Crystal forensics: constraining the timescales of magmatic processes leading to volcanic eruptions

Volcanoes erupt daily and are an everyday hazard for millions of people worldwide. Today, active volcanoes are continuous monitored looking for signs of imminent eruptions. However, in order for the accurate prediction of future eruptions to be achieved a comprehensive knowledge of the internal workings of volcanoes is required, including the timescales over which molten rock moves through the volcano. Magma is molten rock composed of liquid rock and solid particles called crystals. The migration of magma through the Earth's crust and the eventual eruption is complex, however a record of this is preserved in crystals. Just as your favourite crime drama uses forensic science to solve the crime, we can do the same with the composition of minerals from volcanic eruptions. The life of a crystal is not simple but a complex one involving periods of growth and melting, migration and periods of residence in different magma bodies until eventually the crystal is erupted and the composition frozen in. Each of these experiences are preserved in the crystal as chemical or textural markers, that results in highly zoned crystals. These crystal zones can be treated in just the same way as tree rings which record the growth history of a tree. The chemical composition of individual zones can be used to fingerprint the magmatic process that formed the zone and we can use the difference in the chemical composition between two adjacent zones to determine the timescale over which these magmatic processes occurred. Immediately after formation the compositional difference between two adjacent zones is sharp, but with time diffusion of elements (migration of small particles that the crystal is made from) smoothes this compositional boundary and this can be used to calculate the timescale of magmatic processes. Importantly, different elements migrate through the crystal structure at differ rates and therefore a whole range of magmatic processes that occur on the timescales of hours to months prior to eruption can be investigated. However, the zoning in these crystals occur on a sub-micron scale (smaller than the width of a human hair). Therefore, for the first time this study will use the new generation of high resolution Secondary Ion Mass Spectrometers permitting the measurement of fine-scale chemical zonation of plagioclase crystals from Mount St. Helen's in the USA and Mount Taranaki in New Zealand. This will allow the magmatic processes and the timescales over which these processes occurred directly prior to eruption to be assessed. These timescales can then be evaluated against the known timescales for the movement of magma prior to the recent eruptions of Mount St. Helen's allowing better models for the future prediction of volcanic eruptions to be constrained. This will provide valuable insights into the working of volcanos directly prior to eruption and could have immense benefits to the millions of people worldwide who live in the shadow of volcanoes and help mitigate the associated hazards of active volcanoes. This research will be conducted in the Department of Earth Sciences, University of Bristol in conjunction with the Interface Analysis Centre, Bristol, Muenster University, Germany, University of Western Australia, Cascades Volcano Observatory and University of Orogen in the USA and Victoria University of Wellington, New Zealand.