Storage, ascent and emplacement of rhyolite lavas
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The physical properties and dynamic processes that control effusions of rhyolitic lavas are poorly constrained because of a paucity of direct observations. To assess the pre-eruptive storage conditions, eruptive ascent, and subaerial emplacement for a suite of volumetrically diverse rhyolitic lavas, I studied 10 obsidian lavas from Yellowstone Caldera, Wyoming and Mono Craters, California. Storage, ascent, and emplacement of those lavas were quantitatively constrained using phenocryst compositions, high temperature experiments, microlite textures, and compositional gradients surrounding spherulites. Compositions of phenocrysts and quartz-hosted glass inclusions indicate the magmas at Yellowstone were stored at 750±25 °C in the shallow crust (<7 km), in agreement with phase equilibria experiments. Following the initiation of an eruption, magma leaves the chamber and ascends in a conduit. Microlite number density can be used to quantify eruptive ascent rates. To generate the observed microlite number densities (10⁸·¹¹±⁰·⁰³) to 10⁹·⁴⁵±⁰·¹⁵ cm⁻³), the magmas decompressed at ~1 MPa hour⁻¹, equivalent to ascent rates of ~10 mm s⁻¹. Upon subaerial emplacement, microlites act as rigid particles in a deforming fluid (lava), and hence their 3D orientations could indicate flow direction and how strain accumulates in the fluid during flow. Microlites are strongly aligned in samples from all flows, but variations in alignment were found to be independent of flow volume or distance travelled. Together, those observations suggest that strains accumulated during subaerial transport must be small (<2). Instead, microlites most likely aligned in response to strain in the conduit, which can be generated by collapse and flattening. Upon reaching the surface, the cooling history and longevity of rhyolitic lavas are critical for developing models of emplacement and hazard assessment. Compositional gradients surrounding spherulites provide one method to assess such temporal characteristics. Spherulites, crystalline spheres of radiating quartz and feldspar, form by crystallization of obsidian glass in response to cooling. An advection-diffusion model was developed to simulate the growth of spherulites and compositional gradients that develop in the surrounding glass during spherulite growth. Observed gradients are consistent with spherulites growing between ~700 and ~400 °C, and cooling at rates of 10⁻⁵·²±⁰·³) °C s⁻¹.