Yonaguni Knoll IV is an active seafloor hydrothermal system containing massive Zn–Pb sulfides located in the southwestern Okinawa Trough. Previous research has suggested that the metal elements Zn and Pb were dominantly derived from the local volcanic rocks via water–rock interactions. However, it is not known whether or why the volcanic rocks are rich in these metals. Here, we performed a detailed in situ analysis of the major and trace elements in amphiboles in the rhyolite from the hydrothermal field to decipher the physical-chemical conditions of the silicic magma, including temperature (T), pressure (P), oxygen fugacity (fO2), volatile contents (Cl, H2O) and metal contents (Pb, Zn), and investigate the behavior of the metals (Zn and Pb) during melt evolution. Our results show that the amphiboles are euhedral calcic magnesio-hornblende and are in equilibrium with the hydrous and oxidized host rhyolitic melt characterized by high H2Omelt (~5.8–6.5 wt.%), fO2 (NNO +0.1 to +0.6) and SiO2 melt (~71.73–72.62 wt.%) values at 761–797°C and 102–137 MPa, corresponding to an approximate depth of 3.9–5.2 km. In addition, the rare earth element (REE) patterns of the melt reconstructed from the amphiboles are analogous to those of cold-wet-oxidized rhyolites. Thus, the amphiboles crystallized in a cold-wet-oxidized shallow silicic magma chamber. The high oxygen fugacity is inferred to have favored sulfur and metal enrichment in the melt. The most abundant metal in the amphiboles is Zn (237–294 ppm), and the concentrations of Pb are much lower (0.86–3.32 ppm), suggesting that Zn preferentially entered the amphibole and that Pb was retained in the melt. The Cl content of the amphiboles ranges from 0.11 to 0.25 wt.%, and accordingly, the Clmelt content estimated from the amphiboles varies from 0.15 to 0.28 wt.%. The low (Cl/H2O)melt ratios, which are <0.05 (i.e., ~0.02–0.04), suggest that the melt exsolved an H2O-rich vapor rather than a hydrosaline fluid with insufficient Cl to transport metals (e.g., Pb and Zn) into the vapor phase during degassing both in the magma chamber and during ascent. The lack of exsolved hydrosaline fluid and the high oxidation conditions resulted in the metal elements Pb and Zn remaining dissolved in the rhyolitic melt and/or partitioning into crystalline minerals upon eruption. Thus, the large volumes of rhyolitic magma that intruded into the upper crust of the Yonaguni Knoll IV hydrothermal field are a potential fertile source of these elements via leaching by heated seawater.
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