9:30 AM - 9:45 AM
[SCG46-12] Traces of fluid pathway in underplating oceanic crust left at the lithologic boundary of pelitic and mafic schist
Keywords:Subduction zone, Fluid activity, Element mobile
Fluid flow in subduction zones is a key factor associated with seismic activity. Especially, the element transfer toward the overlying plate has been suggested to be relate to the cycle of slow earthquakes (e.g., Audet & Bürgmann, 2014). The details of the fluid flow are recorded in the underplating oceanic crust. In this study, we focused on the lithological boundaries of pelitic-mafic shist, which preserve traces of fluid pathways in the underplating oceanic crust. We performed the mineral mode and bulk composition analysis for the samples, which continuously changes depending on the distance from the lithological boundary.
The study area is in the central part of Shikoku, Japan, within the Sanbagawa metamorphic belt. The sampling points is Karagoshi outcrop (latitude: 33.7950, longitude: 133.5516) in Asemigawa-river region, where the peak metamorphic P-T condition is estimated as 300°C and 0.55-0.65GPa (Kouketsu et al. 2021; Enami et al. 1994). We continuously collected 46 samples over a 10m span across the boundary between pelitic-mafic shist. To obtain whole-rock compositions and mineral mode, we analyzed the prepared samples by XRF, ICP-MS, EPMA, and XRD. In addition, we applied Protolith Reconstruction Models (Matsuno et al., 2022) to quantify element transfer in element concentrations, assuming Th, Nb, Zr, Ti as immobile elements, and estimating for Rb, Ba, U, K, La, Ce, Pb, Sr, P, Nd, Y, Yb, Lu. The Y_actinolite value was obtained from the volume ratio low-P-T amphibole to the total amount of amphibole as an index of retrograde metamorphism (Okamoto & Toriumi 2005)
The mineral mode continuously changed from the host rock to the lithological boundary for both pelitic-mafic shist. The major minerals of the mafic shist are chlorite, epidote, amphibole, plagioclase, muscovite, quartz, and calcite, but sodium amphibole occurs on the host rock side, whereas chlorite actinolite, and muscovite is dominant at the lithologic boundaries. The major minerals of pelitic shist are quartz, chlorite, plagioclase, and muscovite, while muscovite is abundant at the lithologic boundary. Notably, the mineral mode of plagioclase decreased continuously from 33% to 13% within ±20cm of the lithological boundary from pelitic-mafic shist. Furthermore, muscovite in the mafic shist increased continuously from 1% on the host rock side to a maximum of 25% at the boundary, respectively. The Y_actinolite changed continuously from 50% in the host rock located more than 2m away to 100% in all samples within 25cm of the lithological boundary.
There are clear correlations in mineral mode and bulk composition for mafic shist. The relationship between Rb, Ba mobility and Y_actinolite indicates that elemental enrichment of Rb and Ba occurs continuously with increasing Y_actinolite values during retrograde metamorphism at the lithologic boundaries. For the entire mafic schist outcrop, the Rb concentration is about 5 times higher on the host rock side at 50% Y_actinolite values, while it is about 15 times higher at the lithologic boundary at 100% Y_actinolite values. In this context, the mafic schist experiences retrograde metamorphism with the fluid derived from the lithological boundary.
The mineral mode of muscovite in both lithologies correlated with the concentrations and mobility of K, Rb, and Ba. The concentrations of K, Rb, and Ba only increased locally at the lithological boundary compared to the host rocks of both lithologies, suggesting that the element supply was from external fluids rather than internal element redistribution within the outcrop. This implies that the traces of fluid-rock reactions associated with fluids that flowed externally through the lithological boundaries occurred in the underplating oceanic crust. We will further investigate the formation conditions of the reaction zones, including the composition and amounts of fluids flow at the lithological boundaries through geochemical modeling of fluid-rock reactions.
The study area is in the central part of Shikoku, Japan, within the Sanbagawa metamorphic belt. The sampling points is Karagoshi outcrop (latitude: 33.7950, longitude: 133.5516) in Asemigawa-river region, where the peak metamorphic P-T condition is estimated as 300°C and 0.55-0.65GPa (Kouketsu et al. 2021; Enami et al. 1994). We continuously collected 46 samples over a 10m span across the boundary between pelitic-mafic shist. To obtain whole-rock compositions and mineral mode, we analyzed the prepared samples by XRF, ICP-MS, EPMA, and XRD. In addition, we applied Protolith Reconstruction Models (Matsuno et al., 2022) to quantify element transfer in element concentrations, assuming Th, Nb, Zr, Ti as immobile elements, and estimating for Rb, Ba, U, K, La, Ce, Pb, Sr, P, Nd, Y, Yb, Lu. The Y_actinolite value was obtained from the volume ratio low-P-T amphibole to the total amount of amphibole as an index of retrograde metamorphism (Okamoto & Toriumi 2005)
The mineral mode continuously changed from the host rock to the lithological boundary for both pelitic-mafic shist. The major minerals of the mafic shist are chlorite, epidote, amphibole, plagioclase, muscovite, quartz, and calcite, but sodium amphibole occurs on the host rock side, whereas chlorite actinolite, and muscovite is dominant at the lithologic boundaries. The major minerals of pelitic shist are quartz, chlorite, plagioclase, and muscovite, while muscovite is abundant at the lithologic boundary. Notably, the mineral mode of plagioclase decreased continuously from 33% to 13% within ±20cm of the lithological boundary from pelitic-mafic shist. Furthermore, muscovite in the mafic shist increased continuously from 1% on the host rock side to a maximum of 25% at the boundary, respectively. The Y_actinolite changed continuously from 50% in the host rock located more than 2m away to 100% in all samples within 25cm of the lithological boundary.
There are clear correlations in mineral mode and bulk composition for mafic shist. The relationship between Rb, Ba mobility and Y_actinolite indicates that elemental enrichment of Rb and Ba occurs continuously with increasing Y_actinolite values during retrograde metamorphism at the lithologic boundaries. For the entire mafic schist outcrop, the Rb concentration is about 5 times higher on the host rock side at 50% Y_actinolite values, while it is about 15 times higher at the lithologic boundary at 100% Y_actinolite values. In this context, the mafic schist experiences retrograde metamorphism with the fluid derived from the lithological boundary.
The mineral mode of muscovite in both lithologies correlated with the concentrations and mobility of K, Rb, and Ba. The concentrations of K, Rb, and Ba only increased locally at the lithological boundary compared to the host rocks of both lithologies, suggesting that the element supply was from external fluids rather than internal element redistribution within the outcrop. This implies that the traces of fluid-rock reactions associated with fluids that flowed externally through the lithological boundaries occurred in the underplating oceanic crust. We will further investigate the formation conditions of the reaction zones, including the composition and amounts of fluids flow at the lithological boundaries through geochemical modeling of fluid-rock reactions.