Geochemical constraints on the evolution of the magmatic system leading to catastrophic eruptions at Aira Caldera, Japan

Abstract

Caldera-forming eruptions of Aira volcano at 30 ka were among the most explosive eruptions in Japan during the late Pleistocene. In this study, new data on whole-rock major and trace elements, Sr–Nd–Pb isotopes, and 238U-230Th disequilibrium were obtained for representative volcanic products of caldera-forming and pre-caldera eruptions (31–90 ka) to acquire insights on the evolution of the felsic magma system leading to catastrophic eruptions. Voluminous rhyolitic pumices from the caldera-forming eruptions at 30 ka and rhyolitic lavas and pumices of the younger pre-caldera eruptions (31–36 ka) have similar Sr, Nd, and Pb isotopic compositions, and differ from those of dacitic and rhyolitic pumices produced by eruptions at 90 and 60 ka, respectively. These three types of felsic magmas are considered to have formed by partial melting of hornblende gabbroic rocks with distinct isotopic compositions in the middle-lower crust. Most felsic products from the pre-caldera and caldera-forming eruptions are in secular 238U-230Th equilibrium, suggesting that the original magmas were generated > ∼ 400 ka and existed as long-lived crystalline mush in the deep hot crust. Felsic magmas originating from the deep crustal mush zones ascended to form the shallow felsic magma system, which was replaced by a new one after the explosive eruptions at 90 and 60 ka. The voluminous rhyolitic magmas for the caldera-forming eruptions accumulated in a shallow magma reservoir between 60 and 30 ka. Rhyolitic magmas with compositions similar to those of the voluminous magmas erupted catastrophically at 30 ka began to erupt effusively at 36 ka and 33 ka as lava flows with significant 238U-230Th disequilibrium, suggesting that these magmas were generated in the deep crust much later than the other main rhyolitic magmas. The effusive magma eruptions presumably resulted from the relatively low content of dissolved H2O in the melt due to the low crystallinity because of the shorter residence time in the shallow felsic magma system.