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Geology

Heat flux from active crust

Arnoux, G. M., Toomey, D. R., Hooft, E. E., Wilcock, W. S., Morgan, J., Warner, M., & VanderBeek, B. P. (2017). Seismic evidence that black smoker heat flux is influenced by localized magma replenishment and associated increases in crustal permeability. Geophysical Research Letters, 44(4), 1687-1695. DOI: 10.1002/2016GL071990

Did you know that heat from inside Earth is exiting through the surface?  Researches describe the amount of heat flowing out of Earth as heat flux (the amount of heat exiting an area in a given time), but they still have much to learn about what controls how much heat is flowing.

Figure 1: Black smoker on the seafloor (Accessed August 3, 2017 from http://www.photolib.noaa.gov/htmls/nur04506.htm)

There is agreement that roughly a quarter of Earth’s interior heat is transported to the surface by the process of hydrothermal (high-temperature-water) circulation. At vent fields, heat exits Earth as super-heated water through geothermal structures called black smokers (Figure 1).  Based on field observations coupled with computer models, scientists suspect that the mechanisms which control how much heat flows through black smokers are the supply of magma and crustal permeability (how easily water flows through the crust). However, a limitation to scientists’ knowledge about these mechanisms is the uncertainty of the structure of the magma chambers below vent fields.  Arnoux et al. addressed this set-back by employing seismic data to generate a high-resolution image of the upper crust, and analyzing it with respect to the number of earthquakes and heat flux.

What was done:

Arnoux et al. focused on the Endeavour segment of the Juan de Fuca ridge, 300 miles off the coast of Washington and Oregon.  The heat lose from a magma chamber between 1 and 2 miles below the seafloor fuels a row of five black smoker vent fields. From north to south the fields are Sasquatch, Salty Dawg, High Rise, Main Endeavour, and Mothra. Researchers generated a high-resolution image of the magma chamber by using 3-D full-waveform inversion (FWI).  FWI is designed to use all sound recorded in a seismic array to generate a 3-D model of a space at a higher resolution than other imaging techniques.  Small changes in the velocity of sound are caused by changes in the material it encounters; for instance, sound travels more slowly through a liquid than through a solid.  The technique was advantageous to their study because it output figures that capture fine-scale structure that can be used to infer spatial variation in crustal permeability.

What was found:

FWI enabled scientists to decipher structures .20-24 cubic miles in size within in the upper 2 miles of the crust. Although resolution decreased with depth, FWI proved to be four times better than other methods. Along the row of vent fields, FWI revealed a heterogeneous structure.  A large low velocity anomaly found 1.5 miles below the area between the Main Endeavour and High Rise fields was interpreted as the magma chamber.  The other vent fields, Sasquatch, Salty Dawg to the north and Mothra to the south, all appeared to overlay high velocity anomalies (reference figure 4 in the featured article).

Cross-sections of the vent field row revealed low velocity anomalies under abyssal hills 1 mile from the ridge (reference figure 4 in the featured article).

Researchers found that low velocity anomalies below Main Endeavour and High Rise fields where correlated with abundant earthquakes.  They also observed that these areas have higher venting temperatures and heat flux compared than the other three sites. Below Main Endeavour and High Rise, black smokers are supported by heat transferred from the magma chamber to water circulating in the crust through a 10-100 m thermal boundary layer (TBL).  High velocities under Sasquatch, Salty Dawg, and Mothra vent fields are attributed to a decrease in permeability attributed to a cease in magma replenishment, which would result in TBL thickening,  magma crystalizing, and the vents becoming clogged.

Low velocity anomalies below the abyssal hills and on either end of the vent field row are attributed to permeability. Permeability is thought to increase by faulting and cracks caused by dike propagation away from the magmatic chamber outward during replenishment.

The work by Arnoux et al. is direct insight into the crustal structure below the Juan de Fuca Ridge.  The images they produced reveal a complex, variable structure of low velocity and high velocity anomolies below the vent fields. Their findings support the hypothesis that heat flux from black smokers is a function of magma supply and permeability and suggest there is a cycle of evolution that exists at hydrothermal vent fields.

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