10:00 〜 10:15
[SVC36-05] Voluminous silicic magma formation for the 30 ka Aira catastrophic caldera-forming eruption: contributions of crust-derived mafic and felsic magmas
キーワード：Aira caldera、caldera-forming eruption、Sr isotope ratio、magma mixing、crustal assimilation
Aira catastrophic caldera-forming (CCF) eruption occurred at 30 ka and discharged voluminous silicic magma with a small amount of less silicic magma. In order to understand the genesis of magmas in the Aira CCF eruption, petrological and geochemical characteristics of the juvenile clasts and plagioclase phenocrysts in them were investigated. Important petrological and geochemical features of the Aira ejecta and related rocks are described as follows:
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.
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.