Trace and volatile element variations in slab liquids across the northern Japan arcs

Subduction zone magmatism is generated by the interaction of the hot mantle wedge with liquids released from the subducting slab. These liquids carry geochemical signatures reflecting the partitioning behaviour of elements between mineral phases and the liquid. Studies of natural high grade metamorphic suites from old subduction systems, combined with dehydration/melting experiments of basaltic and pelitic rocks and thermodynamic modelling has help predicting the mineralogical composition of the slab and the partitioning behaviour of trace elements at PT conditions relevant to subduction systems, whereas geodynamic and seismic models have allowed the estimation of the slab PT paths for many subduction systems around the world. Olivine-hosted melt inclusions from seven volcanic systems in northern Japan that cover a wide range of PT slab conditions (c. 2.8 to 7.9 GPa and c. 710 to 860 °C) are studied to identify geochemical variations related to progressive reactions in the slab. The trace and volatile elements systematics are correlated with predicted prograde metamorphic reactions occurring in the different layers of the subducting slab. Progressive increase of halogens/HREE and LILE/HREE ratios from arc to back-arc is related to increasing antigorite breakdown from the hydrated lithospheric mantle. Reduction of most LILE/HREE and Cl/HREE at slab conditions of very high PT in the back-arc indicate exhaustion of lithospheric serpentine, whereas roughly constant inputs of Pb, Sr and F along all PT slab conditions reflect the constant participation of lawsonite. The increasing LREE/HREE with slab surface temperature is the result of the increasing mobility of LREE throufh allanite and monazite dissolution. HFSE enrichment is observed at PT conditions that are expected to generate supercritical fluids (> 6 GPa). Fluid flux models of F and Cl systematics indicate that halogen abundances with similar Cl/F ratios in arc magmas are explained by a homogeneous slab liquid composition at different fluid amounts added to the mantle wedge and similar degrees of melting. Slab liquid composition varies to the back-arc though enhancement of LILE, halogen and H2O contents provided by antigorite breakdown. Serpentine exhaustion at >6 GPa conditions produces depletion of LILE and Cl, generating low Cl/F ratios.