11:00 〜 11:15
[SMP28-08] 沈み込み帯における炭素挙動の岩石学的研究: 含石墨ひすい輝石岩からのアプローチ
キーワード：ひすい輝石岩、Deep Earth Water (DEW) モデル、炭素、沈み込み帯、石墨
Subduction zones play a crucial role in Earth's deep carbon cycle, acting as key pathways for carbon transfer between surface reservoirs and the mantle over geological timescales. In this study, we investigate graphite-bearing jadeitite collected from the upper Kanayamadani Valley, Itoigawa. A combination of petrographic analyses and phase equilibrium modeling with Deep Earth Water (DEW) model was employed to elucidate the required conditions of pressure, temperature, and fluid compositions, for the formation of jadeite and associated graphite. We calculated fluid compositions based on the chemical potential of rocks at equilibrium by using a back-calculated electrolyte speciation method (Galvez et al., 2015). This approach replicates rock-dominated conditions with extremely high rock/water ratios, thereby providing insights into fluid evolution across various metamorphic settings. Our modeling explored a range of parameters, including temperatures of 300-600°C, pressures of 0.5-2.5 GPa, and oxygen fugacities (fO2) from –40 to –10.
Based on the petrographic features, the jadeitite samples were classified into three types: (1) a pale black matrix rich in graphite, (2) graphite-poor jadeite veins crosscutting the pale black matrix, and (3) very pale green jadeite with no graphite aggregates. Our Raman spectroscopic measurements confirmed the presence of graphite in both the pale black and pale green portions, with characteristic bands at ~1350 cm–1, 1580 cm–1, 2720 cm–1, and 3240 cm–1. Graphite occurs within jadeite crystals and along grain boundaries, suggesting co-precipitation of jadeite and graphite.
Our phase equilibrium modeling indicates that both of jadeite and graphite are stable across a broad range of conditions, implying that pressure-temperature effects alone do not control the graphite abundance. Carbon-rich fluids promote abundant graphite in pale black jadeitite under relatively low-temperature, low-pressure conditions, while carbon-poor fluids enriched in Ca, Mg, and Fe contribute to the formation of the jadeite vein and the very pale green domains at higher-temperature, higher-pressure conditions. DEW modeling further reveals that fluid compositions can significantly change with depth in subduction zones. In MORB-dominated lithologies, fluids become increasingly acidic under high-temperature and high-pressure conditions. In serpentinite-dominated lithologies (akin to DMM + H2O), water-undersaturated conditions can stabilize brucite, which acts as a water sink. Overall, these findings highlight the complex interplay of redox conditions, fluid composition, and mineral stability in subduction environments. This study underscores the critical role of fluid-rock interactions in carbon sequestration and mineralization processes, providing new insights into how graphite-bearing jadeitite records metamorphic fluid evolution in convergent plate settings.
Based on the petrographic features, the jadeitite samples were classified into three types: (1) a pale black matrix rich in graphite, (2) graphite-poor jadeite veins crosscutting the pale black matrix, and (3) very pale green jadeite with no graphite aggregates. Our Raman spectroscopic measurements confirmed the presence of graphite in both the pale black and pale green portions, with characteristic bands at ~1350 cm–1, 1580 cm–1, 2720 cm–1, and 3240 cm–1. Graphite occurs within jadeite crystals and along grain boundaries, suggesting co-precipitation of jadeite and graphite.
Our phase equilibrium modeling indicates that both of jadeite and graphite are stable across a broad range of conditions, implying that pressure-temperature effects alone do not control the graphite abundance. Carbon-rich fluids promote abundant graphite in pale black jadeitite under relatively low-temperature, low-pressure conditions, while carbon-poor fluids enriched in Ca, Mg, and Fe contribute to the formation of the jadeite vein and the very pale green domains at higher-temperature, higher-pressure conditions. DEW modeling further reveals that fluid compositions can significantly change with depth in subduction zones. In MORB-dominated lithologies, fluids become increasingly acidic under high-temperature and high-pressure conditions. In serpentinite-dominated lithologies (akin to DMM + H2O), water-undersaturated conditions can stabilize brucite, which acts as a water sink. Overall, these findings highlight the complex interplay of redox conditions, fluid composition, and mineral stability in subduction environments. This study underscores the critical role of fluid-rock interactions in carbon sequestration and mineralization processes, providing new insights into how graphite-bearing jadeitite records metamorphic fluid evolution in convergent plate settings.