Melt pockets in phenocrysts and decompression rates of silicic magmas before fragmentation

[1] A new method is presented to estimate the decompression rate of silicic magma prior to fragmentation and is applicable to rates of 1 - 100 kPa/s. This method uses the remaining concentrations of H2O and CO2 in bubble-free cylindrical melt ( now glass) pockets in phenocrysts that are connected to the surrounding bubbly melt. These melt pockets are common in volcanic rocks. During magma ascent, bubbles grow in the surrounding melt, and the concentrations of dissolved H2O and CO2 decrease, establishing a boundary condition of low volatile concentration at the outlet of the melt pocket. In turn, volatiles diffuse from the melt pocket into the surrounding bubbly magma and form a gradient that is time dependent. The volatile concentration at the inner limit of the melt pocket likewise diminishes with time. Knowing the diffusivity and solubility of H2O and CO2 permits the remaining H2O and CO2 contents to be estimated for different decompression rates using diffusion equations. We apply this approach to the measured H2O and CO2 contents in quartz-hosted melt pockets in pumice from the phreatomagmatic 26.5 ka Oruanui eruption, Taupo, New Zealand. A decompression rate of 1 - 7 kPa/s (similar to 5-35 cm/s) was obtained for Oruanui magma.