Temporal-spatial micro-scale investigations of shallow silicic conduits: late-stage degassing, crystallization, and alteration

Abstract

"Conduit samples from Unzen Volcano, obtained a decade after the 1991-1995 eruption, exhibit important physical and mineralogical differences, and subtle differences in bulk chemistry from erupted samples. These differences reflect emplacement confining pressures, maintenance at hypersolidus temperature, and subsequent subsolidus hydrothermal alteration. In contrast, extruded lava underwent decompression to ~1 atm., ~complete loss of magmatic water and rapid cooling. The resulting hypabyssal conduit texture is distinct from both eruptive and plutonic rocks. The low temperature of the conduit, <200°C when sampled by drilling, requires swift post-emplacement textural development. Significant changes in bulk composition were Mg, Fe and Na depletion and C and S enrichment. Trace-element concentrations of the conduit and the last-emplaced lava of the spine indicate a common derivation. Investigating three aspects of magma transport and post-emplacement evolution at Unzen, in conjunction with observations from comparable dome-forming volcanoes, quantifies the processes working in concert to produce the resultant textures. First, we constrain magmatic ascent rates and crystallization depth of the Unzen dacite via decompression crystallization experiments. Our results indicate that slow effusion rates (<̲20 m/hr) are required for both spine and conduit. Second, we present mineralogical evidence for extremely rapid alteration beneath Unzen, related to temperature, permeability within and volatile flux through the volcanic edifice. We describe meter-scale variations in alteration conditions, and conclude that maintenance of permeability to low porosity allows basalt-derived volatiles to traverse the conduit zone promoting extremely rapid alteration, orders of magnitude greater than predicted by models for larger/deeper intrusions. This suggests that despite convective hydrothermal heat removal, chemical effects were limited to early loss of magmatic water and later addition of magmatic volatiles. The extensive alteration within Unzen's conduit contrasts the minimal alteration beneath Obsidian Dome, considered as a 'control'. Finally, we present anisotropy measurements and three-dimensional visualizations of degassing structures from four volcanoes: Bezymianny, Unzen, Mount St. Helens and Obsidian Dome. Our novel approach, employing X-ray computed tomography and percolation models, indicates that gas loss at depth is more efficient in sheared regions (anisotropic), than in those that contain no evidence for shearing (isotropic)"--Leaves iii-iv