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

Abstract

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 refl ect a complex process of tectonic cycling of shallow mantle wedge serpentinized peridotites to depth and then back again to the surface.

A large mantle water source for the northern San Andreas fault system: a ghost of subduction past

“…The nearcoastal region of the former forearc broadly coincides with the surface distribution of the serpentinite tectonic blocks and slivers and diapirs in forearc crustal rocks (Figure 2) as well as 'sedimentary' or 'mud' serpentinites that were thought to be part of a mélange of serpentinites erupted on the seafloor in the oceanic basins (Barnes et al 2013), like those observed in some modern subduction zones, such as the Marianas (Fryer et al 1985;Murata et al 2009;Wakabayashi 2011). Field relations in California indicate some of the diapiric or fault-bounded serpentinites were emplaced in the late Cenozoic (Bailey and Everhart 1964;Dickinson 1966;Coleman 1971Coleman , 1977Coleman , 2000Murata et al 1979;Page et al 1999;Wakabayashi 2004Wakabayashi , 2005Wakabayashi , 2012Wakabayashi and Dilek 2011;Wakabayashi et al 2010).…”

Cleavable olivine in serpentinite mylonites from the Oeyama ophiolite

“…The commonness of cleavable olivine at supra -subduction zones is also suggested by recent reports of its occurrence in serpentinized peridotites from the seafloor (Ishii et al, 1992;Ohara and Ishii, 1998;Niida et al, 2001;Murata et al, 2009). Cleavable olivine in these peridotites was probably formed by a similar mechanism.…”

“…Others ascribed the cleavable olivine to deformation related to tectonic movement (Velinsky and Pinus, 1969;Aikawa, 1981;Kutty et al 1983) or annealing after thermal metamorphism (Uda, 1984). Recently, the occurrence of cleavable olivine has been reported from serpentinized peridotites collected from the seafloor (Ishii et al, 1992;Ohara and Ishii, 1998;Niida et al, 2001;Murata et al, 2009). The lack of evidence for prograde metamorphism in these peridotites and for existence of heat source at the seafloor is inconsistent with the newest hypothesis that relates the formation of cleavable olivine to thermal metamorphism.…”

Petrogenesis and geotectonics of the Mikame ultramafic body, western Shikoku, Japan

“…The isotherm curves are from Obata et al (1974). For comparison, the plots of the serpentinites from the Mariana forearc (Murata et al 2009) and the Kurosegawa belt (Yokoyama, 1987) are also shown. The symbols are the same as those in Figure 4.…”

“…The Mikame ultramafic body composed of layered ultramafic cumulates without any residual peridotites probably originated from a lowermost part of the lower crust. Depleted ultramafic cumulates and residual peridotites, which are derived from the lower crust to upper mantle, have been collected from landward slopes in trenches and from serpentinite seamounts in forearc regions in the Izu-Bonin-Mariana (IBM) arcs (Bloomer and Hawkins, 1983;Ishii, et al, 1992;Parkinson and Pearce, 1998;Ishiwatari et al, 2006;Murata et al, 2009;Morishita et al, 2011). Moreover, meta-serpentinites also occur in the IBM forearc.…”