High field strength element depletions in the mantle beneath island arc

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The results obtained from this research project are summarized as follows.1) On the basis of Sr and Nd isotopic data for Tertiary and Quaternary volcanic rocks from the NE Japan arc, two isotopically different mantle sources have been proposed by the present workers. One is an undepleted sub-island arc upper mantle from which -35 Ma to -15 Ma basaltic magmas in the back-arc side and -21 Ma to the present basalts in the trench side and transitional zone were extracted. The other is depleted asthenosphere which upwelled and intruded the undepleted subisland arc upper mantle beneath the back-arc side of the NE Japan arc at around 15 Ma.2) The temporal change in HFS element (high field strength element) character of two different parts of the mantle beneath the NE Japan arc was investigated using Nb, Ti, Y and Zr in 103 basaltic samples (MgO*5%, SiO2*53%) from the trench side (including some samples from the transitional zone) and 127 samples from the back-arc side. Systematic differences … More are observed in the HFS element contents of older basalts relative to younger ones. Basaltic rocks derived from the subisland arc upper mantle exhibit a gradual decrease in Zr/Y and a gradual increase in Ti/Zr with decreasing age from -35 Ma to the present, whereas those from the upwelled asthenospheric mantle show different HFS element variations with decreasing age from -15 Ma to the present.3) Basalts with the lowest Zr/Y ratios have been plotted on the 2Nb-Zr/4-Y diagram of Meschede (1986), which shows that the sub-island arc upper mantle of the NE Japan arc has gradually changed its HFS element character from a highly incompatible HFS element-enriched OIB source, to an N-MORB source, to a highly incompatible HFS element-depleted IAB source, with decreasing in age. The gradual depletion in HFS elements with greater incompatibility in the sub-island arc upper mantle may be due to the successive extraction of basaltic magmas from the source since -35 Ma ago. On of Meschede's (1986) diagram basalts derived from the upwelled asthenospheric mantle show a change in the source from slightly depleted in Zr and slightly enriched in Nb compared to N-MORB,to a source slightly depleted both in Zr and Nb compared to N-MORB during the last 15 Ma. Less