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.