Experimental Petrology of the 1991-1995 Unzen Dacite, Japan. Part II: Cl/OH Partitioning between Hornblende and Melt and its Implications for the Origin of Oscillatory Zoning of Hornblende Phenocrysts |
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Authors: | SATO HIROAKI; HOLTZ FRANCOIS; BEHRENS HARALD; BOTCHARNIKOV ROMAN; NAKADA SETSUYA |
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Affiliation: | 1 DEPARTMENT OF EARTH AND PLANETARY SCIENCES, FACULTY OF SCIENCE AND GRADUATE SCHOOL OF SCIENCE AND TECHNOLOGY, KOBE UNIVERSITY, KOBE, 657-8501 JAPAN
2 INSTITUTE FOR MINERALOGY, UNIVERSITY OF HANNOVER, WELFENGARTEN 1, D-30167, HANNOVER, GERMANY
3 EARTHQUAKE RESEARCH INSTITUTE, UNIVERSITY OF TOKYO, YAYOI 1-1-1, BUNKYO, TOKYO, 113-0032 JAPAN |
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Abstract: | High-temperaturepressure experiments were carried outto determine the chlorinehydroxyl exchange partitioncoefficient between hornblende and melt in the 1992 Unzen dacite.Cl in hornblende and melt was analyzed by electron microprobe,whereas OH in hornblende and melt was calculated assuming anionstoichiometry of hornblende and utilizing the dissociation reactionconstant for H2O + O = 2(OH) in water-saturated melt, respectively.The partition coefficient strongly depends on the Mg/(Mg + Fe)ratio of hornblende, and is expressed as ln K1 = (Cl/OH)hb/(Cl/OH)melt= 2·37 4·6Mg/(Mg + Fe)]hb at 23kbar and 800850°C. The twofold variation in Cl contentin the oscillatory zoned cores of hornblende phenocrysts inthe 19911995 dacite cannot be explained by the dependenceof the Cl/OH partition coefficient on the Mg/(Mg + Fe)hb ratio,and requires c. 80% variation of the Cl/OH ratio of the coexistingmelt. Available experimental data at 200 MPa on Cl/OH fractionationbetween fluid and melt suggest that c. 1·21·8wt % degassing of water from the magma can explain the required80% variation in the Cl/OH ratio of the melt. The negative correlationbetween Al content and Mg/(Mg + Fe) ratio in the oscillatoryzoned cores of the hornblende phenocrysts is consistent withrepeated influx and convective degassing of the fluid phasein the magma chamber. KEY WORDS: chlorine; element partitioning; hornblende; oscillatory zoning; Unzen volcano |
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Keywords: | : chlorine element partitioning hornblende oscillatory zoning Unzen volcano |
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