17:15 〜 18:30
[SGC32-P01] Secular variation of helium isotope ratios in Izu Oshima: What will be observed in the next eruption?
キーワード：ヘリウム同位体比、伊豆大島火山、噴火
The helium isotope ratio (3He/4He ratio) differs significantly in geochemical reservoirs (3He/4He in air and meteoric water: 1 Ra = 1.4 × 10-6, crust: 0.02 Ra, MORB source mantle: 8±1 Ra), and because helium is chemically inert, the 3He/4He ratio does not change by degassing from magma or by dissolution/degassing in water. Therefore, helium in volcanic gases, hot spring water, and hot spring gases can be used as a sensitive tracer of magma activity, and in fact, it has been reported that the 3He/4He ratio increased or decreased before and after eruptions at several volcanoes (Padrón et al., Geology 2013; Sano et al., Sci. Rep. 2015; Paonita et al., Geology 2016). Izu-Oshima volcano, an active volcanic island located around 100 km SSW of Tokyo, regularly erupted with 30 to 40 years intervals, but no eruption has occurred since the last activity in 1986-1990. In this presentation, we report the 3He/4He ratios of volcanic gases and olivines in pyroclastic materials collected in Izu Oshima, and discuss the long-term secular variation of 3He/4He ratios combining the reports of previous studies (Sano et al., EPSL 1991; Shimoike and Notsu, JVGR 2000).
The 3He/4He ratio of the observation steam well located in the north of the caldera in the center of Izu Oshima increased sharply immediately after the 1986 eruption from a low value of 1.7 Ra, peaked in 1988 at 5.5 Ra, and has been decreasing until now. The present 3He/4He ratio is around 1.5 Ra, which is lower than that before the last eruptive activity. The 3He/4He ratio corrected for atmospheric contamination using 4He/20Ne ratio was about 6.2 Ra throughout the last active period from 1986 to 1990. On the other hand, air-corrected 3He/4He ratios of hot spring gas collected from a well 500 m away from the steam well have been constant at 6.2 Ra since 2001. These results suggest that the mixing ratio of magma-derived and atmospheric helium in the steam well has changed in response to the decrease of supply of magma-derived helium, and the magmatic contribution has now almost completely decayed, while the 3He/4He ratio of magmatic gas itself has been constant over the past 30 years.
The observation that the helium released from the present magma is significantly lower than the value of 8 Ra, which is considered to be the value of the mantle wedge in subduction zone, suggests the significant contribution crustal helium in the magma reservoir. On the other hand, the 3He/4He ratio of olivines separated from lava flows of the Senzu Group, which erupted 20,000-40,000 years ago, and that from scoria in the lowest part (O95) of pyroclastic layers of the Older Oshima group, which was active after 20,000 years ago, are 7.4±0.4 Ra and 7.1±0.7 Ra, respectively, suggesting that 3He/4He ratios of the magma at these stages were almost equal to that of the mantle. However, the 3He/4He ratio of olivines in the N4 scoria erupted about 1300 years ago is 4.6±0.2 Ra, indicating that crustal helium was significantly contaminated into the magma.
Based on these results, two possible changes in the 3He/4He ratios of the steam and hot-spring wells can be expected: first, if the magma reservoir erupted in 1986 is reactivated, the 3He/4He ratio of hot spring gas will remain unchanged at about 6 Ra, and only the contribution of magma-derived helium observed in the steam well will increase. This means that the change in the 3He/4He ratio observed during the previous activity in 1986-1990 will be reproduced. The second scenario, in which the reactivation of the magma reservoir is caused by the supply of primitive magma with a high 3He/4He ratio derived from the mantle, is that not only the contribution of magmatic helium in the steam well increases, but also the air-corrected 3He/4He ratio of the steam and hot-spring wells themselves become higher than 6 Ra. In other words, it is expected that the 3He/4He ratio can be used to clarify whether the next eruption results from new magma supply from greater depths or just reactivation of the old magma.
The 3He/4He ratio of the observation steam well located in the north of the caldera in the center of Izu Oshima increased sharply immediately after the 1986 eruption from a low value of 1.7 Ra, peaked in 1988 at 5.5 Ra, and has been decreasing until now. The present 3He/4He ratio is around 1.5 Ra, which is lower than that before the last eruptive activity. The 3He/4He ratio corrected for atmospheric contamination using 4He/20Ne ratio was about 6.2 Ra throughout the last active period from 1986 to 1990. On the other hand, air-corrected 3He/4He ratios of hot spring gas collected from a well 500 m away from the steam well have been constant at 6.2 Ra since 2001. These results suggest that the mixing ratio of magma-derived and atmospheric helium in the steam well has changed in response to the decrease of supply of magma-derived helium, and the magmatic contribution has now almost completely decayed, while the 3He/4He ratio of magmatic gas itself has been constant over the past 30 years.
The observation that the helium released from the present magma is significantly lower than the value of 8 Ra, which is considered to be the value of the mantle wedge in subduction zone, suggests the significant contribution crustal helium in the magma reservoir. On the other hand, the 3He/4He ratio of olivines separated from lava flows of the Senzu Group, which erupted 20,000-40,000 years ago, and that from scoria in the lowest part (O95) of pyroclastic layers of the Older Oshima group, which was active after 20,000 years ago, are 7.4±0.4 Ra and 7.1±0.7 Ra, respectively, suggesting that 3He/4He ratios of the magma at these stages were almost equal to that of the mantle. However, the 3He/4He ratio of olivines in the N4 scoria erupted about 1300 years ago is 4.6±0.2 Ra, indicating that crustal helium was significantly contaminated into the magma.
Based on these results, two possible changes in the 3He/4He ratios of the steam and hot-spring wells can be expected: first, if the magma reservoir erupted in 1986 is reactivated, the 3He/4He ratio of hot spring gas will remain unchanged at about 6 Ra, and only the contribution of magma-derived helium observed in the steam well will increase. This means that the change in the 3He/4He ratio observed during the previous activity in 1986-1990 will be reproduced. The second scenario, in which the reactivation of the magma reservoir is caused by the supply of primitive magma with a high 3He/4He ratio derived from the mantle, is that not only the contribution of magmatic helium in the steam well increases, but also the air-corrected 3He/4He ratio of the steam and hot-spring wells themselves become higher than 6 Ra. In other words, it is expected that the 3He/4He ratio can be used to clarify whether the next eruption results from new magma supply from greater depths or just reactivation of the old magma.