The Middle Valley segment at the northern end of the Juan de Fuca Ridge is a deep extensional rift blanketed with 200–500 m of Pleistocene turbiditic sediment. Sites 857 and 858 were drilled during Ocean Drilling Program Leg 139 to determine whether these two sites were hydrologically linked end members of an active hydrothermal circulation system. Site 858 was placed in an area of active hydrothermal discharge with fluids up to 270 °C venting through anhydrite-bearing mounds on top of altered sediment. The shallow basement of fine-grained basalt that underlies the vents at Site 858 is interpreted as a seamount that was subsequently buried by turbidites. Site 857 was placed 1.6 km south of the Site 858 vents in a zone of high heat flow and numerous seismically imaged ridge-parallel faults. Drilling at Site 857 encountered sediments that are increasingly altered with depth and that overlie a series of mafic sills at depths of 460–940 m below sea floor.

Sill margins and adjacent baked sediment are highly altered to magnesian chlorite and crosscut with veins filled with quartz, chlorite, sulfides, epidote, and wairakite. The sill interiors vary from slightly altered, with unaltered plagioclase and clinopyroxene in a mesostasis replaced by chlorite, to local zones of intense alteration and brecciation. In these latter zones, the sill interiors are pervasively replaced by chlorite, epidote, quartz, pyrite, titanite, and rare actinolite. The most complete replacement is associated with brecciated horizons with low recovery and slickensides on fracture surfaces, which we interpret as intersections between faults and the sills. Geochemically, the alteration of the sill complex is reflected in significant whole-rock depletions in Ca, Sr, and Na with corresponding enrichments in Mg, Al, and most metals. The latter results from the formation of conspicuous sulfide poikiloblasts. In contrast, metamorphism of the Site 858 seamount includes incomplete albitization of plagioclase phenocrysts and replacement of sparse mafic phenocrysts. Much of the basement alteration at Site 858 is confined to crosscutting veins except for a highly altered and veined horizon at the contact between basaltic basement and the overlying sediment.

The sill complex at Site 857 is more highly depleted in 18O (δ18O = 2.4‰ − 4.7‰) and more pervasively replaced by secondary minerals relative to the extrusives at Site 858 (δ18O = 4.5‰ – 5.5‰). There is no evidence of significant albitization of the plagioclase at Site 857, suggesting high Ca/Na in the pore fluids. Fluid-inclusion data from hydrothermal minerals in altered mafic rocks and veins at Sites 857 and 858 show a consistency of homogenization temperatures, varying from 245 to 270 °C, which is within the range of temperatures observed for the fluids venting at Site 858. The consistency of the fluid inclusion temperatures, the lack of albitization within the Site 857 sills, and the apparently low water/rock ratio collectively suggest that the sill complex at Site 857 is in thermal equilibrium and being altered by a highly evolved Ca-rich fluid similar to the fluids now venting at Site 858.

The alteration evident in these two deep crustal drillsites is a result of the ongoing hydrothermal circulation and is consistent with downhole logging results, instrumented borehole results, and hydrothermal fluid chemistry. The pervasive alteration of the laterally extensive sill-sediment complex at Site 857 determines the chemistry of the fluids that are venting at Site 858. The limited alteration of the Site 858 lavas suggests that this basement edifice acts as a penetrator or ventilator for the regional hydrothermal reservoir with much of the flow focussed at the highly altered and veined sediment-basalt contact.

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