An eruption chronometer based on experimentally determined H-Li and H-Na diffusion in quartz applied to the Bishop Tuff
- Journal Article
© 2020 The Author(s) The diffusion of hydrogen in natural hydrothermal quartz crystals was studied between 657–956 °C at atmospheric pressure and various oxygen fugacity (fO2) conditions. Single crystals of OH-bearing quartz were dehydrated in the presence of Li or Na-enriched powders to induce Li-H or Na-H exchange, with the resulting diffusion profiles measured by both Fourier transform infrared (FTIR) spectroscopy and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Diffusion parallel to , i.e. the crystallographic c-axis, is described by: [Formula presented] where log10D is the base 10 logarithm of the diffusion coefficient, R is the gas constant and T is the temperature in kelvins, and uncertainties are 1 σ. Diffusion is not affected by (fO2) in this system. Diffusion perpendicular to  is consistently slower, but quantitative constraints cannot be obtained from our data given experimental limitations. This diffusivity is primarily associated with H+-Li+ (or H+-Na+, Na+-Li+) exchange, where the H+ and Li+ are charge-balanced by tetrahedrally-coordinated Al3+. A faster mechanism may also exist where the monovalent cations are charge balanced by excess oxygen. The final defect population, as imaged by FTIR spectroscopy and LA-ICP-MS, likely results from a combination of diffusion and inter-site rearrangement of the monovalent cations. Regardless of such complexities, the determined Arrhenius relationship should be applicable for natural volcanic quartz crystals from the Bishop Tuff, California, wherein H+ is associated with Al3+, and H loss from quartz preceding and/or accompanying the eruption is charge-balanced by Li-gain, without Al movement. Li and H profiles from an example quartz crystal suggest that it experienced eruption/cooling timescales of just 16 minutes to 2.4 hours, showing the considerable promise of using frozen H diffusion profiles in quartz to extract timescales and thus elucidate the last moments of such explosive silicic eruptions. Show more
Journal / seriesEarth and Planetary Science Letters
Pages / Article No.
166281 - Understanding eruptive and post-eruptive processes in rhyolites via lithium (Li) isotopes (SNF)
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