MECHANISMS OF MG-PHYLLOSILICATE FORMATION IN A HYDROTHERMAL SYSTEM ATA SEDIMENTED RIDGE (MIDDLE-VALLEY, JUAN-DE-FUCA)

We present results of a detailed mineralogical and geochemical study o f the progressive hydrothermal alteration of elastic sediments recover ed at ODP Site 858 in an area of active hydrothermal venting at the se dimented, axial rift valley of Middle Valley (northern Juan de Fuca Ri dge). These results allow a characterization of newly formed phyllosil icates and provide constraints on the mechanisms of clay formation and controls of mineral reactions on the chemical and isotopic compositio n of hydrothermal fluids. Hydrothermal alteration at Site 858 is chara cterized by a progressive change in phyllosilicate assemblages with de pth. In the immediate vent area, at Hole 858B, detrital layers are int ercalated with pure hydrothermal precipitates at the top of the sectio n, with a predominance of hydrothermal phases at depth. Sequentially d ownhole in Hole 858B, the clay fraction of the pure hydrothermal layer s changes from smectite to corrensite to swelling chlorite and finally to chlorite. In three pure hydrothermal layers in the deepest part of Hole 858B, the clay minerals coexist with neoformed quartz. Neoformed and detrital components are clearly distinguished on the basis of mor phology, as seen by SEM and TEM, and by their chemical and stable isot ope compositions. Corrensite is characterized by a 24 Angstrom stackin g sequence and high Si- and Mg-contents, with Fe/(Fe+Mg) ratio of appr oximate to 0.08. We propose that corrensite is a unique, possibly meta stable, mineralogical phase and was precipitated directly from seawate r-dominated hydrothermal fluids. Hydrothermal chlorite in Hole 858B ha s a stacking sequence of 14 Angstrom with Fe/(Fe+Mg) ratios of approxi mate to 0.35. The chemistry and structure of swelling chlorite suggest that it is a corrensite/chlorite mixed-layer phase. The mineralogical zonation in Hole 858B is accompanied by a systematic decrease in delt a(18)O, reflecting both the high thermal gradients that prevail at Sit e 858 and extensive sediment-fluid interaction. Precipitation of the M g-phyllosilicates in the vent region directly controls the chemical an d isotopic compositions of the pore fluids. This is particularly evide nt by decreases in ME and enrichments in deuterium and salinity in the pore fluids at depths at which corrensite and chlorite are formed. St ructural formulae calculated from TEM-EDX analyses were used to constr uct clay-H2O oxygen isotope fractionation curves based on oxygen bond models. Our results suggest isotopic disequilibrium conditions for cor rensite-quartz and swelling chlorite-quartz precipitation, but yield a n equilibrium temperature of 300 degrees C+/-30 degrees for chlorite-q uartz at 32 m below the surface. This estimate is consistent with inde pendent estimates and indicates steep thermal gradients of 10-11 degre es/m in the vent region.