ON THE TEMPORAL EVOLUTION OF HIGH-TEMPERATURE HYDROTHERMAL SYSTEMS ATOCEAN RIDGE CRESTS

The results of heat balance calculations for a single-pass hydrotherma l system overlying an axial magma body, based on a given rate of heat output of 10(2)-10(3) MW at a time t(o), predict that vent temperature s should decay rapidly for t > t(o), as the magma freezes and the boun dary layer between the hydrothermal system and liquid magma thickens. The model may describe a declining phase of high-temperature, high-hea t-output hydrothermal activity. The model shows that for systems with heat output approximately 100 MW or greater, the boundary layer betwee n the magma and hydrothermal system must remain thin, if vent temperat ures remain relatively constant on a decadal timescale. A thin boundar y layer can be maintained as a result of downward migration of the hyd rothermal system into freshly frozen magma or by some mechanism that m aintains high heat flux from liquid magma to the base of the boundary layer. Some combination of these factors is likely to operate. Downwar d migration of the hydrothermal system into freshly frozen magma may o ccur in conjunction with fracturing resulting from dike injection and the propagation of these cracks laterally away from the dike as a resu lt of thermal stresses. High heat flux from liquid magma to the base o f the hydrothermal system cannot be maintained simply by convection wi thin the magma chamber. High heat flux might be maintained as a result of magma chamber replenishment or by latent beat 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 t hermal output declines. Better time series data on thermal output of t he vents, and not just on the vent temperature, could help distinguish whether the permeability is decreasing or whether heat flux as well a s vent temperatures are relatively constant.