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 (Fo86-90) occurs as overgrowth on the rim or along the cleavage traces of primary olivine (Fo90-92). 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.
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