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[SVC36-05] Voluminous silicic magma formation for the 30 ka Aira catastrophic caldera-forming eruption: contributions of crust-derived mafic and felsic magmas
Keywords:Aira caldera, caldera-forming eruption, Sr isotope ratio, magma mixing, crustal assimilation
1. Juvenile ejecta discharged by the Aira CCF eruption consist of three types of clasts; white pumice, dark pumice, and scoria clasts. Whole-rock chemical compositions of the dark pumices are less SiO2 contents (67.5-75.7 wt.%) than those of the white pumices (74.3-77.2 wt.%). The scoria shows andesitic composition (SiO2=59.0 wt.%).
2. In whole-rock characteristics, the SiO2-poor juvenile clasts have higher 87Sr/86Sr (0.70856±0.00001 of the dark pumice and 0.70738±0.00001 of the scoria) than the white pumice (0.70597±0.00001).
3. An-contents of plagioclase cores in the white and dark pumices show bimodal distributions with peaks of An45 and An80, while the scoria contains solely high-An (>An65) plagioclase.
4. In plagioclase phenocrysts of the Aira juvenile clasts, Sr isotopic ratios of the high- and low-An plagioclase cores are different (0.70680±0.00117 and 0.70575±0.00040, respectively).
5. Basement rocks beneath the Aira caldera consist of Shimanto sediments and Miocene granites. These materials have much higher 87Sr/86Sr (Shimanto sediments: >0.70965; Miocene granites: >0.70885) than the Aira juvenile clasts.
6. Pre-caldera basaltic lavas, which are the most SiO2-poor ejecta around the Aira caldera, have lower 87Sr/86Sr (0.70425-0.70469) than the Aira juvenile clasts.
The above features suggest that voluminous silicic magma bearing the white pumices in the Aira CCF eruption was produced by mixing between low-87Sr/86Sr felsic and high-87Sr/86Sr mafic magmas, crystallizing the low- and high-An plagioclase, respectively. The low-87Sr/86Sr felsic magma was distinguished from both the upper crust and pre-caldera basaltic materials in 87Sr/86Sr characteristics. If the basaltic materials show the mantle signature, the low-87Sr/86Sr felsic magma was derived from a lower crust material. The geochemical signature of the high-87Sr/86Sr mafic magma can be explained by assimilation of high 87Sr/86Sr upper crust materials. The lowest 87Sr/86Sr recorded in the high-An plagioclase is higher than the whole-rock 87Sr/86Sr of the pre-caldera basalts, suggesting the mafic end-member magma for the Aira CCF eruption was the lower crust origin. A frequency ratio of the low- to high-An plagioclase in the white pumice proposed that the mixing ratio of the felsic and mafic magmas was estimated at a ratio of ~8:2, which is consistent with whole-rock 87Sr/86Sr variations of the white pumice. In conclusion, the crust-derived low-87Sr/86Sr felsic and high-87Sr/86Sr mafic magmas had contributed to the formation of voluminous magma discharged by the Aira CCF eruption.