A KINETIC BARRIER TO PLAGIOCLASE NUCLEATION IN DEGASSING ANDESITE AND DACITE LIQUIDS DURING RAPID ASCENT

Although relatively uncommon, crystal-poor (1-5%) andesites and dacites are found throughout the Mexican volcanic arc, typically at peripheral vents. They are often multiply saturated with five to six mineral phases (plagioclase, pyroxenes, two Fe-Ti oxides, ± hornblende), which display euhedral and/or rapid-growth textures. The two Fe-Ti oxides permit temperatures to be calculated (Ghiorso and Evans, 2008), which in turn allows application of the plagioclase-liquid hygrometer (Lange et al., 2009). These calculations were applied to 13 crystal-poor andesites and dacites from the Mexican arc; temperatures range from 1085 to 760 °C, and maximum melt water concentrations range from 2.8 to 8.3 wt%. These results match phase equilibrium diagrams for hydrous andesites and dacites in the literature. There is a wide, continuous range in plagioclase composition within each sample (20-50 mol%An), which can be attributed to a loss of dissolved water (1-3 wt%) in the melt phase during degassing as the magma ascends to the surface. The most surprising result is that the minimum melt water concentration recorded by the most sodic plagioclase in each sample is highly variable (0.8 to 6.0 wt%) and displays a strong correlation with temperature, with cooler magmas recording higher minimum water contents. Using the viscosity model of Hui and Zhang (2007), we show that this relationship is due to a common viscosity limit on the nucleation and growth of plagioclase in these rapidly ascending magmas (~3.9 ± 0.6 log₁₀Pa-s). As the magmas quickly ascend and degas, the loss of water in the melt phase induces an abrupt increase in melt viscosity. For cooler magmas, the viscosity limit occurs at a higher melt water concentration than for hotter magmas. Some of these cool, hydrous magmas crossed the plagioclase liquidus at depths of > 9-12 km; therefore, in order to erupt with < 5% crystals, they must have traversed the upper crust very rapidly, presumably along a fracture, without stalling.