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High field strength element depletions in the mantle beneath island arc

岛弧下方地幔中的高场强元素损耗

基本信息

批准号：
07404026
负责人：
SHUTO Kenji
金额：
$ 22.85万
依托单位：
Niigata University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (A)
财政年份：
1995
资助国家：
日本
起止时间：
1995 至 1996
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-07404026/
关键词：
Northeast Japan arc Sr and Nd isotopes source mantle HFS element depletion basalt magma OIB source MORB source 北海道北部 HFS元素玄武岩 Sr同位体比 Nb同位対比マントルプリューム Nd同位体比

项目摘要

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

本研究项目取得的成果总结如下：1)根据日本东北弧第三纪和第四纪火山岩的Sr和Nd同位素数据，目前的工作人员提出了两种同位素不同的地幔源。一是未枯竭的亚岛弧上地幔，从中提取出弧后侧-35 Ma至-15 Ma玄武岩浆，以及海沟侧和过渡带-21 Ma至现今玄武岩。另一种是贫化软流圈，它在 15 Ma 左右上涌并侵入日本东北弧弧后侧下方未贫化的岛弧上地幔。 2）利用 103 个玄武岩样品（MgO*5%，沟槽侧的 SiO2*53%）（包括过渡区的一些样品）和弧后侧的 127 个样品。相对于较年轻的玄武岩，较古老的玄武岩的 HFS 元素含量存在系统差异。源自岛弧上地幔的玄武岩，从-35 Ma至今，随着年龄的降低，Zr/Y逐渐减少，Ti/Zr逐渐增加，而来自上升的软流圈地幔的玄武岩，从-15 Ma至今，随着年龄的降低，HFS元素呈现出不同的变化。 3）Zr/Y比值最低的玄武岩绘制在 Meschede(1986)的2Nb-Zr/4-Y图显示，日本东北弧的亚岛弧上地幔的HFS元素特征随着年龄的减小，逐渐从高度不相容的富含HFS元素的OIB源转变为N-MORB源，再转变为高度不相容的贫HFS元素IAB源。亚岛弧上地幔中HFS元素逐渐贫化且不相容性较大，可能是由于-35Ma前玄武质岩浆不断从源区抽取所致。 Meschede (1986) 的来自上涌软流圈地幔的玄武岩图显示，在过去 15 Ma 中，来源从与 N-MORB 相比稍微贫 Zr 和稍微富 Nb，到与 N-MORB 相比 Zr 和 Nb 都轻微贫。较少的