16:30 〜 16:45
[SGC37-15] MORB-like mantle noble gases beneath the oldest oceanic plate
★Invited Papers
キーワード：マントル、希ガス同位体、プチスポット火山、リサイクル物質
The Earth’s mantle has been homogenized by convection. Nevertheless, the mantle has also undergone heterogenization through subduction of surface material and incorporation of deep mantle components by upwelling plumes. Noble gases serve as unique tracers of mantle heterogeneity because of their large isotopic variations in geochemical reservoirs, high volatility, and chemical inertness. The elemental and isotopic compositions of noble gases from uniquely gas-rich MORB sample “popping rock” and magmatic CO2 well gases suggest that the substantial amounts of non-radiogenic isotopes of Ar, Kr, and Xe in the upper mantle are derived from recycling of atmosphere by oceanic plate subduction, while Ne is almost primordial in origin (Holland & Ballentine 2006; Parai & Mukhopadhyay 2021).
Petit-spot volcanoes have been discovered on the old Pacific plate near the subduction zone. The major and trace element compositions of petit-spot basalts suggest that the magma was formed by a small degree of partial melting of mantle material (Hirano et al. 2006). The EM1-like Sr–Nd–Pb isotopic compositions likely reflect preferential melting of recycled material in the mantle due to the lower melting temperature than depleted peridotite (Machida et al. 2009). If the atmospheric Ar, Kr, and Xe are concentrated in the recycled materials, the heavy noble gases would be significantly enriched in the melt relative to Ne. Petit-spot volcanic samples therefore provide a unique opportunity to investigate the origin of atmospheric noble gases in the upper mantle.
A fresh, glassy, vesicular basaltic sample 6K#1466R3-003 (hereafter R3-003) from a petit-spot volcano near the Minamitorishima (Marcus) island was used for noble gas analysis. We measured all noble gas isotopic compositions and CO2 abundance of the vesicle gas using two different extraction techniques to minimize atmospheric contamination: stepwise crushing and laser ablation at the University of Tokyo.
The 20Ne/22Ne ratios from some measurements are the upper mantle value of 12.5. This indicates that post-eruptive atmospheric contamination is negligible. Here, we define 21Ne*, 40Ar*, and 136Xe* as the nucleogenic or radiogenic abundance of each isotope. 136Xe*U is the abundance of 136Xe produced by the spontaneous fission of 238U. The 40Ar*/21Ne* and 136Xe*U/21Ne* ratios for R3-003 are consistent with the production rate ratios in the mantle. This suggests that the magmatic noble gases in R3-003 avoided significant elemental fractionation during degassing, thus preserving the source mantle melt composition. The element abundance pattern for non-radiogenic isotopes (22Ne, 36Ar, 84Kr, and 130Xe) in the source melt is similar to those of the popping rock. The MORB-like noble gas composition of the isotopically EM1-like magma may indicate that the recycled atmospheric noble gases are homogenized with the primordial noble gases in mantle peridotite by diffusion. In a previous study, the isotopic composition of upper mantle Xe suggested that the recycling of atmospheric noble gases started at ~2.5 Gyr ago (Parai & Mukhopadhyay 2018). The average diffusion distance of Ar in the mantle olivine during 2.5 Gyr is ~102 m, calculated from the experimentally determined diffusion coefficient (Delon et al. 2019). This length may be the upper limit for the size of recycled material in the petit-spot source mantle. Furthermore, the MORB-like noble gas composition of the petit-spot source mantle melt, formed at a different location and by a different mechanism than mid-ocean ridges, suggests that recycled material subducted since 2.5 Ga is homogeneously distributed in the upper mantle on a global scale.
Petit-spot volcanoes have been discovered on the old Pacific plate near the subduction zone. The major and trace element compositions of petit-spot basalts suggest that the magma was formed by a small degree of partial melting of mantle material (Hirano et al. 2006). The EM1-like Sr–Nd–Pb isotopic compositions likely reflect preferential melting of recycled material in the mantle due to the lower melting temperature than depleted peridotite (Machida et al. 2009). If the atmospheric Ar, Kr, and Xe are concentrated in the recycled materials, the heavy noble gases would be significantly enriched in the melt relative to Ne. Petit-spot volcanic samples therefore provide a unique opportunity to investigate the origin of atmospheric noble gases in the upper mantle.
A fresh, glassy, vesicular basaltic sample 6K#1466R3-003 (hereafter R3-003) from a petit-spot volcano near the Minamitorishima (Marcus) island was used for noble gas analysis. We measured all noble gas isotopic compositions and CO2 abundance of the vesicle gas using two different extraction techniques to minimize atmospheric contamination: stepwise crushing and laser ablation at the University of Tokyo.
The 20Ne/22Ne ratios from some measurements are the upper mantle value of 12.5. This indicates that post-eruptive atmospheric contamination is negligible. Here, we define 21Ne*, 40Ar*, and 136Xe* as the nucleogenic or radiogenic abundance of each isotope. 136Xe*U is the abundance of 136Xe produced by the spontaneous fission of 238U. The 40Ar*/21Ne* and 136Xe*U/21Ne* ratios for R3-003 are consistent with the production rate ratios in the mantle. This suggests that the magmatic noble gases in R3-003 avoided significant elemental fractionation during degassing, thus preserving the source mantle melt composition. The element abundance pattern for non-radiogenic isotopes (22Ne, 36Ar, 84Kr, and 130Xe) in the source melt is similar to those of the popping rock. The MORB-like noble gas composition of the isotopically EM1-like magma may indicate that the recycled atmospheric noble gases are homogenized with the primordial noble gases in mantle peridotite by diffusion. In a previous study, the isotopic composition of upper mantle Xe suggested that the recycling of atmospheric noble gases started at ~2.5 Gyr ago (Parai & Mukhopadhyay 2018). The average diffusion distance of Ar in the mantle olivine during 2.5 Gyr is ~102 m, calculated from the experimentally determined diffusion coefficient (Delon et al. 2019). This length may be the upper limit for the size of recycled material in the petit-spot source mantle. Furthermore, the MORB-like noble gas composition of the petit-spot source mantle melt, formed at a different location and by a different mechanism than mid-ocean ridges, suggests that recycled material subducted since 2.5 Ga is homogeneously distributed in the upper mantle on a global scale.