The heat of a volcano at a plate boundary

The Lucky Strike hydrothermal field covers 1 km2 at the top of one of the volcanoes of the Mid-Atlantic Ridge, on the boundary between the Africa and North America tectonic plates. Hydrothermal fluids escape at temperatures over 300 °C, loaded with metals that they have leached from the basement rocks at depth. How do these hot fluids form? We estimate that the energy they extract from the basement and release into the ocean, corresponds to the cooling of more than 200m3 of magma per hour. Is the hydrothermal system cooling the volcano's magma chamber? And in this case should we expect to see a decrease in temperature and fluid flow with time? Or is this chamber regularly re-supplied with new magma? And what is the relationship at depth between the magma, and the hydrothermal system?

To answer these questions, we have been operating an instrumental network made up of 5 seismometers, pressure probes (to measure any swelling of the volcano under the influx of new magma), and temperature probes in more than 15 smokers. We have already shown that the seismicity is probably maximum where the fluids extract the most heat at a given moment: the rocks probably fracture because they cool so fast. These zones of maximum heat extraction are located about 3 km below the summit of the volcano, just above a magma lens that we have detected using acoustic waves to probe the substratum (active seismic methods). We also show that these zones of maximum heat extraction have moved by a few hundred meters over time, perhaps following the most recent injections of magma, or else because faults have opened up new permeable paths to the water-derived descending fluids. On the other hand, we also show that the average temperature of smokers has not changed over the 10 years of observation; at this time scale, the activity of the hydrothermal system therefore remained stable. To better interpret these observations, we formulate hypotheses (on the permeability structure of the volcano, on the location and volume of magmatic injections, etc.) that we test with digital models.

(right) a map of seafloor depths at the top of the Lucky Strike volcano. The red colors are above 1600m, the blue colors below 1750 m. The main hydrothermal vents are represented by small black stars.

(left) a numerical model of the hydrothermal circulation in a permeable domain located along the ridge and above a hot domain rich in magma (the axial magma lens that has been identified by seismics below the top of the Lucky Strike volcano). The seawater-derived fluids descend and extract the heat at depth; as they heat up, they become less dense and therefore rise back to the seafloor and vent at a hydrothermal field.

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Reference

Barreyre, T., Olive, J.A., Crone, T.J., Sohn, R.A., 2018. Depth-Dependent Permeability and Heat Output at Basalt-Hosted Hydrothermal Systems Across Mid-Ocean Ridge Spreading Rates. Geochemistry, Geophysics, Geosystems 2, 1259–1281. https://doi.org/10.1002/2017GC007152

Barreyre, T., Sohn, R.A., 2016. Poroelastic response of mid-ocean ridge hydrothermal systems to ocean tidal loading: Implications for shallow permeability structure. Geophysical Research Letters 43, 1660–1668. https://doi.org/10.1007/978-3-319-42975-5_25

Chavagnac, V., Leleu, T., Fontaine, F., Cannat, M., Ceuleneer, G., and Castillo, A., 2018, Spatial Variations in Vent Chemistry at the Lucky Strike Hydrothermal Field, Mid-Atlantic Ridge (37°N): Updates for Subseafloor Flow Geometry From the Newly Discovered Capelinhos Vent: Geochemistry, Geophysics, Geosystems, v. 55, no. 2, p. 229–15, doi: 10.1029/2018GC007765

Escartin, J., Barreyre, T., Cannat, M., Garcia, R., Gracias, N., Deschamps, A., Salocchi, A., Sarradin, P.M., Ballu, V., 2015. Hydrothermal activity along the slow-spreading Lucky Strike ridge segment (Mid-Atlantic Ridge): Distribution, heatflux, and geological controls. Earth and Planetary Science Letters. https://doi.org/10.1016/j.epsl.2015.09.025

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