演題概要

1P-19

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シベリア産ダイヤモンドの揮発性成分分析

(¹東大・ ²IGM SBRAS・ ³Novosibirsk State Univ.)
^o佐南谷光¹・ 福島菜奈絵¹・ Zedgenizov, Dmitry^2,3・ 角野浩史¹

We analyzed volatile compositions of diamonds from the Nyurbinskaya kimberlite pipe in Siberia. There were three groups based on the morphology of inclusions: cloudy (inclusions are localized in the core of the diamond), coated (inclusions are concentrated in the rim of the diamond), and cubic (numerous inclusions homogeneously distributed in the diamond). The cubic samples showed the highest 3He/4He among the three groups, while the coated and cloudy samples showed 3He/4He close to or lower than that of the subcontinental lithospheric mantle. Some of the coated samples showed the longest mantle residence times of 200 Ma. The cubic, cloudy, and the rest of the coated samples showed residence times of around 80 Ma with no significant difference. The following model can be suggested based on the results. Some of the coated samples are the oldest, showing the longest mantle residence time and the influence of subduction. After the plume rose, the cubic samples were formed in the subcontinental lithospheric mantle, of which the plume influence had modified the helium isotope ratio to a higher value. Around the same time, the cloudy and the rest of the coated samples were formed in the subcontinental lithospheric mantle, relatively distant from the plume.