TY - JOUR

T1 - Experimental petrology of the 1991-1995 Unzen dacite, Japan. Part I

T2 - Phase relations, phase composition and pre-eruptive conditions

AU - Holtz, Francois

AU - Sato, Hiroaki

AU - Lewis, Jared

AU - Behrens, Harald

AU - Nakada, Setsuya

N1 - Funding Information: Thanks are due to W. Johannes, M. Nowak and K. Suzuki-Kamata for discussions during this work. We appreciated the technical assistance of J. Koepke, D. Ziegenbein, O. Diedrich and the technical staff of the workshop at the Institut fu€r Mineralogie at Hannover. I. Kannewischer and M. Freise are thanked for assistance in the preparation of the manuscript. Raman spectroscopic measurements were made at the Institut fu€r Mineralogie at the University of Clausthal– Zellerfeld (Institut fu€r Mineralogie, Professor K. Mengel). Critical comments of J. Barclay and M. Rutherford and the final review of J. Gamble helped to improve the manuscript. This work was supported by the DFG projects Ho1337/7 and Ho1337/11. The collaboration between Kobe and Hannover Universities was initiated by a grant of the JSPS foundation to F.H. Copyright: Copyright 2008 Elsevier B.V., All rights reserved.

PY - 2004/10/28

Y1 - 2004/10/28

N2 - Crystallization experiments were conducted on dry glasses from the Unzen 1992 dacite at 100 300 MPa, 775-875°C, various water activities, and fO2 buffered by the Ni-NiO buffer. The compositions of the experimental products and natural phases are used to constrain the temperature and water contents of the low-temperature and high-temperature magmas prior to the magma mixing event leading to the 1991-1995 eruption. A temperature of 1050 ± 75°C is determined for the high-temperature magma based on two-pyroxene thermometry. The investigation of glass inclusions suggests that the water content of the rhyolitic low-temperature magma could be as high as 8 wt % H2O. The phase relations at 300 MPa and in the temperature range 870-900°C, which are conditions assumed to be representative of the main magma chamber after mixing, show that the main phenocrysts (orthopyroxene, plagioclase, hornblende) coexist only at reduced water activity; the water content of the post-mixing dacitic melt is estimated to be 6 ± 1 wt % H2O. Quartz and biotite, also present as phenocrysts in the dacite, are observed only at low temperature (below 800-775°C). It is concluded that the erupted dacitic magma resulted from the mixing of c. 35 wt % of an almost aphyric pyroxene-bearing andesitic magma (1050 ± 75°C; 4 ± 1 wt % H2O in the melt) with 65 wt % of a phenocryst-rich low-temperature magma (760-780°C) in which the melt phase was rhyolitic, containing up to 8 ± 1 wt % H2O. The proportions of rhyolitic melt and phenocrysts in the low-temperature magma are estimated to be 65% and 35%, respectively. It is emphasized that the strong variations of phenocryst compositions, especially plagioclase, can be explained only if there were variations of temperature and /or water activity (in time and/or space) in the low-temperature magma.

AB - Crystallization experiments were conducted on dry glasses from the Unzen 1992 dacite at 100 300 MPa, 775-875°C, various water activities, and fO2 buffered by the Ni-NiO buffer. The compositions of the experimental products and natural phases are used to constrain the temperature and water contents of the low-temperature and high-temperature magmas prior to the magma mixing event leading to the 1991-1995 eruption. A temperature of 1050 ± 75°C is determined for the high-temperature magma based on two-pyroxene thermometry. The investigation of glass inclusions suggests that the water content of the rhyolitic low-temperature magma could be as high as 8 wt % H2O. The phase relations at 300 MPa and in the temperature range 870-900°C, which are conditions assumed to be representative of the main magma chamber after mixing, show that the main phenocrysts (orthopyroxene, plagioclase, hornblende) coexist only at reduced water activity; the water content of the post-mixing dacitic melt is estimated to be 6 ± 1 wt % H2O. Quartz and biotite, also present as phenocrysts in the dacite, are observed only at low temperature (below 800-775°C). It is concluded that the erupted dacitic magma resulted from the mixing of c. 35 wt % of an almost aphyric pyroxene-bearing andesitic magma (1050 ± 75°C; 4 ± 1 wt % H2O in the melt) with 65 wt % of a phenocryst-rich low-temperature magma (760-780°C) in which the melt phase was rhyolitic, containing up to 8 ± 1 wt % H2O. The proportions of rhyolitic melt and phenocrysts in the low-temperature magma are estimated to be 65% and 35%, respectively. It is emphasized that the strong variations of phenocryst compositions, especially plagioclase, can be explained only if there were variations of temperature and /or water activity (in time and/or space) in the low-temperature magma.

KW - Experimental study

KW - Magma mixing

KW - Unzen volcano

UR - http://www.scopus.com/inward/record.url?scp=13344259964&partnerID=8YFLogxK

U2 - 10.1093/petrology/egh077

DO - 10.1093/petrology/egh077

M3 - Article

AN - SCOPUS:13344259964

VL - 46

SP - 319

EP - 337

JO - Journal of Petrology

JF - Journal of Petrology

SN - 0022-3530

IS - 2

ER -