Controls of crystal shape on degassing mechanisms in crystal-rich magmas with rhyolitic groundmass melts

Abstract

Understanding the mechanical effects of crystals on degassing kinetics and permeability development in silicic magmas is important for modeling eruptions and examining first order controls on eruption style. We conducted high-pressure-high-temperature isothermal decompression experiments to investigate the role of crystal shape on permeability development and pore pathway geometry. Experiments were performed on hydrous rhyolitic glass (76.3 wt.% SiO2) seeded with variable amounts of equant (aspect ratio ∼1.8 ± 0.6) corundum crystals and elongate (aspect ratio ∼10 ± 5.5) wollastonite crystals to approximate natural phenocryst and microphenocryst/microlite populations, respectively. We measured total porosity, connected pore volume and permeability directly from the experimental charges by applying Archimedes' principle to determine bulk density, helium-pycnometry to measure connected porosity, and a custom-made permeameter to measure permeability. The experimental samples developed permeability at a critical melt porosity (ϕc-melt, above which degassing is enhanced due to bubble coalescence) of ∼55 vol.% vesicles for the corundum experiments and ∼48 vol.% for the wollastonite-bearing experiments; these values are considerably lower than the ϕc-melt>63 vol.% for prior crystal-free experiments (Lindoo et al., 2016; deGraffenried et al., 2019). Critical porosity is reduced when crystals comprise at least ∼20 vol.%, regardless of shape. Connected porosity increases and average bubble size decreases with increasing abundance of elongate wollastonite crystals, explained by the onset of yield strength behavior induced by loosely touching crystal frameworks that form at decreasing crystallinities with increasing elongation of crystals comprising the network. When the population of high-aspect-ratio crystals reaches random loose packing, the resulting reduction in interstitial melt available for unimpeded bubble expansion forces the bubbles to connect at lower total vesicularity. It therefore seems likely that crystal-bearing intermediate magmas experience an abrupt increase in degassing efficiency when crystallinity attains random loose packing. In hydrous magmas, the efficiency of decompression-driven degassing and resulting formation of anisotropic groundmass crystals is controlled by magma composition and decompression rate. Enhanced gas loss in slowly ascending (and crystallizing) magma may aid the development of dense conduit plugs, thus increasing the possibility of violent Vulcanian explosions.