Hydrous and anhydrous melting experiments of an alkali basalt and a transitional tholeiite from the Oginosen volcano,Southwest Japan: The possible influence of melt depolymerization on Ca-Na partitioning between plagioclase and the melt

摘要

It has been well established that the exchange partition coefficient for the exchange of Ca and Na between plagioclase and silicate melts [Kd = (Ca/Na)_PL/(Ca/Na)_melt] increases with increasing water content in the melt, but its atomistic interpretation is not well developed. This work presents new experimental data on the partition coefficient in an alkali basalt and a transitional tholeiite from the Oginosen volcano, southwest Japan and discusses the possible role of melt polymerization in the variations of the partition coefficient. The experiments were conducted at 0.1 MPa, and hydrous 100 MPa and 200 MPa conditions. The partition coefficient at 0.1 MPa increases from 0.8-1.4 to 1.5-1.8 over a temperature increase from 1090 ℃ to 1190 ℃ in the transitional tholeiite, and from 1.2 to 1.7 for 1090 ℃ to 1150 ℃ in the alkali basalt. The partition coefficient increases up to 4.1-4.2 in the presence of 3.4-3.5 wt% water in the melt in both basalts. The variations of the Ca-Na partition coefficient between the plagioclase and the melt is interpreted in terms of the degree of polymerization of the melt. The degree of polymerization of the melt decreases with the increase of temperature and water content, both of which increase the Ca-Na partition coefficient. The anorthite content of the core of natural plagioclase phenocrysts in the alkali basalt is 63-66 mol%, suggesting crystallization under water-undersaturated conditions. In the transitional tholeiite, some of the cores of the plagioclase phenocrysts have An_81-85, which is formed in a water-supersaturated run at 100 MPa and 1085 ℃; however, the significance of the presence of An_81-85 is unknown and must be investigated further.