Remobilization of Highly Crystalline Felsic Magma by Injection of Mafic Magma: Constraints from the Middle Sixth Century Eruption at Haruna Volcano, Honshu, Japan

JOURNAL OF PETROLOGY VOLUME 48 NUMBER 8 PAGES 1543^1567 2007 doi:10.1093/petrology/egm029 Remobilization of Highly Crystalline Felsic Magma by Injection of Mafic Magma: Constraints from the Middle Sixth Century Eruption at HarunaVolcano, Honshu, Japan YUKI SUZUKI1* AND SETSUYA NAKADA2 1 INSTITUTE OF MINERALOGY, PETROLOGY AND ECONOMIC GEOLOGY, GRADUATE SCHOOL OF SCIENCE, TOHOKU UNIVERSITY, AOBA-KU, SENDAI, 980-8578, JAPAN 2 EARTHQUAKE RESEARCH INSTITUTE, UNIVERSITY OF TOKYO, 1-1-1, YAYOI, BUNKYO-KU, TOKYO, 113-0032, JAPAN RECEIVED JULY 21, 2006; ACCEPTED MAY 15, 2007 ADVANCE ACCESS PUBLICATION JUNE 22, 2007 The latest eruption of Haruna volcano at Futatsudake took place in the middle of the sixth century, starting with a Plinian fall, followed by pyroclastic flows, and ending with lava dome formation. Gray pumices found in the first Plinian phase (lower fall) and the dome lavas are the products of mixing between felsic (andesitic) magma having 50 vol. % phenocrysts and mafic magma.The mafic magma was aphyric in the initial phase, whereas it was relatively phyric during the final phase.The aphyric magma is chemically equivalent to the melt part of the phyric mafic magma and probably resulted from the separation of phenocrysts at their storage depth of
15 km. The major part of the felsic magma erupted as white pumice, without mixing and heating prior to the eruption, after the mixed magma (gray pumice) and heated felsic magma (white pumice) of the lower fall deposit. Although the mafic magma was injected into the felsic magma reservoir (at
7 km depth), part of the product (lower fall ejecta) preceded eruption of the felsic reservoir magma, as a consequence of upward dragging by the convecting reservoir of felsic magma. The mafic magma injection made the nearly rigid felsic magma erupt, letting low-viscosity mixed and heated magmas open the conduit and vent. Indeed the lower fall white pumices preserve a record of syneruptive slow ascent of magma to 2 km depth, probably associated with conduit formation. I N T RO D U C T I O N KEY WORDS: high-crystallinity felsic magma; magma plumbing system; multistage magma mixing; upward dragging of injected magma; vent opening by low-viscosity magma Knowledge of the triggering mechanisms of eruptions is important for the forecasting of future eruptions. The injection of new magma into a magma reservoir is one of the major mechanisms. Theoretical aspects of the triggering process include the increase in pressure as a result of (1) an increase in the magma reservoir resulting from the simple volumetric addition of injected magma (Blake, 1984), and (2) the intrinsic increase in volume owing to the exsolution of volatile phases from the melt. Exsolution can be generated either by cooling-induced crystallization of the injected high-temperature magma (Folch & Martı́, 1998) or by heating of the low-temperature reservoir magma (Sparks et al., 1977). Furthermore, petrological studies on the ejecta have focused on magma mixing that results from the injection. To link the magma injection and eruption, some recent petrological studies have investigated the timescale from mixing to ejection (e.g. Nakamura, 1995; Venezky & Rutherford, 1999). Also, petrological studies have proposed a new triggering process for the case of highly crystalline felsic magma eruption: the injected low-viscosity mafic magma remobilizes the highly viscous felsic magma, reducing the magma viscosity as a result of mixing (e.g. Pallister et al., 1996; Venezky & Rutherford, 1997; Murphy et al., 2000; Takeuchi & Nakamura, 2001). *Corresponding author. Telphone: þ81-22-795-5786. Fax: þ81-22795-7763. E-mail: ß The Author 2007. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@ oxfordjournals.org JOURNAL OF PETROLOGY VOLUME 48 NUMBER 8 AUGUST 2007 Fig. 1. Distribution of ejecta from the Futatsudake eruptions and earlier ejecta of Haruna volcano (after Oshima (1986) and Soda (1989)). Earlier ejecta include those of all stages (Stage 1^5) in Haruna activity. Inset shows the location of Haruna volcano in the NE Japan arc. Samples of the middle sixth century eruption were collected at Owazawa and Ohinata. Only fall deposits are observed at Ohinata. Py-flow, pyroclastic flow; VF (inset), volcanic front. Although petrological study of ejecta is a powerful tool for studying the triggering mechanisms of eruptions, it has certain drawbacks. There are only a few studies that consider the migration of magma around the reservoir (e.g. Venezky & Rutherford, 1997; Cottrell et al., 1999). Although studies of groundmass microlites can provide information about magma ascent at shallow levels in the conduit (e.g. Hammer et al., 1999; Nakada & Motomura, 1999; Suzuki et al., 2007), they do not address migration around the reservoir. When an eruption is triggered by magma injection, the migration of magma around the reservoir is associated with chemical modification of the magmas, and is thus important for understanding the whole picture of the eruption triggering processes. The most important factors that characterize and control the migration of the magma are the depth, shape and dimension of the magma storage system. However, it is sometimes difficult to resolve these storage systems, because of the still poor spatial resolution of geophysical imaging methods and incomplete knowledge of the endmember magmas that are mixed (especially the mafic magma, which comes from a deeper level than the felsic magma). The poor resolution of the magma storage systems makes it difficult to evaluate magma migration around the reservoir. To ascertain the locations of magma storage we need to determine the bulk and melt compositions of the end-member magmas, for example, using mass balance of phenocrysts for the mixing of phyric and aphyric magmas (Nakamura, 1995), and the compositions of melt inclusions (Cottrell et al., 1999). If magma storage conditions can be better ascertained, this would help in understanding the interaction of different magmas at the time of injection, through the estimation of their physical properties. At the same time, detailed study of phenocryst zoning could help us to reveal the progress of magma migration and mixing. If no modern observation record exists for a volcano, eruption triggering processes and the depths of magma storage reservoirs, which are obtained from petrological approaches, are particularly important in predicting the future eruptions based on geophysical observations. With this motivation and background, we have investigated the mid sixth century Futatsudake eruption of the Haruna volcano in central Honshu (Fig. 1). Although no historical documentation is available, the eruption sequence is well known (Oshima, 1983; Soda, 1989, 1996). In this study we show that a highly crystalline (50 vol. %) felsic (andesitic) magma and two mafic magmas (aphyric and phyric) were involved in (...truncated)