A record core drilling 1.2 kilometers deep in the Earth’s mantle

The team of Andrew McCaig, from the University of Leeds, and Susan Lang, from the Woods Hole Oceanographic Institute, in the United States, achieved an exceptional feat. These geologists succeeded in taking an almost continuous core of 1,268 meters in the upper mantle of the Earth.

The upper mantle plays a major role in the dynamics of the planet: magmatism, formation of the crust, constitution of the oceans and the atmosphere, cycle of numerous elements… Located between 6 and 70 kilometers below the surface, the mantle is difficult to ‘access. The knowledge that specialists have is often indirect or comes from samples collected from the surface of the ocean floor by dredging or scuba diving. However, these samples find themselves devoid of an overall geochemical context and are altered by reactions with seawater which produce a metamorphic rock, serpentinite, itself strongly oxidized when it approaches the ocean floor. This alteration confuses all the information that could be extracted from the analysis of these rocks.

In the mid-oceanic ridges, mantle convection leads to a rise of the latter as close as possible below the ocean floor. Samples had been taken from such sites, but never as large as that of the team surrounding Johan Lissenberg, first author of the study. These researchers operated on the Atlantis massif, in the middle of the Atlantic Ocean, thanks to the drilling ship specialized in scientific research, the Joides Resolution. This underwater mountain is not composed of black basalt, which typically constitutes the ocean floor, but of green serpentinized peridotites characteristic of the mantle and its hydration by sea water. This observation suggests that the massif was formed under the mid-Atlantic ridge before extracting itself from it due to the tectonic movements of the ridge. In 2016, a previous sampling campaign was carried out there. And earlier, in 2000, a mission there discovered a vast hydrothermal field called “the Lost City”.

Johan Lissenberg and his colleagues carried out their drilling 800 meters north of the Lost City. The sample shows that 80-90% of the peridotite has undergone serpentinization. The alterations are compatible with the composition of the fluids emitted by the chimneys of the Lost City, indicating that these rocks were subjected to strong hydrothermal activity.

“On the other hand, only the first 200 meters of the sample were oxidized,” notes Rémi Coltat, from the Andalusian Institute of Earth Sciences and member of the team. The non-oxidized portion made it possible to highlight another phenomenon, that of the partial melting of the rock when it rises from the depths towards the surface. Indeed, under the ridge at the level of the Atlantis massif, the mantle rock migrates at a speed of 1 centimeter per year. In doing so, the pressure decreases and the rock undergoes adiabatic decompression which causes it to partially melt. The magma produced escapes and feeds the magma chambers. But to explain this dynamic, specialists only had theoretical models. Serpentinized samples recovered from the ocean floor were too altered to show how the fluid migrated. With the new sample, Johan Lissenberg and his colleagues have clearly identified fluid transport channels by percolation between the solid grains. And contrary to what was expected, these channels are oriented at an angle to the direction of movement of the rock.

The researchers confirm that, despite the significant serpentinization of peridotites along the entire length of the sample, the latter has a less altered composition than those taken from the seabed. A result which encourages further drilling even deeper to create an ever more precise image of the crucial processes taking place beneath our feet!