Melt inclusion shapes: Timekeepers of short-lived giant magma bodies
Wednesday, 17 December 2014: 10:20 AM
Supereruptions have the potential to be devastating events, and constraining the longevity of the giant pools of magma (>450 km3) discharged in such eruptions is critical. Much work has focused on this problem, but no consensus has been reached, and estimates vary considerably depending on the approach used (e.g. Bishop Tuff, zircon geochronology: 103-106 a; oxygen isotopes: 104 a; thermal modeling: 103-105 a; textures, diffusion chronometry: 103 a). This discrepancy exists partly because different timekeepers in eruptive products record different magmatic processes, and methods vary in how well they can resolve timescales related to these processes. We describe a new method using textures (sizes, shapes, positions) of quartz-hosted melt inclusions, determined from propagation phase-contrast x-ray tomography, to estimate crystallization times. The premise of this geochronometer is that, over time, initially round melt inclusions become more faceted through diffusion, and their current shape can be used to estimate their magmatic residence time. This method is distinct in that it illuminates the time that a magma was melt-rich and eruptible rather than crystal-rich and uneruptible; many geochronometers can record these latter, more protracted histories, making it challenging to interpret the timescales that result from them. We also use this method to estimate quartz growth rates, an important but currently poorly constrained quantity. We determine growth rates by relating inclusion residence times and their positions within a crystal: Growth Rate = Length (distance from crystal edge) / Time (residence time). We apply this method to three large-volume high-silica rhyolite eruptions: the paired 240 ka Ohakuri-Mamaku (245 km3 combined volume; central Taupo Volcanic Zone [TVZ], New Zealand), the 26.5 ka Oruanui (530 km3; central TVZ), and the 760 ka Bishop Tuff (1000 km3; California, USA). To validate this method, we compare a subset of our results to those obtained from Ti diffusion profiles in the same crystals. Results show that: (a) the two methods give comparable timescale estimates; (b) quartz growth rates average 10-12 m/s; and (c) quartz melt inclusions give decadal to centennial timescales, revealing that giant magma bodies develop over notably short, historical timescales.