Here we present the first permeability data from an actively venting, felsic hydrothermal environment. The variability in the data set is large, spanning five orders of magnitude. The variability could be because of micro-fracturing within the core samples, varying amounts or types of alteration that has taken place in a particular sample, or other heterogeneities that are not readily apparent in thin section analysis. Permeability decreases with depth, with near-surface values ranging from 10–17 to 10–15 m2 and deeper values ranging from 10–19 to 10–15 m2. Permeability also decreases with decreasing porosity. When the amount of alteration is considered, trends between both porosity and permeability with depth are more apparent. Completely altered samples decrease in both permeability and porosity as depth increases. Less altered samples retain a relatively constant permeability and porosity, with several outlying values. Correlations to other physical properties such as velocity or thermal conductivity are not readily apparent.
The average permeability of the PACMANUS hydrothermal field is higher than measured permeabilities in other seafloor environments, based on similar laboratory measurements. The cause for this elevated permeability is undetermined; however, it may be related to the felsic nature of the rock or alteration features, both defining characteristics of the minicore samples. In situ permeabilities would likely be greater than values measured in the laboratory tests because of macro-scale fracturing, which would serve as flow conduits. Fluid flow velocities observed in the PACMANUS hydrothermal vents are larger than could be obtained from rocks with a bulk permeability of 10–16 m2 as computed by simplified analytical calculations. Also, permeability values for each site do not correspond to observed differences in surface flow velocities. For example, Site 1189, which has rapid, focused venting, does not have a higher average permeability than Site 1188, which exhibits slow, diffusive venting. Therefore, the fluid flow processes in the basin are likely to be strongly influenced by the large-scale faulting and macro-scale fracturing. Future analysis of the fracture field based on logging information together with these core-scale data can be used to further establish the permeability field in the PACMANUS hydrothermal field.