Izu-Bonin rear-arc magmatism: Geochemical investigation of volcanoclastic material
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Studied samples from the Izu Bonin rear arc show a distinct geochemical pattern that resemble the modern continental crust. In contrast to the volcanic front, samples from the Izu Bonin rear arc show enrichment of LREE (La, Ce, Pr, Nd) and higher K2O at a given SiO2. This suggest that processes leading up to the geochemistry observed in the rear arc is fundamental in creating the modern continental crust. Additional isotopic and trace element analysis from volcanic material recovered in the rear arc basin between Enpo and Manji seamount chain, will give new insight in the temporal variation in this magmatic regime. The drilled core from Izu Bonin rear arc represent a time interval of 6 Ma, where the oldest unit (VII) is 12 Ma and the youngest unit (IV) is 6 Ma. Analysis of trace element show a progressively enriched mantle signature. Going from a flat, MORB like pattern in unit VII to an enriched LREE pattern in unit IV. Elemental ratios such as Nb/Zr are higher for unit IV compared to unit VII. For all units analyzed, a high LILE/HFSE ratio is observed. The isotopic composition of Nd and Hf show a decreasing trend towards unit IV, where 143^Nd/144^Nd is going from 0,513068 to 0,512911 and 176^Hf/177^Hf from 0,2832460 to 0,2831803. Better correlation between Pb isotopic composition and Nb/Zr than for Sr and Nb/Zr suggest that the slab derived material is progressively added to the mantle wedge as the mantle becomes more enriched. These results suggest that the sources involved in rear arc magmatism has undergone a gradual change in the components contributing to the magma. Going from a depleted MORB like magmatism to a more enriched rear arc magmatism coincide with the cessation of back arc spreading in 15 My. A subduction component is added to the magma source as it becomes progressively more enriched.