Permeability-Porosity Relationships in Deep Sea Hydrothermal Vent Deposits

To map out the thermal and chemical regimes within vent deposits where micro-and macro-organisms reside requires accurate modeling of mixing and reaction between hydrothermal fluid and seawater within the vent structures. However, a critical piece of information, quantitative knowledge of the permeability of vent deposits, and how it relates to porosity and pore geometry, is still missing. To address this, systematic laboratory measurements of permeability and porosity were conducted on 3 large vent structures from the Mothra Hydrothermal vent field on the Endeavor Segment of the Juan de Fuca Ridge. Twenty-five cylindrical cores with diameters of 2.54 cm and various lengths were taken from Phang (a tall sulfide-dominated spire that was not actively venting when sampled), Roane (a lower temperature spire with dense macrofaunal communities growing on its sides that was venting diffuse fluid of < 300° C) and Finn (an active black smoker with a well-defined inner conduit that was venting 302° C fluids prior to recovery (Delaney et al., 2000; Kelley et al, 2000)). Measurements were made to obtain porosity and permeability of these drill cores using a helium porosimeter (UltraPoreTM300) and a nitrogen permeameter (UltrapermTM400) from Core Laboratories Instruments. The porosimeter uses Boyle's law to determine pore volume from the expansion of a know mass of helium into a calibrated sample holder, whereas the permeameter uses Darcy's law to determine permeability by measuring the steady-state flow rate through the sample under a given pressure gradient. A moderate confining pressure of 1.38 MPa was applied during the measurements to prevent leakage between the sample surface and the sample holder. The permeability and porosity relationship is best described by two different power law relationships with exponents of ∼9 (group I) and ∼3 (group II), respectively. Microstructural observations suggest that the difference in the two permeability-porosity relationships reflects different evolution processes as pores are sealed within different parts of the vent structures. Our data suggest that correctly identifying the processes of pore space evolution in seafloor vent deposits is the key to successfully relating permeability to porosity.