[S-GC32] Volatiles in the Earth - from Surface to Deep Mantle

[SGC32-P08] Evolution of magma toward caldera-forming eruption in Kikai Caldera; melt inclusion study of the submarine core

*Takeshi Hanyu¹, Katsuya Kaneko², Kenji Shimizu³, Takayuki Ushikubo³, Junji Yamamoto⁴, Morihisa Hamada¹, Qing Chang¹ (1.Japan Agency for Marine-Earth Science and Technology, Research Institute for Marine Geodynamics, 2.Kobe University, Graduate School of Science, 3.Japan Agency for Marine-Earth Science and Technology, Kochi Institute for Core Sample Research, 4.Kyushu University, Graduate School of Science)

Keywords:Kikai Caldera, Melt inclusion, Submarine core, Volatile

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.