11:00 AM - 11:15 AM
[SVC32-17] Tracing the Magma Plumbing System of Kirishima Volcano through Helium Isotopes in Phenocrysts of the Volcanic Ejecta
Keywords:Kirishima, Inclusion, Helium, Magma plumbing system, isotope
Noble gases are chemically inert and have different isotopic ratios in different geochemical reservoirs, making them useful tracers for the origin of Earth’s materials. For example, isotopic ratio of helium, 3He/4He, is 1.4 × 10-6 ≡ 1 RA in the atmosphere, while the ratio is higher in the mantle because a significant portion of primordial helium having high 3He/4He is still preserved there. 3He/4He = 8 (± 1) RAis the representative value in the convecting mantle. On the other hand, the crust has a low isotopic ratio of 3He/4He < 0.02 RA because of 4He addition from decays of U and Th. These properties make helium a valuable tool for monitoring volcanic activity, as isotopic anomalies in volcanic gases often correlate with eruptions (e.g.,1). However, volcanic gas data do not constrain the eruption processes because they provide only time-series information. In addition, it can be influenced by interactions with crustal and atmospheric components, which hampers estimating magma 3He/4He composition from volcanic gases.
To overcome these limitations, noble gas isotopic compositions in olivine and pyroxene phenocrysts from volcanic rocks were analyzed. These minerals can trap melt inclusions during their crystallization, preserving helium isotopic composition due to slow diffusion. In this study, isotopic composition of minerals that erupted in the Kirishima volcano group, which includes the Shinmoe-dake, Iwo-yama, and Ohachi volcanoes, were analyzed. A superiority using of this volcano for the isotope study is under three points. (1) Juvenile materials at the Shinmoe-dake 2011 eruption were sampled. (2) The subsurface structure was proposed by geophysical and petrological studies. (3) Helium isotope data of volcanic gas monitoring at the Iwo-yama were available.
Olivines (>0.5 mm, 200–700 mg) and pyroxenes (>1 mm, 500–700 mg) were extracted from pumice from the 2011 Shinmoe-dake eruption, and olivines (>0.25 mm, 100–300 mg) were separated from three lavas of Ohachi (erupted in 788, 1235, and 1350 AD). Noble gases were released by crushing minerals in a vacuum, purified, and analyzed using a mass spectrometer.
Helium isotopic ratios were ~7.4 RA in olivines and ~8.0 RA in pyroxenes from Shinmoe-dake. Although olivines and pyroxenes in the Shinmoe-dake 2011 pumice are considered to have been crystallized in the deep and shallow magma chambers, respectively2,3, helium isotopic ratios indicated that melt inclusions in pyroxenes have more mantle-like properties. Therefore, it is necessary to consider not only differences in depth of crystallization, but also differences in time to explain this. While relatively fresh magma supplied from the mantle would have 3He/4He of 8 RA, as the magma is stored in a magma chamber for a long time, 3He/4He would decrease with time due to degassing of the magma and an addition of helium with low 3He/4He from the surrounding crust. The results suggest that olivines crystallized when 3He/4He of magma chambers were low, while pyroxenes crystallized when they were high. In addition, the fact that there was magma with 3He/4He of ~8 RA provides the criterion for the monitoring. The 3He/4He in Iwo-yama, which is thought to share the magma conduit with Shinmoe-dake, was ~7.0 RA in 2016, but it has increased for 8 years to reach ~7.8 RA. This suggests that the recent volcanic gases are supplied from magma with 3He/4He of ~8 RA along with the geodetic survey that indicated the expansion of Iwo-yama4.
Olivines in the lavas of Ohachi had higher 3He/4He (7.9-8.4 RA) than the olivines from Shinmoe-dake. Ohachi is thought to have a different magma conduit from Shinmoe-dake that directly leads to the deep source5. This is consistent with the helium results, suggesting a deeper origin.
[1] Padrón, E. et al. (2013) Geology. [2] Suzuki, Y. et al. (2013) JVGR. [3] Aizawa, K. et al. (2014) JGR Solid Earth. [4] JMA (2023) Volcanic activity explanation materials. [5] Kagiyama, T. et al. (1997) Bull. Volcanol. Soc. Jpn.
To overcome these limitations, noble gas isotopic compositions in olivine and pyroxene phenocrysts from volcanic rocks were analyzed. These minerals can trap melt inclusions during their crystallization, preserving helium isotopic composition due to slow diffusion. In this study, isotopic composition of minerals that erupted in the Kirishima volcano group, which includes the Shinmoe-dake, Iwo-yama, and Ohachi volcanoes, were analyzed. A superiority using of this volcano for the isotope study is under three points. (1) Juvenile materials at the Shinmoe-dake 2011 eruption were sampled. (2) The subsurface structure was proposed by geophysical and petrological studies. (3) Helium isotope data of volcanic gas monitoring at the Iwo-yama were available.
Olivines (>0.5 mm, 200–700 mg) and pyroxenes (>1 mm, 500–700 mg) were extracted from pumice from the 2011 Shinmoe-dake eruption, and olivines (>0.25 mm, 100–300 mg) were separated from three lavas of Ohachi (erupted in 788, 1235, and 1350 AD). Noble gases were released by crushing minerals in a vacuum, purified, and analyzed using a mass spectrometer.
Helium isotopic ratios were ~7.4 RA in olivines and ~8.0 RA in pyroxenes from Shinmoe-dake. Although olivines and pyroxenes in the Shinmoe-dake 2011 pumice are considered to have been crystallized in the deep and shallow magma chambers, respectively2,3, helium isotopic ratios indicated that melt inclusions in pyroxenes have more mantle-like properties. Therefore, it is necessary to consider not only differences in depth of crystallization, but also differences in time to explain this. While relatively fresh magma supplied from the mantle would have 3He/4He of 8 RA, as the magma is stored in a magma chamber for a long time, 3He/4He would decrease with time due to degassing of the magma and an addition of helium with low 3He/4He from the surrounding crust. The results suggest that olivines crystallized when 3He/4He of magma chambers were low, while pyroxenes crystallized when they were high. In addition, the fact that there was magma with 3He/4He of ~8 RA provides the criterion for the monitoring. The 3He/4He in Iwo-yama, which is thought to share the magma conduit with Shinmoe-dake, was ~7.0 RA in 2016, but it has increased for 8 years to reach ~7.8 RA. This suggests that the recent volcanic gases are supplied from magma with 3He/4He of ~8 RA along with the geodetic survey that indicated the expansion of Iwo-yama4.
Olivines in the lavas of Ohachi had higher 3He/4He (7.9-8.4 RA) than the olivines from Shinmoe-dake. Ohachi is thought to have a different magma conduit from Shinmoe-dake that directly leads to the deep source5. This is consistent with the helium results, suggesting a deeper origin.
[1] Padrón, E. et al. (2013) Geology. [2] Suzuki, Y. et al. (2013) JVGR. [3] Aizawa, K. et al. (2014) JGR Solid Earth. [4] JMA (2023) Volcanic activity explanation materials. [5] Kagiyama, T. et al. (1997) Bull. Volcanol. Soc. Jpn.