Cl/OH Partitioning Between Hornblende and Melt and its Implications for the Origin of Oscillatory Zoning of Hornblende Phenocrysts in Unzen Dacite

HR: 1340h
AN: V33D-1488 [Abstracts]
TI: Cl/OH Partitioning Between Hornblende and Melt and its Implications for the Origin of Oscillatory Zoning of Hornblende Phenocrysts in Unzen Dacite
AU: * SATO, H
EM: hsato@kobe-u.ac.jp
AF: Dept.Earth & Plan.Sci., Kobe University, Rokko-dai 1-1, Nada-ku, Kobe, 657-8501 Japan
AU: HOLTZ, F
EM: f.holtz@mineralogie.uni-hannover.de
AF: Institute for Mineralogy, Univ. of Hannover, Callinstr.3, 30167, Hannover, 30167 Germany
AU: BEHRENS, H
EM: H.Behrens@mineralogie.uni-hannover.de
AF: Institute for Mineralogy, Univ. of Hannover, Callinstr.3, 30167, Hannover, 30167 Germany
AU: BOTCHARNIKOV, R
EM: R.Botcharnikov@mineralogie.uni-hannover.de
AF: Institute for Mineralogy, Univ. of Hannover, Callinstr.3, 30167, Hannover, 30167 Germany
AU: NAKADA, S
EM: nakada@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, Univ. of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-0032 Japan
AB: We carried out high temperature-pressure experiments to determine the chlorine-hydroxyl exchange partition coefficient between hornblende and melt on the 1992 dacite of Unzen volcano. Cl in hornblende and melt are analyzed by electron microprobe, whereas OH in hornblende and melt are 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 ln K1 = (Cl/OH)hb/(Cl/OH)melt = 2.37-4.6*[Mg/(Mg+Fe)]hb at 2-3 kb and 800-850讈. It is shown that twofold variation of Cl content in the oscillatory zoned core of hornblende phenocrysts in the 1991-1995 dacite of Unzen volcano is not explained by the dependence of Cl/OH partition coefficient on the Mg/(Mg+Fe)hb, and requires ca. 80% variation of Cl/OH ratio of the coexisting melt. Available experimental data at 200 MPa on Cl/OH fractionation between fluid/melt suggest that ca. 1.2-1.8 wt% degassing of water from the magma explains the required 80% variation of the Cl/OH ratio of the melt. The negative correlation between Al content and Mg/(Mg+Fe) ratio in the oscillatory zoned core of hornblende phenocrysts is consistent with repeated influx and convective degassing of the fluid phase in the magma chamber.
DE: 8414 Eruption mechanisms
DE: 8434 Magma migration
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting