Wed 12th July @ noon, Rm 312.222
Abstract:
Cristobalite is a low-P, high-T polymorph of crystalline silica (SiO2). Despite having a nominal stability field far outside the conditions that exist in crustal magma bodies, it forms and persists as metastable vapour-phase crystals in the pores of many volcanic rocks. Interest in volcanic cristobalite has been driven by: concerns that volcanic cristobalite may be a respiratory hazard, the possibility that it may exacerbate lava dome instability, and the fact that its persistence challenges ideas of equilibrium phase stability from geochemical and crystallographic perspectives.
We examine the occurrence, morphology, chemistry, and structure of vapour phase cristobalite. In an effort to unravel its formation mechanisms, significance, and possible utility, we have employed a large range of techniques, ranging from fieldwork to Atom Probe Tomography. We find that volcanic cristobalite is far more common than previously recognized. Textural (µ-cT) evidence points to local (e.g., individual pore scale) SiO2 redistribution, whereby acidic volcanic gases corrode magmatic melt and redistribute its components (mainly SiO2) as cristobalite. Coupled microchemical and microstructural information indicate temporal purification during the vapour deposition process, with progressively lower levels of impurities (mainly Al3+ and Na+), corresponding to progressively greater structural order.
We discuss the concept that volcanic cristobalite represents a natural proxy for the same type of epitaxial chemical vapour deposition used in thin film manufacturing processes. We argue that its link to particular volcanic degassing regimes and its high preservation potential make cristobalite a useful, durable, and underappreciated indicator of crucial magmatic processes.
Short Bio:
Ian Schipper is a lecturer in igneous processes at Victoria University of Wellington. His background is in submarine explosive volcanism and the analysis of textures in volcanic rocks. In recent years, his enthusiasm for all things volcanic have (overly?) broadened his interests to include all stages of magmatic evolution, degassing and eruption. His continual search for adventurous fieldwork has taken him from the bottom of the Pacific Ocean to the tops of the Andes.
