi.org/10.1111/j.1525-1314.1992.tb00100.x
5 Goto et al., 2002, J. Metamorph. Geol. 20, 255–262. https://doi.org/10.1046/j.0263-4929.2001.00365.x
6 Bottinga, 1969, Geochim. Cosmochim. Acta 33, 49–64. https://doi.org/10.1016/0016-7037(69)90092-1
7 Yoshida et al., 2015, Lithos 226, 50–64. http://dx.doi.org/10.1016/j.lithos.2015.03.002
The carbon isotope composition of a total of 51 samples showed a wide range (δ13C = –28.7 to –21.4‰). A slight increase in δ13C values was observed with increasing metamorphic grade (chlorite zone: δ13C = –28.7 to –23.4‰, garnet zone: δ13C = –27.6 to –22.5‰, albite-biotite zone: δ13C = –23.0 to –21.9‰, oligoclase-biotite zone: δ13C = –25.8 to –21.4‰). We interpret the δ13C trend as being influenced by two processes that modify the CMs carbon isotope composition: (1) carbon isotope exchange between carbonate minerals and CMs and (2) devolatilization. Based on the carbon isotope fractionation between carbonate minerals and graphite4, carbon isotope exchange between them can result in an increase in δ13C values of graphite derived from organic carbon. Considering the common occurrence of minor calcite in Sambagawa pelitic schists5, the isotope exchange could impact the δ13C values. However, significant changes in δ13C of carbonaceous material require large amounts of CMs. The effect of devolatilization on δ13C of CMs depends on whether carbon is released as CO2 or CH4; note that CO2 release can decrease the δ13C of CMs, while CH4 release can cause an increase6. The observed high δ13C values imply the release of CH4-rich fluid during prograde metamorphism. This CH4 release aligns with the presence of CH4-bearing fluid inclusions in the Sambagawa pelitic schists7. However, a significant modification in δ13C requires the degassing of a substantial amount of carbon as CH4. Attempting to attribute the obtained δ13C trend to each process individually is unrealistic. Therefore, a combination of both processes may offer an explanation for the carbon isotope compositions of CMs in pelitic schists of the Sambagawa Belt.
Reference
1 Galvez et al., 2013, Nat. Geosci. 6, 473–477. https://doi.org/10.1038/ngeo1827
2 Vitale Brovarone et al., 2020, Chem. Geol. 549, 119682. https://doi.org/10.1016/j.chemgeo.2020.119682
3 Zhang et al., 2018, Chem. Geol. 490, 30–44. https://doi.org/10.1016/j.chemgeo.2018.05.003
4 Dunn & Valley, 1992, J. Metamorph. Geol. 10, 487–501. https://doi.org/10.1111/j.1525-1314.1992.tb00100.x
5 Goto et al., 2002, J. Metamorph. Geol. 20, 255–262. https://doi.org/10.1046/j.0263-4929.2001.00365.x
6 Bottinga, 1969, Geochim. Cosmochim. Acta 33, 49–64. https://doi.org/10.1016/0016-7037(69)90092-1
7 Yoshida et al., 2015, Lithos 226, 50–64. http://dx.doi.org/10.1016/j.lithos.2015.03.002