16:45 〜 17:00
[SVC30-12] 斜長石の地球化学的性質から推定される姶良カルデラ地域10万年間の珪長質マグマ形成過程
キーワード：姶良カルデラ、Sr同位体比、珪長質マグマ、カルデラ形成噴火
Plagioclase phenocrysts record geochemical characteristics of magmas when they crystallize and grow. Chemical and isotopic variations on the plagioclase micro area enable us to unravel the hidden magma formation processes. The Aira caldera was formed by the 30 ka VEI-7 supereruption (the AT eruption), and two VEI-6 eruptions occurred at 90 and 60 ka as the pre-AT volcanism. After the AT eruption, two eruptive activities of the Sakurajima volcano and the Wakamiko volcano have continued, and their largest eruptions occurred at 12.8 ka and 13 ka, respectively. The latest plinian eruption of the Sakurajima volcano occurred in AD 1914. These eruptions have mainly discharged silicic (>60 SiO2 wt.%) magmas having different geochemical characteristics in whole-rock compositions. In order to clarify the genesis of the silicic magmas produced in the Aira caldera region, we examined 87Sr/86Sr values of the plagioclase phenocrysts contained in juvenile pumice clasts of the above six eruptions.
All the clasts contain plagioclase with cores of both low-An (<An60) and high-An (>An60). Except for the 13 ka Wakamiko clasts, 87Sr/86Sr values of the low-An plagioclase in the clasts of the five eruptions are within the range of 0.7049-0.7062. On the other hand, the high-An plagioclases can be classified into three types based on the 87Sr/86Sr values; the identical to the low-An (0.7049-0.7061), the lower 87Sr/86Sr (0.7043-0.7049), and the higher 87Sr/86Sr (0.7063-0.7076). The high-An plagioclases having 87Sr/86Sr identical to the low-An are contained in the 90 ka, 60 ka, 30 ka, and 12.8 ka clasts. The 30 ka clasts also contain the high-An plagioclase with high 87Sr/86Sr. The low-87Sr/86Sr and high-An plagioclase are contained in the AD1914 clasts. The 13 ka Wakamiko clasts contain plagioclase of high-87Sr/86Sr (0.7073±3, 1SD).
Bimodal distributions of An-contents observed in all the clasts indicate that the silicic magmas in the Aira caldera region experienced mixing between felsic and mafic magmas crystallizing the low-An and high-An plagioclases, respectively. The similar 87Sr/86Sr values of the felsic magmas suggest that, except for the Wakamiko clasts, the felsic magmas in the Aira caldera region have a same source material during the last 100,000 years. The identical 87Sr/86Sr values of the low-An and high-An plagioclase imply that a dominant process producing the magmas is an anatexis of a mafic crustal rock, which can produce both felsic and mafic magmas by difference of degree of partial melting. This process produced the silicic magmas discharged by the 90 ka, 60 ka, and 12.8 ka eruptions. The geochemical variation observed in the 30 ka clast indicates that the silicic magma discharged by the 30 ka supereruption was produced by mixing between the crust-derived low-87Sr/86Sr magma and a high-87Sr/86Sr mafic magma. The low-87Sr/86Sr plagioclases in the AD 1914 clasts suggest that the AD 1914 silicic magma experienced mixing with a low-87Sr/86Sr mantle-derived basaltic magma. Origin of the high-87Sr/86Sr magmas discharged by the 13 ka Wakamiko eruption can be explained by fractionation of the high-87Sr/86Sr mafic magma or partial melting of another crustal material with high-87Sr/86Sr. In conclusion, crustal melting is the crucial process to produce magmas in the Aira caldera region, and the variable origin of the mafic magmas generates the geochemical variations of the silicic magmas.
All the clasts contain plagioclase with cores of both low-An (<An60) and high-An (>An60). Except for the 13 ka Wakamiko clasts, 87Sr/86Sr values of the low-An plagioclase in the clasts of the five eruptions are within the range of 0.7049-0.7062. On the other hand, the high-An plagioclases can be classified into three types based on the 87Sr/86Sr values; the identical to the low-An (0.7049-0.7061), the lower 87Sr/86Sr (0.7043-0.7049), and the higher 87Sr/86Sr (0.7063-0.7076). The high-An plagioclases having 87Sr/86Sr identical to the low-An are contained in the 90 ka, 60 ka, 30 ka, and 12.8 ka clasts. The 30 ka clasts also contain the high-An plagioclase with high 87Sr/86Sr. The low-87Sr/86Sr and high-An plagioclase are contained in the AD1914 clasts. The 13 ka Wakamiko clasts contain plagioclase of high-87Sr/86Sr (0.7073±3, 1SD).
Bimodal distributions of An-contents observed in all the clasts indicate that the silicic magmas in the Aira caldera region experienced mixing between felsic and mafic magmas crystallizing the low-An and high-An plagioclases, respectively. The similar 87Sr/86Sr values of the felsic magmas suggest that, except for the Wakamiko clasts, the felsic magmas in the Aira caldera region have a same source material during the last 100,000 years. The identical 87Sr/86Sr values of the low-An and high-An plagioclase imply that a dominant process producing the magmas is an anatexis of a mafic crustal rock, which can produce both felsic and mafic magmas by difference of degree of partial melting. This process produced the silicic magmas discharged by the 90 ka, 60 ka, and 12.8 ka eruptions. The geochemical variation observed in the 30 ka clast indicates that the silicic magma discharged by the 30 ka supereruption was produced by mixing between the crust-derived low-87Sr/86Sr magma and a high-87Sr/86Sr mafic magma. The low-87Sr/86Sr plagioclases in the AD 1914 clasts suggest that the AD 1914 silicic magma experienced mixing with a low-87Sr/86Sr mantle-derived basaltic magma. Origin of the high-87Sr/86Sr magmas discharged by the 13 ka Wakamiko eruption can be explained by fractionation of the high-87Sr/86Sr mafic magma or partial melting of another crustal material with high-87Sr/86Sr. In conclusion, crustal melting is the crucial process to produce magmas in the Aira caldera region, and the variable origin of the mafic magmas generates the geochemical variations of the silicic magmas.