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

建立了深海橄榄岩中单斜辉石的多同位素测量技术。这可能适用于海底的火成岩。深海橄榄岩是现今地幔的碎片。然而，严重的风化和蚀变使我们无法获得详细的地球化学信息。在这里，分离和收集单斜辉石矿物是有效的。由于单斜辉石在海底的蚀变和风化作用相对较强，即使是在强风化或蚀变的橄榄岩中，也不是不可能采集到足够数量和高质量的单斜辉石。这一结果还表明，西南印度洋海岭10°E和55° E玄武岩中氦同位素的不均一性实际上反映了地幔性质。作为一种局部尺度的现象，即使在同位素均匀的样品组中，丰度的不均匀性也是存在的。即使在单斜辉石岩中，许多蚀变/风化敏感元素（如锂）也不是不受蚀变/风化作用的影响。模式计算表明，在大洋中脊区，当^<40>Ar/^<36>Ar比值大于1000时，这种影响可以忽略不计。虽然这样的样品很难找到，Dupal异常，发现在玄武岩从印度洋，没有观察到的单斜辉石的橄榄岩在印度洋。这意味着，在难熔元素中观察到的某些同位素异常是作为一种外来成分在岩浆过程中引起的，因为岩浆通过喷发形成：残留地幔不持有这种异常的同位素组成。作为一种可能的情况，古老的俯冲下地壳物质分散在浅地幔，并选择性地产生具有异常同位素组成的岩浆，并诱导成MORE岩浆。

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.