A Comparison of Oceanic Crustal Permeability at the Outcrop, Hand Sample and Thin Section Scales

Abstract:

Hydrothermal flow through the oceanic crust controls heat and mass fluxes between the crust and the ocean, with implications for element cycling and Earth’s heat budget. Permeability in oceanic crust is generated by a combination of high-porosity faults and volcanic deposits. Whereas faults generally increase permeability in the vertical direction, volcanic deposits impart an isotropic permeability. This study is focused on evaluating the permeability in volcanic rocks from the outcrop to thin-section scales including: large inter-pillow voids and hyaloclastite zones (m-cm), vesicles (cm-mm), and micro-scale pore networks (mm-μm). Quantifying hydrothermal fluxes requires measurements of crustal permeability. However, most physical measurements of oceanic crustal permeability are made on core pieces and rock fragments. The importance of large pore space for controlling formation-scale permeability is not known in detail but has been interpreted from in situ measurements of bulk permeability at boreholes. To better quantify formation-scale permeability of oceanic crust we integrate 2D and 3D permeability measurements at the outcrop, hand sample and thin section scales. We estimate outcrop-scale permeability using LiDAR scans and photographs of a pillow lava outcrop in the Talcott basalt of southeastern Connecticut. Permeability is quantified from a 2D classified image of dense pillow cores and permeable inter-pillow hyaloclastite zones using equations based on the equivalent channel model of Walsh and Brace (1984). We apply the same method to 2D images of porous rock hand samples and thin sections that have likewise been segmented into pore and matrix space. Our results will help evaluate how porosity and permeability measurements made at the sample and thin section scale can be adapted to understand magnitudes of fluid flow through oceanic crust at the regional and global scales.