Geochemical mapping of slab-derived fluid and source mantle along Japan arcs

Although slab-derived fluid significantly affects melt generation and dynamics within subduction zones, its amount and distribution are not sufficiently constrained at present. Herein, we use isotopic systematics of arc volcanic rocks, subducting materials, and intrinsic mantle components prior to metasomatism, to quantify the contribution of the slab-derived fluid that metasomatizes the overlying mantle wedge beneath the entire area of Japan arcs. Simultaneous application of several multivariate statistical analyses (clustering analysis and principal/independent component analyses) to the isotopic data set allows Japan arcs to be broadly divided into eastern and western parts at the first order. Moreover, a clear higher-order inter-arc segmentation is observed, together with some intra-arc variations that possibly correspond to the heterogeneity of incoming plates. Inter-arc segmentation is shown to be primarily controlled by the geometrical parameters of the slab and the arc (e.g., subduction of a single plate or double plates beneath either oceanic or continental crust), which results in differences between mantle wedge and slab thermal conditions. Accordingly, the Kuril and Izu arcs, which have thin arc crusts (~20 km), exhibit the lowest extent of slab-derived fluid addition (0.1 wt%) to the mantle wedge, while the NE Japan arc, with a thicker arc crust (up to 36 km), features a higher value of 0.2 wt%, although the slab thermal parameters for these three arcs are essentially the same. The Central Japan arc shows the highest extent of slab-derived fluid addition (>1.0 wt%) because of the overlapping subduction of Pacific and Philippine Sea slabs, while the SW Japan and Ryukyu arcs feature moderate values of ~0.5 wt%. Moreover, a clear exotic plume zone and spots are observed in SW Japan and the Japan Sea. In addition to the variability of slab-derived fluid composition, the intrinsic mantle composition (before slab-derived fluid–induced metasomatism) shows a clear along-arc variation that is possibly caused by a large-scale mantle flow from the continental side. Thus, slab-derived fluid addition and mantle composition variability equally contribute to inter-arc segmentation, which highlights the importance of both local and regional thermal flow structures of slab-mantle systems.