5:15 PM - 6:45 PM
[SGC32-P02] Volatile analyses of diamonds from the Nyurbinskaya kimberlite pipe, Siberia
Keywords:noble gas, diamonds, subcontinental lithospheric mantle
Diamonds are formed in the mantle deeper than 150-200 km below the surface, and their chemical stability and physical hardness suggest that they preserve information about volatiles deep in the mantle that were captured internally during their formation. In this study, we analyzed noble gas isotope and halogen 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: 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 diamonds were investigated with the Fourier transform infrared spectrometer. Based on CO2 and H2O concentrations obtained with the infrared absorption spectra, the samples were further divided into nine subgroups. Each group was crushed in a 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. After that, we also measured the heavy halogen (Cl, Br, and I) in the recovered sample after the crushing experiment using neutron irradiation and noble gas mass spectrometry (Kobayashi et al., 2021). Besides, we estimated the mantle residence times of each sample from the nitrogen-defect aggregation states determined by the infrared spectra (Taylor et al., 1990).
The cubic samples showed the highest 3He/4He (up to 9.5 Ra, where Ra denotes the atmospheric 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 (6.1±0.9 Ra, Gautheron et al., 2002). 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.
Interaction of a plume, which was derived from the deep mantle and associated with high 3He/4He, to the Siberian subcontinental lithospheric mantle prior to the eruption of the Nyurbinskaya kimberlite has been proposed (Broadley et al., 2018).
The following model can be suggested based on this and the experimental results. Some of the coated samples are the oldest, showing the longest mantle residence time and the influence of subduction (low 3He/4He and high 40Ar/36Ar). 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. Halogen compositions will also be presented to see if they are consistent with the model.
Twenty-two diamonds were divided into 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 diamonds were investigated with the Fourier transform infrared spectrometer. Based on CO2 and H2O concentrations obtained with the infrared absorption spectra, the samples were further divided into nine subgroups. Each group was crushed in a 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. After that, we also measured the heavy halogen (Cl, Br, and I) in the recovered sample after the crushing experiment using neutron irradiation and noble gas mass spectrometry (Kobayashi et al., 2021). Besides, we estimated the mantle residence times of each sample from the nitrogen-defect aggregation states determined by the infrared spectra (Taylor et al., 1990).
The cubic samples showed the highest 3He/4He (up to 9.5 Ra, where Ra denotes the atmospheric 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 (6.1±0.9 Ra, Gautheron et al., 2002). 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.
Interaction of a plume, which was derived from the deep mantle and associated with high 3He/4He, to the Siberian subcontinental lithospheric mantle prior to the eruption of the Nyurbinskaya kimberlite has been proposed (Broadley et al., 2018).
The following model can be suggested based on this and the experimental results. Some of the coated samples are the oldest, showing the longest mantle residence time and the influence of subduction (low 3He/4He and high 40Ar/36Ar). 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. Halogen compositions will also be presented to see if they are consistent with the model.