Investigation of geochemical heterogeneity in mantle from abyssal peridotites using new isotopes

使用新同位素研究深海橄榄岩地幔的地球化学异质性

基本信息

批准号：
14540457
负责人：
KUMAGAI Hidenori
金额：
$ 2.37万
依托单位：
Independent Administrative Institution, Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
2002
资助国家：
日本
起止时间：
2002 至 2004
项目状态：
已结题

项目摘要

The technique of multi-isotope measurement for clinopyroxene separates from abyssal peridotites has been established. This could apply to the igneous rock on the seafloor. The abyssal peridotites are the very fragment of present mantle. However, serious weathering and alteration prevented us from obtaining detailed geochemical information. Here, to separate and collect clinopyroxene minerals are valid. Because clinopyroxene is relatively robust for alteration and weathering on the seafloor, even in highly weathered or altered peridotites, it is not impossible to collect sufficient amount with high quality.The agreement of helium isotopes between peridotites and basalts has been reported as the world premiere. This result also suggest that heterogeneity of helium isotopes found in basalts from 10°E and 55° E of the Southwest Indian Ridge actually reflects mantle nature. As a local scale phenomenon, the abundance heterogeneity even in an isotopically homogeneous sample suite. This signature may record that the magmatic process within the uppermost mantle.Even in clinopyroxene separates, much alteration/weathering sensitive elements (e.g.Lithium) are not free from alteration/weathering. A model calculation suggests that such an effect may be negligible in the case that ^<40>Ar/^<36>Ar ratio are greater than 1000 in the Mid-Oceanic Ridge area. Although such a sample is hardly found, the Dupal anomaly, found in basalt from the Indian Ocean, was not observed in the clinopyroxenes of peridotites in the Indian Ocean. This implies that some of isotopic anomaly observed in the refractory elements are induced as an exotic component during magmatic processes since magma genesis through eruption : residual mantle does not hold such anomalous isotopic compositions. As a possible scenario, the ancient subducted lower crustal material is scattered within shallow mantle and selectively generates magma having anomalous isotopic composition and induced into MORE magma.

已经建立了与深渊橄榄岩分离的多同位素测量技术。这可能适用于海底的火成岩。深渊橄榄岩是当前地幔的非常碎片。但是，严重的风化和改变使我们无法获得详细的地球化学信息。在这里，要分离并收集壳氧烯矿物是有效的。由于簇氧烯对于在海底的改变和风化相对强大，即使在高度风化或改变的橄榄岩中，也不可能以高质量收集足够数量。橄榄岩与玄武岩之间的氦同位素一致。该结果还表明，从10°E和西南印度山脊的玄武岩中发现的氦同位素的异质性实际上反映了地幔性质。作为局部规模现象，即使在同位素均匀的样品套件中，丰度异质性也是如此。该签名可能会记录最上层内的岩浆过程。即使在斜氧烯中分开，许多改变/风化敏感元件（例如，lithium）并非没有改变/风化。一个模型计算表明，在 ^<40> ar/ ^<36> ar的比率大于1000的情况下，这种效应可能可以忽略不计。尽管几乎找不到这样的样本，但是在印度洋的橄榄岩的clinopoyroxenes中未观察到在印度洋的玄武岩中发现的Dupal异常。这意味着在岩浆过程中，在岩浆过程中观察到的某些同位素异常是作为外来成分诱导的，因为岩浆发生了通过喷发：残留地幔没有这种异常的同位素组成。作为可能的情况，古老的俯冲下层地壳材料散落在浅套中，并有选择地产生具有异位异位组成的岩浆，并诱导更多的岩浆。