Anisotropy in seafloor flange, slab, and crust samples from measurements of permeability and porosity : implications for fluid flow and deposit evolution

Abstract

Seafloor hydrothermal vents accommodate the convective transfer of fluids from subsurface environments to the oceans. In addition to black smoker chimneys, a variety of other deposit-types form. Flanges protrude from the sides of edifices as horizontal ledges, below which vent fluids pool. Slabs are hydrothermally silicified layered volcaniclastic deposits. Crusts are deposits composed of previously deposited material underlain by hot fluids. Permeability and porosity measurements were conducted on flanges from Guaymas Basin and the Main Endeavour Vent Field, slabs from the Lucky Strike Vent Field, and a crust sample from the Trans-Atlantic Geotraverse (TAG) active mound. Cores taken parallel to textural layers have high permeabilities (≈10−12 m2) and porosities (30–40%) that follow a power law relationship with exponent α ≈ 1 to 2. Cores taken perpendicular to layering have permeabilities from 10−16 to 10−12 m2 and porosities from 20 to 45%, with α ≈ 5 to 8. The two distinct trends result from the heterogeneity of textural layers within these deposits. Microstructural observations show large variations in grain packing and pore distributions between layers, consistent with flow perpendicular to layering being more susceptible to changes in permeability that result from mineral precipitation than flow parallel to layering. These results imply that the primary flow direction in these deposits is parallel to layering, whereas flow perpendicular to layering is more restricted. Quantification of anisotropic permeability provides important constraints for determination of fluid flux from these layered deposits, and temperatures, chemistry, and availability of nutrients to organisms living in and at exteriors of deposits.