On the temporal evolution of high‐temperature hydrothermal systems at ocean ridge crests

Abstract

The results of heat balance calculations for a single‐pass hydrothermal system overlying an axial magma body, based on a given rate of heat output of 102–103 MW at a time t0, predict that vent temperatures should decay rapidly for t > t0, as the magma freezes and the boundary layer between the hydrothermal system and liquid magma thickens. The model may describe a declining phase of high‐temperature, high‐heat‐output hydrothermal activity. The model shows that for systems with heat output ∼100 MW or greater, the boundary layer between the magma and hydrothermal system must remain thin, if vent temperatures remain relatively constant on a decadal timescale. A thin boundary layer can be maintained as a result of downward migration of the hydrothermal system into freshly frozen magma or by some mechanism that maintains high heat flux from liquid magma to the base of the boundary layer. Some combination of these factors is likely to operate. Downward migration of the hydrothermal system into freshly frozen magma may occur in conjunction with fracturing resulting from dike injection and the propagation of these cracks laterally away from the dike as a result of thermal stresses. High heat flux from liquid magma to the base of the hydrothermal system cannot be maintained simply by convection within the magma chamber. High heat flux might be maintained as a result of magma chamber replenishment or by latent heat transfer during the formation of a cumulate mush at the base of the magma chamber, however. A hydrothermal system, in which the permeability decreases with time, can maintain relatively constant vent temperatures even though the thermal output declines. Better time series data on thermal output of the vents, and not just on the vent temperature, could help distinguish whether the permeability is decreasing or whether heat flux as well as vent temperatures are relatively constant.

The response of two-phase hydrothermal systems to changing magmatic heat input at mid-ocean ridges

“…Initially, single-phase fluid with the properties of pure water was considered as the convecting fluid for hydrothermal systems at mid-ocean ridges [e.g., Brikowski and Norton, 1989;Lowell and Burnell, 1991;Lowell and Germanovich, 1994;Wilcock, 1998]. Later boiling was considered, but the fluid was still single pure water [Lowell and Xu, 2000;Coumou et al, 2006].…”

Anhydrite precipitation and the relationship between focused and diffuse flow in seafloor hydrothermal systems

“…Thus equations (17) and (18) indicate that crustal sealing as a result of precipitation of anhydrite upon mixing between hot sulfate-free hydrothermal fluids and cooler, sulfate-rich seawater can occur on a timescale of years provided the permeability of the pillows (where mixing occurs) and the sheeted dikes are roughly equal and range between 10 À11 and 10 À12 m 2 . These values of ridge axis crustal permeability are of the same order as estimated from measurements in ophiolites [Nehlig and Juteau, 1988;Nehlig, 1994;van Everdingen, 1995] and from hydrothermal modeling of black smoker flow [e.g., Lowell and Germanovich, 1994].…”

“…150°C. It is generally thought that flow resistance in the recharge zone is much less than that of the discharge zone [e.g., Lowell and Germanovich, 1994;Pascoe and Cann, 1995]. If we ignore the resistance in the recharge zones and write the term on the right hand side of equation (8) as f(c, x), we find the relation between k m and the permeability in the deep discharge zone k d…”

“…Although this is a necessary condition for anhydrite precipitation to occur at all, it is not an overly stringent condition. Measurements of permeability in boreholes suggest that permeability in pillows is considerably higher than in sheeted dikes [Fisher, 1998]; however, estimates of the permeability in sheeted dikes from ophiolites [e.g., Nehlig and Juteau, 1988;Nehlig, 1994;van Everdingen, 1995] and from hydrothermal models of black smoker flow [e.g., Lowell and Germanovich, 1994] suggest that permeability of the sheeted dikes may be roughly the same as that measured in pillows.…”

On thermoelasticity and silica precipitation in hydrothermal systems: Numerical modeling of laboratory experiments

“…Thus temporal variability of seafloor hydrothermal discharge occurs on relatively short time scales and careful monitoring of hydrothermal evolution following magmatic events could provide useful constraints on numerical models that address temporal variability. Lowell and Germanovich [1994] provided simple analytical results on the decay of hydrothermal output following magmatic heat input and suggested that thermoelastic stresses could strongly affect the temporal evolution and stability of a black smoker vents on a time scale of years to decades. Germanovich et al [1996] showed that thermoelastic processes in upflow zones following magmafie events could lead to oscillatory hydrothermal output on a timescale of years.…”