Clastic metabasites and authigenic minerals within serpentinite protrusions from the Mariana forearc: Implications for sub-forearc subduction processes

Recent discoveries of actively protruding sedimentary serpentinite deposits from the Mariana forearc have provided a window into metamorphic and devolatilization processes on the subducting slab. The protrusion of these deposits on the seafloor has constructed large km-scale seamounts, probably upwelling through fault-bounded conduits in the suprasubduction-zone mantle before reaching the seafloor.;The fluids venting through these deposits are ultimately derived from the subducting slab and have distinctive chemistries. Authigenic minerals are formed where these fluids interact with seawater, the most common of which are carbonate minerals. delta13C values extend from -27.4 to 10.2‰ VPDB (Vienna PeeDee Belemnite standard) suggesting a range from light values typically associated with microbial oxidation of CH 4-bearing fluids to heavy values common in carbonates from other serpentinite/ultramafic deposits. The delta18O values extend from -10.2 to 12.3‰ VPDB suggesting carbonates that were precipitated in response to fluid temperatures that fluctuate from ambient to slightly warm temperatures and/or were not in equilibrium with seawater.;Clasts of basic metamorphic rocks and minerals, interpreted to come from the subducting Pacific plate, have been sampled in these serpentinites and are typically of the moderate- to high-pressure and low-temperature blueschist facies conditions expected for sub-forearc subduction zone environments. The majority of minerals have no indication of retrograde metamorphism or metasomatism, indicating that most of the clasts ascended relatively rapidly through the suprasubduction-zone mantle wedge and forearc crust without any significant chemical interaction with the surrounding environment. Bulk rock compositions have been calculated for these clasts using a combination of element mapping and modal analysis. Bulk compositions are generally basaltic, but with unusually high Mg and low Al. Geobarometric and geothermometric results are in agreement with previous work, and inferred conditions of metamorphism are approximately 0.45 to 0.6 GPa and 250 to 300°C.