17:15 〜 18:45
[SGC32-P08] Evolution of magma toward caldera-forming eruption in Kikai Caldera; melt inclusion study of the submarine core
キーワード：鬼界カルデラ、メルト包有物、海底コア、揮発性成分
Caldera-forming eruptions, despite rare in occurrence, cause hazardous impacts on the Earth’s environment. Magma reservoir evolution beneath the caldera has been a focus of interest, yet it remains uncertain in many cases because of the lack of successive volcanic records prior to the catastrophic eruption. Submarine cores collected during the Chikyu SCORE cruise (CK20-S01, Leg2) record two caldera-forming eruptions of K-Tz (95 ka) and K-Ah (7.3 ka) and inter-caldera volcanism during the caldera cycle. Here we present geochemical composition of mineral-hosted melt inclusions (MIs) in the volcaniclastic core sequences.
Volatile and lithophile element concentrations in MIs were measured with SIMS, EPMA and LA-ICP-MS. Density of CO2 in shrinkage bubbles in MIs was measured with Raman spectroscopy for some samples. The MIs show chemical trends that overlap with those defined by the coexisting glass fragments in terms of major element composition. The MIs in the inter-caldera samples (referred to as inter-caldera MIs below) varies from dacitic to rhyolitic. The K-Tz and K-Ah MIs show a narrow range in SiO2 that is higher than the inter-caldera MIs. The inter-caldera MIs show some variation in CO2 while the K-Tz and K-Ah MIs are depleted in CO2. Similarly, the K-Tz and K-Ah MIs are more depleted in S than the inter-caldera MIs. This contrasts to the enrichment in H2O in the K-Tz and K-Ah MIs relative to the inter-caldera MIs as a group. Assuming the saturation of H2O in magmas because of low CO2 concentration, the volatile abundance in MIs suggest the presence of shallow magma reservoir during the inter-caldera stage and the expansion of the magma reservoir to the deeper level by storing large volume of magmas.
Volatile and lithophile element concentrations in MIs were measured with SIMS, EPMA and LA-ICP-MS. Density of CO2 in shrinkage bubbles in MIs was measured with Raman spectroscopy for some samples. The MIs show chemical trends that overlap with those defined by the coexisting glass fragments in terms of major element composition. The MIs in the inter-caldera samples (referred to as inter-caldera MIs below) varies from dacitic to rhyolitic. The K-Tz and K-Ah MIs show a narrow range in SiO2 that is higher than the inter-caldera MIs. The inter-caldera MIs show some variation in CO2 while the K-Tz and K-Ah MIs are depleted in CO2. Similarly, the K-Tz and K-Ah MIs are more depleted in S than the inter-caldera MIs. This contrasts to the enrichment in H2O in the K-Tz and K-Ah MIs relative to the inter-caldera MIs as a group. Assuming the saturation of H2O in magmas because of low CO2 concentration, the volatile abundance in MIs suggest the presence of shallow magma reservoir during the inter-caldera stage and the expansion of the magma reservoir to the deeper level by storing large volume of magmas.