Oxygen and hydrogen isotopes compositions of circum-pacific blueschist-facies phengites: Insights into isotopic slab-derived fluid signatures

Phengite is the most common metamorphic mineral in HP-UHP metasedimentary rocks, which can convey H₂O, LILEs (especially K, Ba, Cs and Rb), Li, B and N in their structure formed at depths up to 300 km. The breakdown of phengite in a downgoing oceanic slab would cause fluid-induced element transport into the overlying mantle wedge. We have investigated the ²H/¹H (D/H) and ¹⁸O/¹⁶O ratios of twenty-four phengite separates from pelitic schists of the Devonian-Carboniferous Renge Belt (SW Japan), Permian Shaiginsky Complex (Far East Russia) and Cretaceous Sambagawa Belt (SW Japan).

We found the presence of the very light hydrogen isotope (δD < -95‰) in blueschist-facies phengites in the three different metamorphic belts. For example, phengite from the lawsonite- and epidote-grade metasedimentary schists of the Osayama Serpentinite Mélange (OSM) of the Renge Belt are characterized by negative hydrogen isotope compositions (δD values relative to VSMOW) ranging from -113 to -93.9‰ and oxygen isotope compositions (δ18O values relative to VSMOW) ranging from +12.9 to +14.6‰.

High-Si features and K-Ar ages of the investigated phengites deny the possibility of meteoric-hydrothermal alteration to have caused the low δD values. The light values might be attributed to isotopic fractionation during progressive metamorphic dehydration. Assuming a meamorphic temperatures range of 250-350°C for the OSM schists, the inferred metamorphic fluid compositions in blueschist-facies depth for that fossil slab had a range of δ D = ~-40 to -75‰ and δ18O = ~+13 to +15‰. These values are significantly lighter than the slab-fluid induced from the Arima hot spring water in a forearc region of modern SW Japan subduction zone. Our study suggests that slab-derived fluids in ancient Pacific-type subduction zone are characterized by light hydrogen isotope and that the phengite breakdown can affect hydrogen isotope of nominally anhydrous minerals (NAMs) in the deep mantle.