Structure of the shallow magma chamber of the active volcano Mt. Zao, NE Japan: Implications for its eruptive time scales

Abstract

Zao volcano is one of the representative active stratovolcanoes in northeast Japan, having many historical eruption records, as the last one occurred in 1940. Since 2013 to present, precursory phenomena, such as volcanic tremors and deformation of the summit area, have been detected in this volcano; thus, it is urgently necessary to thoroughly characterize the magma feeding system beneath the volcano. Our aim is to understand the magma chamber structure and dynamics associated with the recent activity (ca. ~2 ka) of Zao volcano in NE Japan. We conducted petrological and geochemical studies in the representative pyroclastic products and youngest lavas of its recent activity. The products are divided into the GSN (Goshikidake-nanbu) pyroclastics, GST (Goshikidake-tobu) pyroclastics and GS lavas (Furikodaki lava and Goshikidake-nanpo lava and associated pyroclastic deposits).The rock samples are calc-alkaline olivine-bearing pyroxene andesites originated as the result of mixing between felsic (containing Mg-poor pyroxenes + anorthite (An)-poor plagioclases) and mafic (containing forsterite (Fo)-rich olivines + An-rich plagioclases magmas). Thermodynamic modelling indicates that the temperature (T)–pressure (P)–water activity (H2O) conditions of the felsic (ca. 60–61 wt% SiO2, 968–1000 °C, 1.6–2.0 kbar and 1.8–2.2 wt% H2O) and the mafic (ca. 48.0–50.5 wt% SiO2, 1113–1170 °C, <2.4 kbar and 1.0–2.2 wt% H2O) end–members are similar between the GSN, GST pyroclastics and GS lavas.Compositional zoning of orthopyroxene phenocrysts suggests the formation of two mixed magmas, namely the main mixed and the Mg-rich mixed ones. The mafic magma from a deeper source entered repeatedly into a shallower felsic magma chamber. As a result, the main mixed magma developed within the chamber. The Mg-rich-mixed magma tentatively developed by the injection and simultaneous mixing of both the main mixed magma and the injected mafic one, merging later on into the main mixed magma.The Fe-Mg diffusion modelling on orthopyroxenes reveal that the injections occurred from <60 days to 1.7 years for thin-darker-zone types, and multiple injections of mafic magma occurred from 2.5 years to 150 years for broad-darker-zone types until the eruptions. The thin-darker-zone type orthopyroxene phenocrysts are abundant in the GSN and GST pyroclastics but rare in the GS lavas. Instead, the broad-darker-zone type orthopyroxenes are rare in the GSN and GST pyroclastics and abundant in the GS lavas. Thus, the main mixed layer was more evolved for the GS lavas than for the GSN and GST pyroclastics. The injected mafic magmas for the GS lavas were merged more extensively than the GSN and GST pyroclastics, resulting in a larger release of volatile components that triggered the effusive episodes. Given that local precursory phenomena have been currently detected in Zao volcano, we are confident that any research to thoroughly characterize the magma feeding system under this active volcano is essential for society to better react to any potential volcanic hazard in the near future.