Vol.:(0123456789) 1 3 Contrib Mineral Petrol (2017) 172:22 DOI 10.1007/s00410-017-1335-3 ORIGINAL PAPER Intermittent generation of mafic enclaves in the 1991–1995 dacite of Unzen Volcano recorded in mineral chemistry Hiroaki Sato 1  · Francois Holtz 2  · Roman E. Botcharnikov 2  · Setsuya Nakada 3   Received: 30 April 2016 / Accepted: 3 February 2017 © Springer-Verlag Berlin Heidelberg 2017 enclaves with a long residence time in the dacitic magma reservoir, whose constituent minerals were annealed at low-temperature to be in equililbrium with the rhyolitic melt, represent type-III enclaves. In contrast, type-I mafic enclaves result from recent mafic injections with a mineral assemblage that still retains the high-temperature mineral chemistry. Taking temperature, Ca/(Ca + Na) ratio of pla- gioclase, and water activity of the hydrous Unzen magma into account, the Mg contents of plagioclase indicate that plagioclase microlites in type-III enclaves initially crystal- lized at high temperature and were subsequently re-equil- ibrated at low-temperature conditions. Compositional pro- files of Mg in plagioclase suggest that older mafic enclaves (Type-III) had a residence time of ~100 years at 800 °C in a stagnant magma reservoir before their incorporation into the mixed dacite of the 1991–1995 Unzen eruption. Pres- ence of different types of mafic enclaves suggests that the 1991–1995 dacite of Unzen volcano tapped mushy magma reservoir intermittently replenished by high-temperature mafic magmas. Keywords Mafic enclaves · Mg in plagioclase · Cl in hornblende · Magma mixing · Diffusion re-equilibration · Unzen volcano Introduction Mafic enclaves (abbreviated as MEs in the followings) are ubiquitously observed in silicic lavas and domes in arc vol- canoes, and their petrography and chemistry are mostly interpreted as quenched mafic magmas formed during replenishment of silicic magma reservoir by mafic mag- mas (e.g., Bacon 1986; Koyaguchi 1986). There are several mechanisms proposed for the quenching of mafic magmas. Abstract Mafic enclaves in the 1991–1995 dacite of Unzen volcano show chemical and textural variability, such as bulk SiO 2 contents ranging from 52 to 62 wt% and fine- to coarse-grained microlite textures. In this paper, we investigated the mineral chemistry of plagioclase and horn- blende microlites and distinguished three enclave types. Type-I mafic enclaves contain high-Mg plagioclase and low-Cl hornblende as microlites, whereas type-III enclaves include low-Mg plagioclase and high-Cl hornblende. Type-II enclaves have an intermediate mineral chemis- try. Type-I mafic enclaves tend to show a finer-grained matrix, have slightly higher bulk rock SiO 2 contents (56– 60 wt%) when compared with the type-III mafic enclaves (SiO 2 = 53–59 wt%), but the overall bulk enclave composi- tions are within the trend of the basalt–dacite eruptive prod- ucts of Quaternary monogenetic volcanoes around Unzen volcano. The origin of the variation of mineral chemistry in mafic enclaves is interpreted to reflect different degree of diffusion-controlled re-equilibration of minerals in a low-temperature mushy dacitic magma reservoir. Mafic Communicated by Timothy L. Grove. Electronic supplementary material The online version of this article (doi:10.1007/s00410-017-1335-3) contains supplementary material, which is available to authorized users. * Hiroaki Sato hsato47@gmail.com 1 Department of Earth and Planetary Sciences, Kobe University, Kobe 657-8501, Japan 2 Leibniz Universität Hannover, Institute of Mineralogy, Callinstr. 3, 30167 Hanover, Germany 3 Earthquake Research Institute, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan