DISCUSSION

When the alteration intensity is considered, the correlations between depth and both porosity and permeability are more clearly defined (Fig. F6). Alteration intensity is categorized as fresh (<2% alteration), slight (2%–10% alteration), moderate (10%–40% alteration), high (40%–80% alteration), very high (80%–95% alteration), and complete (95%–100% alteration), following the classification system described in Binns, Barriga, Miller, et al. (2002). Alteration intensities and mineral compositions listed in Table T1 are from shipboard analyses of thin sections when available. Thin sections are coincident with samples, except where noted in Table T1. For samples without corresponding thin section analyses, alteration intensity is based on alteration logs and graphic logs of the core (Binns, Barriga, and Miller, et al., 2002), and modified using the same criteria and techniques to better characterize the sample because heterogeneities exist within many described rock sections. Unaltered materials, termed fresh for the remainder of this paper, include relict plagioclase, pyroxene, and glass.

In samples with >5% fresh material (all samples not completely altered), porosity is relatively constant with depth (Fig. F6A). With the exception of four outliers, porosity ranges from 11% to 22%. The sample with the lowest porosity, 1%, is a fresh sample from the top portion of Hole 1188A. The three porosity values that are significantly higher than the rest of the samples come from samples that exhibit clastic or hydrothermal breccia textures and flow banding. The only other clastic sample in this suite is the very highly altered sample at 337.48 mbsf, with a midrange porosity of 20%. The clastic nature of the rock could allow higher void space in the rock structure; however, it is not reflected by an increase in permeability. In samples with <5% fresh material, porosity decreases with depth, from ~30% near the seafloor to 15% at ~350 mbsf (Fig. F6B). The maximum porosity measurement was 42% at 117 mbsf, and the minimum was 10% at 336 mbsf. The decrease in porosity corresponds to an increase in the amount of quartz with depth and a disappearance of cristobalite by ~130 mbsf, especially apparent at Site 1188. Both cristobalite at upper levels and quartz at depth can fill void spaces, such as vesicles, so the decrease in porosity with depth is likely related to the crystallization of quartz in the body of the rocks.

Permeability measurements are more variable with depth. For samples with >5% fresh material, permeability generally remains constant with depth (Fig. F6C). Permeability in the majority of samples ranges from 10–18 to 10–15 m2. One sample with a high permeability value of 6.99 x 10–15 m2, located at 19.36 mbsf, is a fresh rock from the uppermost section of Hole 1188A with <10% alteration. The outlying low-permeability sample, 1.37 x 10–19 m2, at 241 mbsf, is a very highly altered sample with only 7% fresh material. This sample seems to better fit the trend of completely altered samples. In completely altered samples, permeability gradually decreases with increasing depth, similar to the trend seen in the porosity-depth profile for completely altered samples. Values range from ~10–16 m2 near the seafloor to ~10–18 m2 below 300 mbsf (Fig F6D). One slightly higher permeability value at 185 mbsf of ~10–15 m2 is from a large silica-magnetite vein, which may contribute to the higher permeability value; however, it is not reflected by the porosity.

When porosity–permeability relationships are considered for subsets of data based on the amount of alteration, the correlation between permeability and porosity is stronger (Fig. F7). For samples with >5% fresh material, there is no obvious trend between porosity and permeability. Values are generally clustered in the middle range for both permeability and porosity, with several outlying data points. For completely altered samples, there is a defined correlation between porosity and depth, in which permeability decreases with decreasing porosity. In completely altered samples, both the permeability and porosity are similarly affected. One possible explanation for the correlation found in altered samples is that the formation of alteration products and subsequent removal of igneous minerals causes the porosity and permeability to be solely dependent on alteration. With less altered samples, more of the relict texture remains in the rock, which can be more variable, thus creating a more scattered permeability–porosity relationship.

