Abstract
<p>Mineralogic, petrographic, chemical and isotopic (O,C,S) data on altered basalts from DSDP Hole 504B are presented with the objectives of: (1) determining the distribution of alteration products and identifying the sequence and conditions of alteration in order to understand the structure and evolution of the hydrothermal system; and (2) determining whole-rock chemical changes for specific stages or processes and to comment on their significance for seawater-crustal chemical fluxes. Hole 504B penetrated 1076m into 5.9m.y. old basement south of the Costa Rica Rift, through 274.5m sediment; a 571.5m pillow section; a 209m transition zone; and 295m into a sheeted dike complex.Alteration of basalts in the transition zone and dikes occurred in multiple stages, predominantly in the upwelling zone of an axial convection cell close to the spreading axis (within (TURN)1km). Conditions evolved from: (1) unaltered to partly reacted seawater (200(--->)300(DEGREES)C); to (2) reacted hydrothermal fluids (200-400(DEGREES)C), probably similar to those sampled from spreading ridges on the seafloor; to (3) more highly evolved and lower temperature fluids (100-250(DEGREES)C) during the cooling and waning of axial convection. Mixing of hydrothermal fluids upwelling through the dikes with seawater circulating in the overlying more permeable pillow section occurred in the upper part of the transition zone. This resulted in the formation of a sulfide-rich stockwork-like zone in the upper transition zone during Stage 2, and deposition of secondary minerals in the lower pillow section (at around 100(DEGREES)C). Axial hydrothermal alteration probably ceased at Site 504 within several thousand years, whereas lower temperature off-axis circulation may be still occurring in the upper pillow section and in adjacent crust today. Relatively unaltered seawater penetrated to at least 1 km subbasement as the crust moved away from the spreading axis. Circulation in the upper pillows was initially open and oxidizing (< 50(DEGREES)C?), but conditions throughout the pillow section evolved to more reducing and rock-dominated. This probably occurred as cracks in the basalt were sealed with secondary minerals and the basement was covered with a layer of sediment. Sealing of the crust to convective cooling also allowed conductive reheating of the crust from below.Calculations of seawater-crustal chemical fluxes from whole-rock data indicate that basalt-seawater interactions are a sink for Mg and K and a source of Si and Ca. They also indicate that the low temperature alteration of the upper crust may be a sink for Mg.</p>