Research into the Afar mantle plume provides new insights into deep Earth processes

Advanced analysis of tiny bubbles of ancient gas trapped in volcanic rocks, combined with new geophysical modelling, has shed new light on a long-held hypothesis about the deep Earth.

An international team of scientists led by researchers from SUERC and the University of Glasgow’s School of Geography and Earth Sciences has discovered surprising results in a new study of volcanic lava that erupted into the Red Sea from the Afar mantle plume.

Mantle plumes are columns of extremely hot rock that rise to the Earth’s surface from the boundary between the core and the mantle, 2,900 km underground. When they reach the surface, they cause local volcanic activity and often have enough energy to split continents apart.

Current scientific consensus is that plumes carry “primitive” material from deep within the mantle to the surface, which was created when the Earth formed. If this is true, the volcanic rocks that form when that magma erupts should contain significant traces of these primordial materials.

But researchers found that volcanic rocks dredged from the bottom of the Red Sea contained much lower concentrations of the primordial gas helium than typical models of Earth would predict.

In a new paper published in the journal Communications Earth and Environment, the team concludes that the Afar volcano’s plume is in fact made up mostly of material that was previously on the Earth’s surface.

Their findings are based on mass analysis of basalt glass samples taken from the Red Sea and the Gulf of Tadjoura, and suggest that the mantle plume is a complex mixture of pristine deep mantle and ocean floor rocks that was recycled back into the Earth’s interior by a process known as “subduction.”

Basalt glass forms when lava erupts into seawater, cools rapidly and traps initially dissolved gases as bubbles.The team from the Scottish Universities Environmental Research Centre (SUERC) used highly sensitive mass spectrometry to measure two helium isotopes (helium-3 and helium-4) of the gas trapped in the Red Sea glass.

Helium isotopes record the amount of primordial gas in basalts, and studies have found that the Afar plume contains up to one-tenth as much primordial helium as it would have if it had originated deep in the mantle.

“The Afar mantle plume is located beneath a thin crust where three plates meet, making it an incredible natural laboratory for studying the deep Earth,” said Ugur Barç, a graduate research student at SUERC and lead author of the paper.

“A surprising result of our study is that the plume is largely made up of rocks that were on the Earth’s surface less than 100 million years ago, which calls into question our previous understanding of how mantle plumes form.”

The team also analyzed seismic tomography models to identify subducted ocean floors within the Earth that may be the source of the geochemical signature of the Afar volcano plume. Seismic tomography is a technique similar to MRI that uses earthquakes to allow scientists to “see” inside the Earth.

Using this information, the team was able to determine the location, direction and surface source of the subducted seafloor, and estimate how fast it was sinking before encountering the Afar volcano’s plume.

Dr Antoniette Greta Grima, from the University of Glasgow’s School of Geography and Earth Sciences, co-author of the paper, said: “The isotopic fingerprints from the rocks give us a glimpse into the formation of the Afar mantle plume, and the seismic tomography model provides another important lens for understanding the interaction between the mantle and the subducted ancient seafloor, which we do not have direct access to.”

“The geochemical data suggest that the rising plume is interacting with newer subducted ocean floor material 660 km below the surface, rather than the very old subducted material at the core-mantle boundary as previously assumed.”

“By combining seismic tomography models, slab subsidence calculations and plate reconstruction models, we identified the subducted ocean floor and linked it to the current active subduction zone beneath the Zagros Mountains.”

“Mantle plumes were first recognised in the early 1960s,” said Professor Finn Stewart, from the Scottish Universities Environmental Research Centre (SUERC), who led the project. “Mantle plumes are fundamental to Earth – they drive plate tectonics, cool the planet, deliver elements essential for life to the surface and provide our best window into the depths of the Earth.”

“This study calls into question the prevailing paradigm that all plumes transport water from deep within the Earth to the surface. The key to unlocking this new insight is combining SUERC’s isotopic geochemistry expertise with the geodynamic modelling capabilities of the School of Geography and Geosciences.”

Researchers from the European Institute of Marine Science and Technology (IUEM) in France and King Abdullah University of Science and Technology in Saudi Arabia also contributed to the paper.