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

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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.