研究業績
Deep Sea Research Part I: Oceanographic Research Papers 180, 103687 (2022)
Re-Os geochemistry of hydrothermally altered dacitic rock in a submarine volcano at Site U1527, IODP Expedition 376: Implications for the Re cycle in intraoceanic arcs
著者
Ishida, M., Nozaki, T.*, Takaya, Y., Ohta, J., Chang, Q., Kimura, J.-I., Nakamura, K. and Kato, Y.
カテゴリ
学術論文
Abstract
Despite the widespread use of Re?Os isotopes in various geoscience fields, relatively little is known about the behavior of Re and Os during fluid-mediated hydrothermal alteration of igneous rocks in the (sub)seafloor, especially in an intraoceanic arc setting. Here, we provide a depth profile of Re?Os geochemistry at Site U1527, located on the NW caldera rim of the Brothers volcano hydrothermal field in the Kermadec arc and drilled during International Ocean Discovery Program (IODP) Expedition 376 in 2018. Volcaniclastic rocks from Hole U1527C that had experienced various degrees of high- and low-temperature hydrothermal alteration were analyzed for bulk chemical composition and Re?Os isotopes. The concentration of Re varied from 0.172 to 18.7 ppb, and that of Os from 9.7 to 147.1 ppt. Hydrothermal alteration usually resulted in Re uptake by rocks, but a part of Re was released into the ocean by later oxidative weathering. Compared with Re, Os mobility resulting from hydrothermal alteration was limited. The relatively homogenous 187Os/188Os values (0.13?0.14) of the original volcaniclastic rocks were altered, becoming slightly more radiogenic (up to 0.212), by the addition of seawater-derived radiogenic Os by (1) Os precipitation induced by seawater?hydrothermal fluid mixing, which was accompanied by enrichment with Ba (barite precipitation) as well as by the highest Re and Os concentrations in the profile, and (2) adsorption of seawater-derived radiogenic Os onto Fe hydroxide without any corresponding enrichment of Ba, Re, or Os. Compared with high temperature-altered zones of mid-ocean ridge basalt, the altered samples in this study showed intense Re enrichment, presumably reflecting abundant pyrite precipitation in a Re-rich magmatic?hydrothermal environment unique to the arc setting. The results at Hole U1527 may suggest that subseafloor high-temperature alteration can be a sink for Re degassed from magma. The altered rock may also be a source of Re in the ocean as a result of subsequent low-temperature alteration.