Determining pre- and syn-eruptive timescales by diffusion methods: case studies from the Oruanui super-eruption
Abstract
Diffusion methods permit investigation of timescales of short-lived processes occurring in magmatic systems before, and in some cases during, eruption. We here present three case studies from the 25.4 ka Oruanui super-eruption looking at diffusion in a variety of minerals that reflects a spectrum of timescales. At one end of the spectrum, the diffusion of Li is exceedingly rapid. Diffusion distances of >100 microns in quartz and plagioclase can be attained in minutes at magmatic conditions. Strong diffusion appears to be induced in the very latest stages of magmatic ascent across most quartz and plagioclase crystals and is interpreted to be due to changes in the melt structure and fluid-melt equilibrium during decompression. With corrections for adiabatic expansion and post-eruptive cooling, the magma ascent rate from depths corresponding to the critical pressure is constrained to be up to 21 m/s. Determining timescales here requires corrections of raw data for local anorthite contents, although Li diffusion itself appears to be largely independent of composition. Fe-Ti oxides can retain significant zonation of Fe and Ti abundance for only a matter of days at magmatic conditions, making them ideal for thermometry to establish pre-eruptive conditions. However, observation of zonation in magnetite grains can be used to infer timescales of processes occurring over days before eruption. Magnetites from the phase 7 Oruanui eruption deposits show zoning, reflecting events which occurred in the hours before quenching of the host pumice. In combination with field and textural evidence, we interpret these events to result from syn-eruptive infiltration of hot mafic magma into Oruanui rhyolite. Diffusion in this scenario is straightforward to model in one dimension but requires consideration of ambient fO2. Structural ions in silicate minerals typically diffuse much more slowly, and so yield information on long-lived and persistent processes. Fe-Mg interdiffusion in orthopyroxene is more complex than either Fe-Ti in magnetite or Li in feldspars and quartz. The diffusion coefficient is both composition- and oxygen fugacity-dependent and so a finite difference model would typically be used to model the diffusion behaviour. Here, we show how a specially-constructed spreadsheet can accurately fit and model diffusion profiles based on certain diffusion laws. Results obtained for Oruanui orthopyroxenes suggest residence times in the range of 300 to 1000 years, reflecting the timescale over which the eruptible portion of the Oruanui magma body was assembled (see also ASR Allan et al. in session V046).
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.V31C2799M
- Keywords:
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- 1036 GEOCHEMISTRY / Magma chamber processes;
- 3618 MINERALOGY AND PETROLOGY / Magma chamber processes;
- 3625 MINERALOGY AND PETROLOGY / Petrography;
- microstructures;
- and textures;
- 3640 MINERALOGY AND PETROLOGY / Igneous petrology