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

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 ln K-1 = (Cl/OH)(hb)/(Cl/OH)(melt) = 2.37 - 4.6[Mg/(Mg + Fe)](hb) at 2-3 kbar and 800-850degreesC. 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.