Magma plumbing system of the 2000 eruption of Miyakejima Volcano, Japan

Abstract

During the 2000 eruption at Miyakejima Volcano, two magmas with different compositions erupted successively from different craters. Magma erupted as spatter from the submarine craters on 27 June is aphyric basaltic andesite (<5 vol% phenocrysts, 51.4-52.2 wt% SiO 2 ), whereas magma issued as volcanic bombs from the summit caldera on 18 August is plagioclase-phyric basalt (20 vol% phenocrysts, 50.8-51.3 wt% SiO 2 ). The submarine spatter contains two types of crystal-clots, A-type and A 0 -type (andesitic type). The phenocryst assemblages (plagioclase, pyroxenes and magnetite) and compositions of clinopyroxene in these clots are nearly the same, but only A 0 -type clots contain Ca-poor plagioclase (An < 70). We consider that the A 0 -type clots could have crystallized from a more differentiated andesitic magma than the Atype clots, because FeO*/MgO is not strongly influenced during shallow andesitic differentiation. The summit bombs contain only B-type (basaltic type) crystal-clots of Ca-rich plagioclase, olivine and clinopyroxene. The Atype and B-type clots have often coexisted in Miyakejima lavas of the period 1469-1983, suggesting that the magma storage system consists of independent batches of andesitic and basaltic magmas. According to the temporal variations of mineral compositions in crystal-clots, the andesitic magma became less evolved, and the basaltic magma more evolved, over the past 500 years. We conclude that gradually differentiating basaltic magma has been repeatedly injected into the shallower andesitic magma over this period, causing the andesitic magma to become less evolved with time. The mineral chemistries in crystal-clots of the submarine spatter and 18 August summit bombs of the 2000 eruption fall on the evolution trends of the A-type and B-type clots respectively, suggesting that the shallow andesitic and deeper basaltic magmas existing since 1469 had successively erupted from different craters. The 2000 summit collapse occurred due to drainage of the andesitic magma from the shallower chamber; as the collapse occurred, it may have caused disruption of crustal cumulates which then contaminated the ascending, deeper basalt. Thus, porphyritic basaltic magma could erupt alone without mixing with the andesitic magma from the summit caldera. The historical magma plumbing system of Miyakejima was probably destroyed during the 2000 eruption, and a new one may now form.

238U-230Th radioactive disequilibrium in the northern Izu arc: (230Th/232Th) in the sub-arc mantle

“…2). Our results are consistent with those of previous studies (Takahashi et al, 1991;Watanabe et al, 2006;Kawanabe, 1991;Amma-Miyasaka and Nakagawa, 2003;Amma-Miyasaka et al, 2005;Koyaguchi, 1986;Yamamoto et al, 1991). Figure 2 shows correlation diagrams of SiO 2 with other major elements.…”

“…It has been recognized that processes, such as crystal fractionation and magma mixing, in shallow crustal levels modified chemical compositions of volcanic rocks (Watanabe et al, 2006;Kawanabe, 1991;AmmaMiyasaka and Nakagawa, 2003;Amma-Miyasaka et al, 2005;Koyaguchi, 1986). Major element correlation diagrams (Fig.…”

Orientation of the Eruption Fissures Controlled by a Shallow Magma Chamber in Miyakejima

“…Several previous works on the petrological analysis of Miyakejima (AmmaMiyasaka and Nakagawa, 2003; Kuritani et al, 2003;Niihori et al, 2003;Amma-Miyasaka et al, 2005) show that the magmas that erupted within the last ∼2800 years are hybrid with basaltic and andesitic end-components. The mixed end components were stored in a duplicated magma chamber system consisting of a deep-seated basaltic chamber and a shallow andesitic chamber.…”

“…Equilibrium relationships between minerals and the groundmass suggest that the less-fractionated magmas with basaltic compositions are stored in a magma chamber at around ∼200 MPa. Whereas the fractionating magmas with andesitic compositions are stored at a shallower depth at around ∼100 MPa (Kuritani et al, 2003;Amma-Miyasaka et al, 2005). These pressures correspond to depth of ∼10 km for the basaltic magma and ∼5 km for the andesitic magma, respectively.…”

Special issue: The 2000 eruption of Miyakejima volcano, Japan

“…The 2000 eruption began with discharge of basaltic andesite magma, but this was subsequently replaced by basalt (Amma-Miyakasa et al 2005). Since eruption-re-S. Nakada ( ) ) Earthquake Research Institute, University of Tokyo, Tokyo, 113-0032, Japan e-mail: nakada@eri.u-tokyo.ac.jp lated earthquakes below the summit were concentrated about 3 km below sea level, it is interpreted that the roof of the shallow magma reservoir lay at this depth.…”