Phengite breakdown and associated fluid flow in Pacific-type subduction zone: Investigating the nature of slab-derived fluids of blueschist-facies metapelites from the Renge and Sambagawa belts (SW. Japan).

White micas breakdown in down-going slabs of subduction zones implies consequent fluids release, inducing element transport into the overlying hanging-wall mantle. Phengite is the most common white mica occurring in HP / UHP metasedimentary rocks, carrying significant amounts of H2O, LILE (K, Ba, Cs and Cr especially), and Li, B or N to the upper mantle. Here, 2H/1H (D/H) and 18O/16O ratios of 23 metapelites samples from the Devonian-Carboniferous Renge and Cretaceous Sambagawa belts are investigated to better understand the O and H isotope signatures of phengites in metapelites of the Pacific-type subduction zone. In addition, we try to constrain the stable isotopic compositions of metamorphic fluids equilibrated with phengites and see their behavior during continuous dehydration reactions.The investigated pelitic blueschist-facies phengite samples presented non negligeable values of ∂D (∂D < -88‰). 14 of them belong to the Osayama serptentinite melange (central Chugoku Mountains, SW. Japan) of the Renge Belt and separated from lawsonite- and epidotes-grade. They presented a significantly negative ∂D composition, ranging from -113.2‰ to -88.3‰, and a ∂O composition ranging from 12.9‰ to 14.6‰ (∂D and ∂O values approximate SMOW). The 9 other samples are garnet-bearing metapelites of the Sarutagawa schists from Sambagawa Belt (central Shikoku, SW. Japan) and presented ∂D = -95.6‰ to -60.5‰ and ∂O = 12,3‰ to 14,4‰.Fluids can be characterized as deep-sourced by looking at previous results on high-Si features and K-Ar ages of the investigated samples (Tsujimori & Itaya, 1999). The consequently low values of ∂D cannot be due to meteoric-hydrothermal alteration but by isotopic fractionation during prograde metamorphic dehydration of a plunging slab. Modelling on the obtained data and muscovite, H2O, H and O factors fractionation for nominal temperatures allowed to estimate an isotopic composition for metamorphic fluids equilibrated with phengites. We unveil through this study that slab-devolatilization derived fluids in Pacific-type subduction zone present low ∂D value, implying a non-negligeable role of the phengite breakdown on H isotope composition of nominally anhydrous minerals (NAMs) in deep mantle.