Hydrothermal circulation, anhydrite precipitation, and thermal structure at ridge axes

Abstract

The thermal structure of the axial region of ridges spreading at 60 mm yr−1 was studied by comparing observations with numerical thermal models. Data from Deep Sea Drilling Project hole 504B constrain the temperature in the pillows and the upper part of the dike complex. Seismic reflection data constrain the depth to the magma chamber. Magnetic anomalies constrain the accumulation rate of pillows. The thermal models assumed steady state and included the kinematics hydrothermal flow and the accumulation of pillows basalts. In the successful thermal models, hydrothermal fluid entered the dike intrusion zone at the ridge axis over a depth range from the base of the pillows to about 0.8 km above the top of the magma chamber. The computed temperature at the axis in this depth range was nearly isothermal and suitable for the source regions of black smoker vents. The computed temperature grid and assumed flow grid were used to model anhydrite precipitation and sulfate transport and thus more fully examine the implications of the flow pattern. Anhydrite was precipitated as seawater approached the hot axial zone. The anhydrite was the carried outward with the spreading seafloor and redissolved when it reacted with pristine downwelling seawater. The net effect was to deposit anhydrite near the base of vigorous hydrothermal circulation where it could not easily be leached by lower temperature off‐axis circulation and returned to the ocean. This is a general feature of flow patterns where cold seawater vigorously enters the axial zone in broad downwellings that are not strongly heated by conduction at shallow depths. Alternatively, the cold recharge is concentrated in the axial region and exchanges heat with nearby upwelling flow in the transition zone beneath the pillows. Anhydrite is then precipitated at shallow depths where it can later be leached back into the ocean. The latter possibility is favored by studies of geochemical mass balances as well as the lack of evidence for a deep anhydrite layer in the oceanic crust.