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Petrographic and mineralogic studies of the sedimentary cover drilled at Middle Valley on the Juan de Fuca Ridge allow characterization of the newly formed silicate and carbonate phases and a determination of their origin. At all drill sites, the sediments are composed of a mixture of detrital and authigenic minerals in various proportions. The unmodified sediments forming the surficial cover consist of detrital minerals, including quartz, feldspar, amphibole, mica, clays (smectite, chlorite), and biogenic remains. Quartz, feldspar, and mica are still present in hydrothermally modified sediments where they coexist with newly formed minerals, except in Hole 858B, which is located close to an active hydrothermal vent where pure hydrothermal layers consist solely of authigenic minerals. The main authigenic phases in the modified sediments are silicate minerals and calcite concretions with minor sulfides. The most common authigenic silicate in the deepest layers downhole is Mg-chlorite. At Site 858, a mineralogic sequence is clearly established from the seawater/sediment interface to the deeper parts of the hole. The shallowest hydrothermal unit from Hole 858B is composed of Mg-smectite and authigenic pyrite, which downhole sequentially overlies corrensite-rich layers which contain some talc and anhydrite. In the deepest unit, chlorite is the dominant phyllosilicate coexisting with authigenic quartz. All of these silicates seem to be the product of a direct precipitation from hydrothermal fluids. This sequence reflects a thermal gradient related to the vertical influx of hydrothermal fluids. Secondary precipitation of carbonate concretions was generated from Ca released by biogenic phases and hydrothermal fluids. The concretions show several stages of crystallization and seem to occur preferentially in oxidizing areas and in the shallowest deposits. The occurrence of the concretions is favored by high porosity in the sediments. The presence of pure hydrothermal layers locally interbedded with unmodified sediments, and the variable mineralogy of the carbonate nodules suggest that convective circulation of fluids acts at various scales in this hydrothermal system and that the sedimentary cover favors lateral fluid circulation.

Mineralogic and Petrographic Records of Sediment-Fluid Interaction in the Sedimentary Sequence at Middle Valley, Juan de Fuca Ridge, Leg 139