Why hydrothermal fluids cool during flow through permeable fault zones

Abstract:

While preferential flow of hot fluids along highly permeable, fractured rocks seems intuitive, such efficient flow leads to the entrainment of cold ambient seawater resulting in a distinct decrease in fluid temperatures. This temperature drop is difficult to reconcile with high-temperature black smoker activity observed at outcropping fault zones on the seafloor. In our recent study (Andersen et al., 2015) we aim to resolve this apparent contradiction by combining newly acquired seismological data (Grevemeyer et al., 2013) from the high-T, off-axis Logatchev 1 hydrothermal field (LHF1) along the Mid-Atlantic Ridge (MAR) with 2D hydrothermal flow modeling. The seismic data shows intense off-axis seismicity with focal mechanisms suggesting a fault zone dipping from LHF1 toward the ridge axis.

In order to explain fault-related high-T hydrothermal discharge as observed at LHF1, our simulations predict that fault zones need to be just permeable and wide enough to capture and redirect hot hydrothermal fluids ascending from above a driving heat source. Very wide or highly permeable fault zones are not sufficiently fed by the limited supply of hot fluids such that cold seawater is entrained to “fill up” the fault zone, causing a decrease in hydrothermal vent temperatures. Our findings illustrate an intrinsic relationship between fault characteristics (width and permeability) and associated hydrothermal fluid flow: the higher the fluid mass flux along a fault zone, the lower the hydrothermal fluid temperature.

The cooling effect observed in hydrothermal fluids at permeability contrast, might explain the sparsity of high-T hydrothermal activity along the heterogeneous, highly faulted Mid-Atlantic Ridge.