Hydrothermal alteration within the basement of the sedimented ridge environment of Middle Valley, northern Juan de Fuca Ridge

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 mem bers of an active hydrothermal circulation system. Site 858 was placed in a n area of active hydrothermal discharge with fluids up to 270 degrees C ven ting through anhydrite-bearing mounds on top of altered sediment. The shall ow 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 no w and numerous seismically imaged ridge-parallel faults. Drilling at Site 8 57 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 ch lorite and crosscut with veins filled with quartz, chlorite, sulfides, epid ote, and wairakite. The sill interiors vary from slightly altered, with una ltered 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 as sociated with brecciated horizons with low recovery and slickensides on fra cture surfaces, which we interpret as intersections between faults and the sills. Geochemically, the alteration of the sill complex is reflected in si gnificant whole-rock depletions in Ca, Sr, and Na with corresponding enrich ments in Mg, Al, and mast metals. The latter results from the formation of conspicuous sulfide poikiloblasts. In contrast, metamorphism of the Site 85 8 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 O-18 (delta(18)O = 2.4 parts per thousand-4.7 parts per thousand) and more pervasively replace d by secondary minerals relative to the extrusives at Site 858 (delta(18)O = 4.5 parts per thousand-5.5 parts per thousand). There is no evidence of s ignificant albitization of the plagioclase at Site 857, suggesting high Ca/ Na in the pore fluids. Fluid-inclusion data from hydrothermal minerals in a ltered mafic rocks and veins at Sites 857 and 858 show a consistency of hom ogenization temperatures, varying from 245 to 270 degrees C, which is withi n the range of temperatures observed for the fluids venting at Site 858. Th e consistency of the fluid inclusion temperatures, the lack of albitization within the Site 857 sills, and the apparently low water/rock ratio collect ively suggest that the sill complex at Site 857 is in thermal equilibrium a nd being altered by a highly evolved Ca-rich fluid similar to the fluids no w 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 loggin g 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 basem ent edifice acts as a penetrator or ventilator far the regional hydrotherma l reservoir with much of the now focussed at the highly altered and veined sediment-basalt contact.