Diagenesis of Amorphous Silica in Middle Valley, Juan de Fuca Ridge

Abstract

Measured profiles of dissolved and amorphous silica from the four sites (855-858) of Ocean Drilling Program Leg 139, Middle Valley, Juan de Fuca Ridge, are compared to profiles calculated from a steady-state advection-diffusion-reaction model to illustrate the effects of hydrothermal processes on diagenesis of amorphous silica. In particular, the effects of pore-water flow and the composition of basement pore water on dissolution of amorphous silica are evaluated. Based on the calculated profiles, measured profiles of dissolved and amorphous silica are interpreted in terms of past and present pore-water flow and the diagenetic transformation of amorphous silica to quartz. Four sites were drilled, each with different patterns of fluid flow, heat flow, and composition of pore water. Site 855 is located in an area with low heat flow near a normal fault that is a conduit for bottom seawater to enter basement. Dissolved silica at this site is governed by diffusion and dissolution of amorphous silica. At the other three sites (856-858), hydrothermal processes, both past and present, promote the reaction of amorphous silica to quartz. This diagenetic transition is characterized by decreases in the concentrations of dissolved and amorphous silica with depth. Concentrations of amorphous silica decrease with depth from values greater than 1 weight percent (wt%) SiO2 typical of sediments at Site 855 to about 0.2 wt% SiO2, whereas concentrations of dissolved silica at depth are generally less than those calculated for equilibrium with chalcedony or quartz. Interpretation of dissolved silica data is limited by the fact that above 50°C, extraction of pore water by squeezing sediment results in lower concentrations of dissolved silica than those obtained using in-situ sampling techniques. Present and past patterns of pore-water circulation in Middle Valley are complex, but the amorphous silica data provide some insights. Site 856 is located on an uplifted hill where sill intrusions and/or hydrothermal flow related to the Site 856 sulfide body probably caused pore-water flow that depleted amorphous silica from all but the uppermost part of the sediment column. The objective at Site 857 was to drill into a hydrothermal reservoir. Based on the dissolved and amorphous silica data, we find no evidence of recent vertical pore-water flow. Lastly, Site 858 is located in an area of active hydrothermal venting. Measured profiles of dissolved and amorphous silica concentrations indicate that lateral, vertical, and past vertical pore-water flow influence the diagenesis of amorphous silica at this site.