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Significance of serpentinization of wedge mantle peridotites beneath Mariana forearc, western Pacific

¹ Department of Literature, Kobe Women's University, Kobe 654-8585, Japan, and Graduate School of Science, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan
² Graduate School of Science, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan
³ Graduate School of Science and Technology, Kumamoto University, Kurokami 2, Kumamoto 860-8555, Japan
⁴ Graduate School of Environmental Studies, Nagoya University, Chikusa-ku, Nagoya 464-8601, Japan
⁵ Japan Agency for Marine-Earth Science and Technology, Natsushima-cho, Yokosuka 237-0061, Japan
⁶ Department of Earth Sciences, Faculty of Science, Okayama University, Tsushima-Naka 3, Okayama 700-8530, Japan
⁷ Department of Earth Sciences, Nara University of Education, Takabatake-cho, Nara 630-8528, Japan

In the Mariana forearc, horst and graben structures are well developed in the outer forearc basement, which is composed of both island arc and oceanic crust-mantle rocks. A zone of dome-shaped diapiric seamounts, which are composed mainly of serpentinized peridotites, formed on the basement in the outer forearc regions. Serpentine minerals in peridotites from both diapiric seamounts and basement are mostly chrysotile and/or lizardite. Antigorite, however, is rarely found in peridotites recovered from Conical, Big Blue, Celestial, and South Chamorro Seamounts. Antigorite-bearing peridotites always contain secondary iron-rich olivine and metamorphic clinopyroxene, and antigorite seems to coexist stably with them. Iron-rich secondary olivine (Fo_86–90) occurs as overgrowth on the rim or along the cleavage traces of primary olivine (Fo_90–92). The assemblage shows high-temperature conditions of serpentinization at ~450–550 °C, whereas chrysotile- and/or lizardite-bearing assemblages occur at ~200–300 °C. In antigorite-bearing samples, chrysotile and/or lizardite veins both predating and postdating antigorite formation are recognized. This may reflect a complex process of tectonic cycling of shallow mantle wedge serpentinized peridotites to depth and then back again to the surface.