The mean value of permeability measurements from the PACMANUS hydrothermal field is significantly greater than in other submarine environments at the core scale (Fig. F8). As compared to samples from both young and old crust in a variety of geophysical settings (Johnson, 1980; Karato, 1983a, 1983b; Christensen and Ramananantoandro, 1988; Iturrino et al., 2000), the average permeability is two to four orders of magnitude greater than in other regions. A fundamental difference between the samples in this study and those of other seafloor environments is that samples from the PACMANUS basin are felsic rocks and in most cases are extremely altered. It is possible that felsic rocks are inherently more permeable or that the alteration process could increase permeability within rocks. However, less altered samples did not have significantly lower permeabilities than completely altered samples. Moreover, the only fresh sample in this study had the highest permeability. While this hints that some characteristic like vesicularity in felsic rocks contributes to the higher permeability, the data provide no clear indication of the relative roles of alteration and precursor rock type.

Although surface fluid venting is quite different between Sites 1188, 1189, and 1191, permeability measurements do not vary systematically between holes. Site 1189, which exhibits the most vigorous flow at chimneys, has core-scale permeability values ranging from 3.37 x 10–18 to 7.69 x 10–16 m2. This range is similar to that of Site 1188, where flow at the seabed is only diffuse, in which permeability varies from 1.37 x 10–19 to 6.99 x 10–15 m2. Because surface fluid flow at the two contrasted sites is not reflected in the core-scale permeabilities, other factors must exert the dominant control.

Core-scale measurements do not include the impacts of larger regional features, such as faulting and macro-scale fractures. We can use a simplified, one-dimensional analytical model to estimate the likely fluid flow velocities in a medium with an isotropic, homogeneous permeability of 10–16 m2. We can then compare the calculated value to observed velocities in the PACMANUS hydrothermal field to determine if fracturing is necessary to obtain the observed velocities. Assuming that the only driving force for fluid flow is buoyancy, pressure is hydrostatic, and fluid flow is in the vertical direction, fluid flow velocity (Darcy velocity = q) can be calculated using a modified version of Darcy's law (Domenico and Schwartz, 1997):

q = –(kgoT/), (1)

where

k = permeability,
g = gravity,
o = reference density for seawater,
= thermal expansivity for seawater,
T = difference in temperature between the bottom of a given borehole and the seafloor surface, and
= fluid viscosity.

In the deepest hole drilled during Leg 193, Hole 1188F, temperature measurements made using an ultra high temperature multisensor memory tool (UHT-MSM) temperature probe recorded a maximum temperature of 313°C at a maximum depth of 387 mbsf. Using a maximum T of 313°C and fluid properties integrated over the range of pressures and temperatures along the flow path, the maximum Darcy velocity is <2 cm/yr. Although flow rates of venting fluids have not been rigorously measured in the PACMANUS hydrothermal field, estimates of flow rates at particular chimney orifices are in the range of 2–5 cm/s (R. Binns, pers. comm., 2003). Thus, with a bulk rock permeability of 10–16 m2, the hydrothermal venting observed in the PACMANUS Basin could not occur.

In order to drive the rapid fluid flow observed in the PACMANUS hydrothermal field and produce chimney structures, particularly evident at the Roman Ruins site, there would likely be a mechanism to focus flow through more permeable zones. A likely fluid flow scenario would channel fluid through large-scale fractures, as seen in the Juan de Fuca Basin. For example, the one laboratory permeability measurement in basaltic rock from Middle Valley, Juan de Fuca Ridge, is up to six orders of magnitude less than that observed in in situ measurements, measured using packer testing (Becker et al., 1994; Iturrino et al., 2000). These results suggest that high-permeability zones are associated with fractures or faults (Iturrino et al., 2000), whereas the rock matrix has a significantly lower permeability. Similar hydrogeologic conditions may exist in the PACMANUS hydrothermal field; however, because of the nature and instability of the submarine environment, in situ permeability measurements were not possible during ODP Leg 193. Fractures may exist in the regions drilled in the PACMANUS hydrothermal field, but the low core recovery and fractured nature of the recovered core made it impossible to determine the amount of fractures from core analyses. However, recovered portions of the core were often brecciated or fractured. Additionally, preliminary downhole logging results indicate that there are a significant number of fractures and brecciated zones (Binns, Barriga, Miller, et al., 2002; Bartetzko et al., 2003) that may serve as conduits for hydrothermal fluid circulation. Future work is still in progress to determine fracture aperture and density parameters and their importance to the PACMANUS system.

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