Fluid-sediment interaction and clay authigenesis along the flank of the Juan de Fuca Ridge

This study focuses on the mineralogy and chemistry of sediments that overly basaltic basement through which seawater circulates (Leg168, flank of the Juan de Fuca Ridge). Silicate authigenesis was observed in the sediment layer just above basement at sites located more than 30 km from the ridge axis. This sediment alteration is particularly abundant at ODP Sites 1031 where the authigenic formation of Fe-Mg rich smectite (montmorillonite and saponite) and zeolite, and the dissolution of biogenic calcite, are observed. Vertical advection of basement fluid through the sediment section is required to produce this alteration. These processes are still active at Site 1031, based on systematic variations in pore-water profiles. A SEM and TEM-EDX study of the authigenic clays and zeolites enable the chemistry and mineralogy of the hydrothermal phases to be characterized and their mechanism of formation to be determined. This mineralogical study has been complemented by REE analyses on clay fractions in order to further constrain the origin of the authigenic clays and their environment of formation.