15:30 〜 17:00
[SGC37-P07] Noble gas isotope compositions of diamonds from the Nyurbinskaya kimberlite pipe, Siberia
キーワード：希ガス、ダイヤモンド、大陸下マントル、プルーム、沈み込み
Diamonds are formed in the mantle 150-200 km below the surface, and their chemical stability and physical hardness suggest that they keep information about noble gases deep in the mantle that were captured internally during their formation. In this study, we analyzed noble gas isotope compositions of diamonds brought to the surface by the eruption of the Nyurbinskaya kimberlite in Siberia.
Twenty-two diamonds were divided into three groups based on the morphology of inclusions: NBRS (cloudy: inclusions localized internally), NBCT (coated: inclusions concentrated around the periphery), and NBCB (cubic: numerous inclusions homogeneously distributed) diamonds were investigated with the Fourier transform infrared spectrometer. Based on the infrared absorption spectra, the samples were further divided into nine groups based on differences in CO2 and H2O concentrations. Each group was crushed in vacuum to extract the noble gases, and the abundances and isotope ratios of helium, neon, and argon were measured using a noble gas mass spectrometer. Depending on the amount of gas, each sample was step-wisely crushed up to four times.
The NBCB samples show the highest 3He/4He, while those of NBCT and NBRS samples are close to those of the subcontinental lithospheric mantle (6 Ra). Interaction of a plume tapping from the deep mantle and associated with high 3He/4He observed in the Siberian flood basalt to the Siberian subcontinental lithospheric mantle prior to the eruption of the Nyurbinskaya kimberlite has been proposed (Broadley et al., 2018). The NBCB diamonds are relatively young and is likely to have been affected by the high 3He/4He of the plume because the diamonds were formed in the mantle just below Siberia after the plume had ascent. NBCT and NBRS diamonds, on the other hand, are thought to have formed at an older age and to have existed in the subcontinental lithospheric mantle prior to the plume's ascent.
Twenty-two diamonds were divided into three groups based on the morphology of inclusions: NBRS (cloudy: inclusions localized internally), NBCT (coated: inclusions concentrated around the periphery), and NBCB (cubic: numerous inclusions homogeneously distributed) diamonds were investigated with the Fourier transform infrared spectrometer. Based on the infrared absorption spectra, the samples were further divided into nine groups based on differences in CO2 and H2O concentrations. Each group was crushed in vacuum to extract the noble gases, and the abundances and isotope ratios of helium, neon, and argon were measured using a noble gas mass spectrometer. Depending on the amount of gas, each sample was step-wisely crushed up to four times.
The NBCB samples show the highest 3He/4He, while those of NBCT and NBRS samples are close to those of the subcontinental lithospheric mantle (6 Ra). Interaction of a plume tapping from the deep mantle and associated with high 3He/4He observed in the Siberian flood basalt to the Siberian subcontinental lithospheric mantle prior to the eruption of the Nyurbinskaya kimberlite has been proposed (Broadley et al., 2018). The NBCB diamonds are relatively young and is likely to have been affected by the high 3He/4He of the plume because the diamonds were formed in the mantle just below Siberia after the plume had ascent. NBCT and NBRS diamonds, on the other hand, are thought to have formed at an older age and to have existed in the subcontinental lithospheric mantle prior to the plume's ascent.