TY - JOUR

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

T2 - Cl/OH partitioning between hornblende and melt and its implications for the origin of oscillatory zoning of hornblende phenocrysts

AU - Sato, Hiroaki

AU - Holtz, Francois

AU - Behrens, Harald

AU - Botcharnikov, Roman

AU - Nakada, Setsuya

N1 - Funding Information: We appreciate discussions with Professor Wilhelm Johannes, Dr Youxue Zhang, Dr Atsushi Goto, Dr Susanne Ohlhorst and Dr Keiko Suzuki-Kamata. We also acknowledge technical assistance by Dr Ju€rgen Koepke, the late Dr Dieter Ziegenbein, Mr Otto Dietrich and other technical staff at the Institute for Mineralogy, University of Hannover. The paper benefited considerably from the critical reviews of Drs J. Barclay, M. Rutherford and Ian Smith, and from the final improvements of Dr J. Gamble. Financial support came from the Unzen Scientific Drilling Project, MECSST, Japan, the DFG projects Ho1337/7 and Ho1337/11, and JSPS project 14340162. Copyright: Copyright 2008 Elsevier B.V., All rights reserved.

PY - 2004/12/10

Y1 - 2004/12/10

N2 - High-temperature-pressure experiments were carried out to determine the chlorine-hydroxyl exchange partition coefficient 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 anion stoichiometry of hornblende and utilizing the dissociation reaction constant 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 In K1 = (Cl/OH)hb/(Cl/OH)melt = 2· 37 - 4·6[Mg/(Mg + Fe)]hb at 2-3kbar and 800-850°C. The twofold variation in Cl content in the oscillatory zoned cores of hornblende phenocrysts in the 1991-1995 dacite cannot be explained by the dependence of the Cl/OH partition coefficient on the Mg/(Mg + Fe) hb ratio, and requires c. 80% variation of the Cl/OH ratio of the coexisting melt. Available experimental data at 200 MPa on Cl/OH fractionation between fluid and melt suggest that c. 1· 2-1·8 wt % degassing of water from the magma can explain the required 80% variation in the Cl/OH ratio of the melt. The negative correlation between Al content and Mg/(Mg + Fe) ratio in the oscillatory zoned cores of the hornblende phenocrysts is consistent with repeated influx and convective degassing of the fluid phase in the magma chamber.

AB - High-temperature-pressure experiments were carried out to determine the chlorine-hydroxyl exchange partition coefficient 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 anion stoichiometry of hornblende and utilizing the dissociation reaction constant 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 In K1 = (Cl/OH)hb/(Cl/OH)melt = 2· 37 - 4·6[Mg/(Mg + Fe)]hb at 2-3kbar and 800-850°C. The twofold variation in Cl content in the oscillatory zoned cores of hornblende phenocrysts in the 1991-1995 dacite cannot be explained by the dependence of the Cl/OH partition coefficient on the Mg/(Mg + Fe) hb ratio, and requires c. 80% variation of the Cl/OH ratio of the coexisting melt. Available experimental data at 200 MPa on Cl/OH fractionation between fluid and melt suggest that c. 1· 2-1·8 wt % degassing of water from the magma can explain the required 80% variation in the Cl/OH ratio of the melt. The negative correlation between Al content and Mg/(Mg + Fe) ratio in the oscillatory zoned cores of the hornblende phenocrysts is consistent with repeated influx and convective degassing of the fluid phase in the magma chamber.

KW - Chlorine

KW - Element partitioning

KW - Hornblende

KW - Oscillatory

KW - Unzen volcano

KW - Zoning

UR - http://www.scopus.com/inward/record.url?scp=13344285318&partnerID=8YFLogxK

U2 - 10.1093/petrology/egh078

DO - 10.1093/petrology/egh078

M3 - Article

AN - SCOPUS:13344285318

VL - 46

SP - 339

EP - 354

JO - Journal of Petrology

JF - Journal of Petrology

SN - 0022-3530

IS - 2

ER -