Crystal Control on Pore Pathways and Degassing of Mafic Volcanic Rocks

The degree to which magma can degas, and thereby relieve gas pressure prior to an eruption, determines whether an eruption will be effusive or explosive. Potential gas-escape pathways can be imaged by combining measurements of connected porosity, permeability, and electrical conductivity of volcanic rocks with analysis of thin section images. Most work on volcanic pore geometries has focused on crystal-poor silicic magmas. Here we examine crystal-rich mafic magmas to see how pore geometries and pathways are affected by the presence of a crystalline network. We measure connected porosity of one-inch diameter, one-inch long cores using helium pycnometry; permeability is measured using a capillary flow porometer; and resistance to electrical flow is measured by an LCR meter attached to a core saturated in a conductive saline solution. Electrical tortuosity (a proxy for sinuosity and/or roughness of connected pathways) is calculated using the connected porosity of the core and its resistance to electrical flow. Plots of Darcian permeability versus connected porosity for a range of rock types reveal that mafic (basalts and basaltic andesites) and silicic (dacites and rhyolites) samples cluster in separate regions with little overlap. Mafic rocks are generally more permeable and have lower electrical tortuosities than silicic rocks with the same connected porosities. These differences most likely reflect some combination of larger pore apertures and lower pore path sinuosities in mafic samples. In a detailed study of a suite of basaltic andesites from Bezymianny volcano, Kamchatka, Russia, we see that permeability and porosity vary widely with sample type (samples include dense cryptodome, dense to vesicular lava and pycroclastic flows, and highly vesicular airfall deposits) and that electrical tortuosity decreases with increasing connected porosity. Preliminary examination of SEM images shows that all samples have abundant phenocrysts and microlites. Low porosity cryptodome samples have isolated large pores that are deformed and elongated around and between crystals. In contrast, high porosity airfall samples have abundant small vesicles, particularly along crystal margins. We suggest that the presence of crystals facilitates bubble nucleation, and that as crystallinity increases, bubble deformation becomes more pronounced as bubbles are able to elongate and coalesce to form crack-like pathways. This implies that even at low connected porosities, permeable networks are able to form and provide effective pathways for degassing.