Geochemists measure the new composition of the Earth's mantle

What is the chemical composition of the Earth's interior? Since it is impossible to drill more than ten kilometers deep in the Earth, volcanic rocks formed by the melting of the deep interior of the Earth often provide such information. Geochemists from the Universities of Münster (Germany) and Amsterdam (Netherlands) have studied the volcanic rocks that make up the group of Portuguese islands of the Azores. Their goal: to gather new information on the evolution of the composition of the Earth's mantle, the layer located at a depth of about 30 to 2,900 kilometers inside the Earth. Using sophisticated analytical techniques, they discovered that the composition of the mantle below the Azores was different from what was previously thought – suggesting that large parts of it contain surprisingly few so-called incompatible elements. They are chemical elements that, due to the constant melting of the Earth's mantle, accumulate in the Earth's crust, which is the outermost solid layer of the Earth.

The researchers conclude that, during the Earth's history, a larger portion of the Earth's mantle melted – and eventually formed the earth's crust – as previously thought. "To maintain the material budget between the mantle and the Earth's crust, the mass fluxes between the surface and the interior of the Earth must have worked faster," says Professor Andreas Stracke of the University of Münster who is leading the study.

While the material beneath the Azores comes from very deep Earth's mantle – and looks unexpectedly at most of its top – the composition of the Earth's entire mantle may differ from current thinking. "Our results have opened a new perspective," Andreas Stracke said, "because we will now have to re-evaluate the composition of most of the Earth – after all, the Earth's mantle is over 80% of its volume." The study was published in the journal Nature Geoscience.

Context and method

In their study, geochemists examined olivine ore and its inclusions of molten material, ie the encapsulated magma during the crystallization of olivine before the eruption of lavas. The researchers isolated these melting inclusions by a few microns, chemically dissolved them and separated some chemical elements. These elements are altered by radioactive decay during their lifetime and move upward from the Earth's interior – traveling thousands of kilometers for hundreds or even thousands of millions of years.

The researchers analyzed the isotopic composition of the melts with very sensitive mass spectrometers. These methods measure the relative abundance of different atoms in an element, called isotopes. "Thanks to the high efficiency of our measurements, we were able to analyze the isotopic composition of a billionth of a gram of the element," says co-author, Felix Genske, of the "Ion-sized element". Institute of Mineralogy of the University of Münster, which has done analytical work. Thus, researchers have indirectly obtained information on the composition of the material contained in the Earth's mantle: isotopic analyzes have shown that it contains much less rare terrestrial elements such as samarium and neodymium, but also chemically similar elements such as thorium and uranium.

"Based on similar geochemical data on volcanic rocks from different regions, for example Hawaii, other parts of the Earth's mantle may also contain a larger proportion of materials that are highly depleted of incompatible elements," says Andreas Stracke. The researchers assume that this global deficit could be offset by a higher rate of recycling the Earth's incompatible crust on its mantle. By continuing their studies, researchers want to confirm their working hypothesis by examining samples from other volcanic islands around the world.