High field strength element depletions in the mantle beneath island arc

本研究项目的研究结果总结如下：1)根据日本东北弧第三系和第四纪火山岩的Sr和Nd同位素资料，提出了两种同位素差异的地幔源。一是未枯竭亚岛弧上地幔，弧后侧-35 Ma ~ -15 Ma玄武岩岩浆，海沟侧和过渡带-21 Ma ~现今玄武岩岩浆。(2)利用103个玄武岩样品(MgO*5%，SiO2*53%)来自海沟侧（包括部分过渡带样品），127个样品来自弧后侧。老玄武岩的HFS元素含量比新玄武岩高。从-35 Ma到现在，源自次岛弧上地幔的玄武岩的Zr/Y值随年龄的减小而逐渐减小，Ti/Zr值随年龄的增大而逐渐增大，而来自上涌软流圈地幔的玄武岩的HFS元素随年龄的减小而不同。3)在Meschede（1986）的2Nb-Zr/4-Y图上绘制了Zr/Y比值最低的玄武岩，表明日本东北弧次岛弧上地幔的HFS元素特征逐渐发生了变化，从高不相容的富HFS元素OIB源，到N-MORB源，再到高不相容的贫HFS元素IAB源，其年龄逐渐减小。亚岛弧上地幔中不相容性较大的HFS元素逐渐枯竭可能是由于-35 Ma以来玄武岩岩浆从源区连续提取所致。在Meschede（1986）的图解中，来自软流圈上浮地幔的玄武岩显示出在过去的15 Ma中，与N-MORB相比，源的Zr和Nb含量略有减少，而与N-MORB相比，源的Zr和Nb含量略有减少。少

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