During the last three decades, the Deep Sea Drilling Project (DSDP) and the Ocean Drilling Program (ODP) have dedicated time and effort in trying to understand the geological, geophysical, and biological processes associated with spreading centers and convergent margins. Recently, significant emphasis has been placed on investigating the role that magmatic fluids and heat fluxes associated with hydrothermal systems play on the genesis of major sulfide deposits and biological communities. Circulation of hydrothermal fluids along oceanic spreading centers is one of the most fundamental processes associated with crustal accretion and localized mineralization. The styles and effects of circulation are partially controlled by the permeability and porosity of the ocean crust. Heat from magmatic intrusions drives circulation of seawater through permeable portions of the oceanic crust and upper mantle, discharging at the seafloor as both focused high-temperature (250°-400°C) flows and diffuse lower temperature (<250°C) flows. The transport of heat through this process has been estimated to comprise almost 25% of the total heat flux of the Earth (Sclater et al., 1980; Stein and Stein, 1994). This complex interaction between the circulating hydrothermal fluids and the oceanic basement greatly influences the physical properties and the composition of the crust (Thompson, 1983; Jacobson, 1992; Johnson and Semyan, 1994).
An attempt to understand the dynamic processes associated with active hydrothermal systems was made by drilling in the Middle Valley of the northern Juan de Fuca Ridge during ODP Legs 139 and 169 and the Trans-Atlantic Geotraverse (TAG) hydrothermal mound during ODP Leg 158. Most of the samples used for this study were from the Bent Hill area of Middle Valley that is located at ~48°26.2'N and 128°40.86'W. This site is in the eastern part of Middle Valley roughly 3 km west of the normal fault scarp that forms the eastern topographic boundary and ~4 km east of the current rift axis (Davis, Mottl, Fisher, et al., 1992). Hydrothermal fluids associated with the formation of the massive sulfides in this area may have been heated either locally by the intrusion of basalt underneath Bent Hill or could have been derived from a more regional, high-temperature reservoir developed by convective circulation in the upper igneous crust beneath the relatively impermeable sediment fill (Davis and Fisher, 1994). The deepest penetration in this area is 500 meters below seafloor (mbsf) in Hole 856H (Fig. F1). This hole consists of a 4-m layer of unconsolidated clastic sulfides overlying a 94-m massive to semimassive sulfide unit. A sulfide feeder zone consisting of sulfide veins and impregnations and a thick sedimentary unit of interbedded hemipelagic and turbiditic sediments underlie this massive sulfide unit. Below the sediments lies a 40-m-thick basaltic sill-sediment complex and basaltic flows (Davis, Mottl, Fisher, et al., 1992; Fouquet, Zierenberg, Miller, et al., 1998).
The TAG hydrothermal area, approximately located at 26°08.22'N and 44°49.55'W, is part of a 40-km-long ridge segment of the Mid-Atlantic Ridge that trends north-northeasterly. This mound is a distinctly circular feature measuring ~200 m in diameter and 50 m in height (Humphris, Herzig, Miller, et al., 1996). It is mainly a large and mature deposit composed of massive sulfide and anhydrite deposits as well as areas with active fluid flow. The observed distribution of silicified wall-rock breccias and chloritized basalt breccias recovered during Leg 158 suggests that the morphology of the mound may include several stages of evolution beginning with volcanic activity within the neovolcanic zone, subsequent spreading-related tectonic activity, brecciation, cementation, hydrothermal reworking, and, finally, sulfide and anhydrite precipitation (Humphris, Herzig, Miller, et al., 1996). A composite stratigraphic section inferred from holes drilled in three areas along a northwest-southeast transect across the mound shows four distinct zones (Fig. F2). Clast-supported massive pyrite breccias dominate the upper 10-20 m of the mound followed by an anhydrite-rich zone composed of matrix-supported, pyrite-anhydrite breccias and pyrite-anhydrite-silica breccias. With increasing depth, the amount of quartz-pyrite mineralization and quartz veining increases. This section represents the top of a quartz-sulfide stockwork zone that overlies a quartz-chlorite stockwork unit (Humphris, Herzig, Miller, et al., 1996).
Because of the low core recovery usually associated with drilling in these active hydrothermal areas (Davis, Mottl, Fisher, et al., 1992; Humphris, Herzig, Miller, et al., 1996; Fouquet, Zierenberg, Miller, et al., 1998), downhole and core physical properties measurements are extremely valuable for reconstructing complete stratigraphic sequences and, thus, an accurate series of events along their geologic history. Electrical resistivities are widely used for determining the size and extent of sulfide deposits from downhole and surface measurements. Permeability measurements are needed for assessing the extent of diffuse fluid flow associated with hydrothermal circulation. Finally, accurate thermal conductivity measurements are required for determining heat flow from temperature measurements and extrapolating thermal regimes to greater depths from near-surface temperature data.
In this manuscript, observations and interpretations are made from a limited set of measurements. We relate lithologic variations to physical parameters using thin-section observations and mineralogical estimates from visual core descriptions. All data are shown in Tables T1, T2, T3, and T4, and the methods used for the different data acquisition techniques are explained in the "Appendix". All measurements made as a function of pressure were obtained at a constant pore pressure of 5 MPa and confining pressures ranging from 10 to 50 MPa. The 50-MPa upper limit was chosen for estimating maximum in situ conditions for boreholes drilled along Middle Valley (see "Appendix").