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Diversity of primary magma types and their cyclic temporal changes beneath an island arc volcano: Geology and petrology of Akita-Komagatake volcano, NE Japan

Shota Taji, Mitsuhiro Nakagawa , Akiko Matsumoto

Abstract

Akita-Komagatake volcano has been active since 15 ka after a long period of dormancy and has erupted two magma types with different Sr-Nd-Pb isotope ratios. One has SiO2 = 51-63 % and 87Sr/86Sr = 0.7041-0.7043 (high-Sr type) and the other has SiO2 = 53-60 % and 87Sr/86Sr=0.7039-0.7040 (low-Sr type). In both types, the isotopic ratios change gradually with increasing SiO2 content, and the ratios of incompatible elements such as Ba/K and Zr/K are not constant. This suggests that the compositional variation of each magma type was not formed by simple fractional crystallization, but by the AFC process of the primary basaltic magma. In addition, these two magma types show distinct trends in SiO2 vs. isotope ratio diagrams, indicating that the primary basaltic magma and associated crustal materials were different between the two magma types. Each magma type was active at different stages of activity without a relatively long dormant period. The high-Sr type during 15 - 13 ka, the low-Sr type during 13 - 9 ka, and the high-Sr type again from 9 ka. In the 20th century, although the high-Sr type was still active in the CE 1932 eruption, the magma type changed to the low-Sr type in the CE 1970 eruption. It is suggested that two primary basaltic magma types with different source mantle originated sequentially and each ascended respectively to form a magma plumbing system beneath the volcano. The cycle from high to low-Sr type has been repeated twice in 15,000 years.