17:54 〜 17:57
[SSS29-P14] 摩擦溶融時のシュードタキライト中の動的な酸化・還元状態について
ポスター講演3分口頭発表枠
We carried out a stable carbon isotopic study of graphite-bearing pseudotachylytes to characterize the origin, role and behavior of different forms of carbon during frictional melting. The study area, located in the Hidaka Metamorphic belt, exposes metasedimentary rocks and various magmatic intrusions and are extensively deformed as evidenced by the presence of various types of fault rocks such as graphite-bearing cataclasite, ultracataclasite and pseudotachylytes. In particular, graphite-bearing pseudotachylytes are observed in the brittle shear zones, which are a few mm to maximum 5 cm in width. They are mainly divided into two types (Pst-I and Pst-II) based on the occurrences, microstructures and mineral assemblages of lithic fragments and secondary minerals. The graphite in each domain of Pst-I and Pst-II were separately analyzed for carbon isotopic composition. Disseminated graphite in protolith has a narrow range of δ13C values between -23.6 and -25.8 ‰ (n = 13), and cataclasite, ultracataclasite and Pst I also have similar values between -24.1 and -27.0 ‰ (n = 25). On the other hand, the graphite separated from Pst II matrix on slab sections show values between -18.2 and -23.6 ‰ (n = 16), shifting the carbon isotope values to 2-3 ‰ higher from host metamorphic graphite. In particular, there is a clear correlation between stable carbon isotope composition and volume fraction of lithic fragments in each domain.
Our data indicate that metamorphic graphite in fault rocks were converted into H2O-CO2 or H2O-CH4 fluids under very-high temperature condition of frictional melting. Subsequently, a part of the COHS fluid re-precipitated as fluid-deposited graphite and the remaining was expelled as COHS fluids into fault zones during quenching stage. On the basis of chemical compositions and mineral assemblages in the pseudotachylytes, we attempt to estimate the P-T-fO2-fS2 phase diagram during frictional melting and crystallization of pseudotachylytes. The thermal decomposition of biotite coexisting with graphite and sulfide minerals are deduced by following reaction;
Annite in biotite + 3Pyrite + 1.5Graphite = Sanidine + 6Pyrrhotite + H2O + 1.5 CO2
The dehydration and decarbonation processes in this reaction are mainly driven by temperature, fS2, and fO2, and the breakdown of biotite that is ferromagnesian mineral change the redox state to the more oxidation state at ranges between ΔFMQ +0.5 to + 3.0. Such high fO2 and fS2 environments are only observed in the domains which show relatively low-temperature condition (Pst I matrix) during frictional melting. On the other hand, in the domains which show high temperature conditions (> 1200 degree C) biotite microlite are observed in pseudotachylytes instead of pyrrhotite. This suggests the negative jump to biotite stable field of ASM buffer by lowering fS2 under high-temperature condition. In addition, we tried to estimate the redox state at precipitation stage using graphite-fluid fractionation model. The difference between disseminated graphite and fluid deposited graphite show the positive 2-3 ‰ shifting by carbon isotope fractionation, suggesting the presence of CH4-rich COHS fluid during precipitation stage. Under CH4-H2O dominant fluid in COH diagram, we can explain the carbon isotope variation of fluid deposited graphite at around ΔFMQ -3.0 by hydration reactions during the crystallization of titanite and hydroxyapatite. Such reducing conditions are only observed in the high temperature frictionally melted domains (Pst II matrix).
Thus, oxidation and precipitation processes of graphite are mainly controlled by the breakdown of ferromagnesian minerals and we revealed that the redox state and TOC values dynamically changed within each pseudotachylyte matrix. In addition, our data imply that redox state in pseudotachylyte sensitively change in melting domains at ranges of ΔFMQ -3.0 to + 3.0 as a function of melting temperature and bulk chemistry.
Our data indicate that metamorphic graphite in fault rocks were converted into H2O-CO2 or H2O-CH4 fluids under very-high temperature condition of frictional melting. Subsequently, a part of the COHS fluid re-precipitated as fluid-deposited graphite and the remaining was expelled as COHS fluids into fault zones during quenching stage. On the basis of chemical compositions and mineral assemblages in the pseudotachylytes, we attempt to estimate the P-T-fO2-fS2 phase diagram during frictional melting and crystallization of pseudotachylytes. The thermal decomposition of biotite coexisting with graphite and sulfide minerals are deduced by following reaction;
Annite in biotite + 3Pyrite + 1.5Graphite = Sanidine + 6Pyrrhotite + H2O + 1.5 CO2
The dehydration and decarbonation processes in this reaction are mainly driven by temperature, fS2, and fO2, and the breakdown of biotite that is ferromagnesian mineral change the redox state to the more oxidation state at ranges between ΔFMQ +0.5 to + 3.0. Such high fO2 and fS2 environments are only observed in the domains which show relatively low-temperature condition (Pst I matrix) during frictional melting. On the other hand, in the domains which show high temperature conditions (> 1200 degree C) biotite microlite are observed in pseudotachylytes instead of pyrrhotite. This suggests the negative jump to biotite stable field of ASM buffer by lowering fS2 under high-temperature condition. In addition, we tried to estimate the redox state at precipitation stage using graphite-fluid fractionation model. The difference between disseminated graphite and fluid deposited graphite show the positive 2-3 ‰ shifting by carbon isotope fractionation, suggesting the presence of CH4-rich COHS fluid during precipitation stage. Under CH4-H2O dominant fluid in COH diagram, we can explain the carbon isotope variation of fluid deposited graphite at around ΔFMQ -3.0 by hydration reactions during the crystallization of titanite and hydroxyapatite. Such reducing conditions are only observed in the high temperature frictionally melted domains (Pst II matrix).
Thus, oxidation and precipitation processes of graphite are mainly controlled by the breakdown of ferromagnesian minerals and we revealed that the redox state and TOC values dynamically changed within each pseudotachylyte matrix. In addition, our data imply that redox state in pseudotachylyte sensitively change in melting domains at ranges of ΔFMQ -3.0 to + 3.0 as a function of melting temperature and bulk chemistry.