A team from the Institut de Physique du Globe de Paris (IPGP) reveals the role of CO
2 degassing in deep earthquakes beneath the Mid-Atlantic Ridge. Their work, published in
Nature Communications on January 10, 2025, opens new perspectives on the dynamics of the Earth's mantle.
a) Bathymetric map of the equatorial Mid-Atlantic Ridge (MAR) showing the discovered deep micro-earthquakes and the location of rock samples near the Romanche-MAR ridge-transform intersection. Solid and dashed red lines indicate the axes of the MAR and non-transform discontinuities (NTD), with the names of the defined segments on the side.
b) CO2 content as a function of barium (Ba) composition. Dashed blue lines and numbers indicate the estimated CO2 content in the primary magma at the RC2 ridge segment.
c) Illustration of three segments south of the Romanche transform fault. Brown and gray areas represent the brittle and ductile lithospheres, respectively. The thick black line represents the brittle-ductile boundary (BDB), constrained by the maximum depth of earthquakes, corresponding to the 750 °C isotherm. Deep earthquakes (10-19 km below the seafloor) beneath the MAR axis are interpreted as resulting from CO2 degassing from ascending magmas in the hot ductile mantle. Colored dashed lines indicate isotherms extracted from a simulated thermal model. Volatile substances such as carbon dioxide (CO
2) and water (H
2O) play a key role in the melting of the Earth's mantle beneath oceanic ridges, where tectonic plates diverge and create new oceanic crust. However, their influence on moving magma has remained a mystery until now. A recent study conducted by Satish Singh's team at the Institut de Physique du Globe de Paris (IPGP) sheds new light on this phenomenon.
Using ocean-bottom seismometers (OBS) installed during the SMARTIES campaign in 2019, researchers recorded surprising micro-earthquakes deep beneath the axis of the equatorial Mid-Atlantic Ridge (MAR), a region of slow spreading. These tremors, detected between 6 and 12 miles (10-20 km) below the ocean floor, occur in the hot mantle, well below the boundary between the rigid lithosphere and the ductile mantle, known as the BDB (brittle-ductile boundary).
Analysis of nearby basaltic rocks revealed exceptionally high concentrations of CO
2 in the primary magma (approximately 0.4 to 3.0% by weight). Researchers suggest that the degassing of this CO
2, by causing rapid volume changes in the magma, could trigger these deep earthquakes. In other words, the magma could stagnate at these depths, where it continues to evolve before rising to form the oceanic crust.
Ocean-bottom seismometers used during the SMARTIES campaign
These findings are crucial: they show that volatile substances can influence not only the formation of oceanic crust but also deep seismic mechanisms. A high CO
2 content could even allow the presence of magma at lower temperatures than expected beneath the boundary between the lithosphere and the asthenosphere, increasing heterogeneities within the lithosphere.
This study opens new perspectives on the internal dynamics of the Earth and the little-known role of degassing in seismic processes. It was conducted with the support of the European Research Council.