Petrology and geochemistry of cross‐chains in the Izu‐Bonin back arc: Three mantle components with contributions of hydrous liquids from a deeply subducted slab

Abstract

Detailed petrological and geochemical analyses of volcanic rocks from back‐arc en echelon seamounts, consisting of four cross‐chains in the Izu‐Bonin back arc, provide insights into the origin of complex magma types for back‐arc volcanism associated with back‐arc spreading in an oceanic arc environment. The sampled volcanic rocks are classified into three distinct rock suites on the basis of trace element characteristics: an “Enriched Suite” with enriched trace element compositions and high Nb/Zr (0.075–0.10); a “Less Enriched Suite” with depleted high‐field‐strength elements (HFSEs), lower Nb/Zr (0.03–0.075), and enriched large ion lithophile elements (LILEs) and light rare earth elements (LREEs); and a “Depleted Suite” with very depleted HFSEs, the lowest Nb/Zr (<0.03), and slightly enriched LILEs and LREEs. Estimated primary melt compositions of each suite and numerical open‐system melting models for trace elements indicate that different HFSE ratios and abundances in primary magmas for the three suites cannot be explained solely by differences in degree of melting or subduction inputs via slab melting and dehydration. Our results indicate the following: (1) 15% partial melting of source mantle for Shikoku Basin basalt and an influx of fluid‐mobile‐element‐poor supercritical liquid, amounting to 0.075% of the source mantle, from residual (predehydrated) subducted slab material can reproduce the geochemistry of the Less Enriched Suite primary melt. (2) The residual (depleted) mantle, derived from melt extraction by back‐arc basin opening and/or previous Less Enriched Suite volcanism, cannot be source mantle for Depleted Suite lavas, which require a distinct mantle source that is more depleted in trace elements (especially Nb and Ta) than >2σ depleted mid‐ocean ridge basalt mantle (D‐DMM). (3) Source mantle for Enriched Suite rocks is distributed independently beneath the Izu‐Bonin back‐arc region. (4) Less Enriched Suite magmas erupted in all seamount chains, indicating that these magmas constitute the major volcanic component of back‐arc cross‐chain volcanism in the northern Izu‐Bonin arc. We conclude that three distinct mantle sources with various contributions from a deeply subducted slab exist beneath the cross‐chains in the Izu‐Bonin back arc.