| Abstract |
Water budget in the arc crust is of critical importance for volcanic activities, rheology and geothermal energy budget of the arc crust. Especially, fluid supply by the arc magma is important source of energy for supercritical geothermal reservoirs. According to the geochemical mass balance model, for 1 m along arc distance, 13 t/yr/m H2O are supplied to sub arc in the form of hydrous melt (Kimura and Nakajima, 2014). However, water budget and its distributions within arc crust have not been well constrained yet. In this study, to understand the re-distribution of H2O within the arc crust, differentiation of H2O during crust-melt reactions and subsequent ascent in magma chamber were constrained from crust-melt reaction zones in East Antarctica and melt inclusions in fossil calderas in NE Japan. The granulite-hosted crust-melt reaction zones in Sør Rondane Mountains, East Antarctica, records granulite–granitic magma reactions under 0.5 GPa, 700°C (Uno et al., 2017). Based on the mineralogical and geochemical analyses of the reaction zones, it is revealed that the fluid infiltrated only ~10 cm from the dike wall towards the crust. The flux of water from the dike to the crust is limited to ~4.2 kg/m2 dike surface. Therefore, it is suggested that the most of the H2O liberated from the granitic dike (more than 65%) was transported as excess fluid toward the upper crust through the dike-related fractures. The subsequent budget of H2O transported to upper crust are estimated from fossil calderas distributed in NE Japan arc. Series of Miocene Shirasawa and Fukano calderas (10–6 Ma) consist one of the major fossil caldera clusters in NE Japan. The analyses of melt inclusions in quartz in the calderas suggest that their magma chambers existed at the depth of 1–12 km with dacite–rhyolite melt composition, the H2O content of melt ranges 3–7 wt% and the magma chamber was water saturated at 1–8 km depth (Suzuki et al., 2017; Amanda et al., 2019). Based on these constraints, the flux of H2O supplied into the caldera cluster is estimated as 0.4–2.2 × 10^5 kg/yr, or 2–11 t/yr/m for a unit along arc distance. This is almost the same order as the subarc H2O flux estimated by Kimura and Nakajima (2014). Based on these analyses, 0th-order H2O budget in the arc crust is as follows: most of H2O liberated from subarc magma (4 wt% H2O; ~13 t/yr/m) do not react with surrounding crust, and transport upwards to magma chambers in upper crust, and get saturated ~8 km (5–6 wt% H2O melt; 2–11 t/yr/m). Further detailed understanding of mass balance of H2O is needed with better knowledge of evolution of H2O content with magma differentiation, and crustal lithology, water content, fracture density and permeability. |