High field strength element depletions in the mantle beneath island arc

主要研究结果如下：1）根据日本东北弧第三纪和第四纪火山岩的Sr、Nd同位素数据，提出了两种同位素不同的地幔源区。一个是未亏损的次岛弧上地幔，弧后侧提取了-35 ~-15Ma的玄武质岩浆，海沟侧和过渡带提取了-21Ma ~现今的玄武岩;另一个是亏损的软流圈，它在15 Ma左右上涌侵入日本东北弧弧后一侧未亏损的岛弧上地幔本文用Nb、Ti、Y、Zr等元素对103个玄武岩样品进行了研究，（MgO* 5%，SiO2*53%）（包括来自过渡区的一些样品）和来自弧后侧的127个样品。系统性差异 ...更多信息 的HFS元素含量的老玄武岩相对于年轻的。从-35Ma到现在，来自岛弧上地幔的玄武质岩石的Zr/Y逐渐减小，Ti/Zr逐渐增大，而上涌软流层地幔中的玄武岩则表现出不同的HFS元素随年龄的变化。3）在Meschede的2Nb-Zr/4-Y图解上，1986），表明日本东北弧次岛弧上地幔的HFS元素特征随着年龄的减小，逐渐由高度不相容的HFS元素富集OIB源，转变为N-MORB源，再转变为高度不相容的HFS元素亏损IAB源。岛弧上地幔中HFS元素的逐渐贫化和不相容性的增强可能是由于-35Ma以来源区玄武质岩浆的连续抽取。在Meschede（1986）的图解中，来自上涌软流圈地幔的玄武岩显示出在过去15 Ma期间源区的变化，从与N-MORB相比轻微亏损Zr和轻微富集Nb，到与N-MORB相比轻微亏损Zr和Nb。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