[SGC48-P05] Noble gas isotope compositions of mafic phenocrysts in the Stage 3 lavas of Kusatsu-Shirane Volcano
Keywords:noble gas, isotope ratio, Kusatsu-Shirane Volcano
We report the noble gas (He, Ne, Ar) isotopic ratios of mafic phenocrysts (ol, opx, cpx) in the andesitic lavas of Kusatsu-Shirane Volcano. The lavas analyzed in this study were Kagusa Lava erupted from Mt. Shirane, and Sessho Lava and Furikozawa Lava both erupted from Mt. Motoshirane. After fragmentation of these lavas using SELFRAG Lab (High-voltage pulsed power selective fragmentation system) installed at JAMSTEC, about 100 mg each phenocryst was separated by hand picking. Noble gas (He, Ne, Ar) isotope analysis was conducted using a mass spectrometer installed at the University of Tokyo. In-vacuo crushing method was used for extraction of noble gases .
The preliminary 3He/4He ratios were as follows: cpx in Furikozawa Lava was 6.0 ± 0.9 Ra; ol in Furikozawa Lava was 11.0 ± 2.6 Ra; and ol in Sessho Lava was 8.3 ± 0.5 Ra. These values are similar to the 3He/4He ratio of volcanic gases such as Kitagawa fumaroles on the northern flank of Mt Shirane (average 7.6 Ra) and Sessho-gawara fumaroles (average 7.2 Ra) . The 4He/40Ar* ratios (Ar* denotes non-atmospheric 40Ar) were as follows: ol and cpx in Furikozawa Lava were 0.14 and 0.62, respectively; opx in Kagusa Lava was 0.22; and ol in Sessho Lava was 0.65. In contrast, Kitagawa fumaroles were 9.4 and Sessho-gawara fumaroles were 3.9. Because He is more soluble than Ar within silicate melt, 4He/40Ar* ratio of magma would increase as it degasses. Using 4He/40Ar* ratios as the index for the degree of magma degassing, Sessho Lava was slightly more degassed than Furikozawa Lava and Kagusa Lava. In contrast, the values observed in current fumaroles suggest that volcanic gas is derived from magma that is significantly degassed compared to magmas that have crystallized mafic minerals. These results suggest that 4He/40Ar* ratio of volcanic gas has a potential to be a good indicator to monitor degassing stage of magma and/or replenishment of undegassed primitive magma to a shallow magma chamber. As the above-mentioned data are preliminary, we will discuss more in details by analyzing more samples.
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