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Home / Science / "Different than expected" – geochemists measure the composition of the mantle

"Different than expected" – geochemists measure the composition of the mantle



  Olivine

The mineral olivine contains only a few microns of melt inclusions (black dots). The geochemists isolated these inclusions and examined the isotopic composition with mass spectrometer Credit: University of Münster ̵

1; Felix Genske

Researchers suspect a greater dynamic than previously assumed between the earth's surface and the Earth's mantle.

What is the chemical composition of the Earth's interior? Because it is impossible to carry out more than ten bores deep into the earth, volcanic rocks originated by melting deep inland, often such information ts at the universities of Münster (Germany) and Amsterdam (Netherlands) have volcanic rocks which form the Portuguese archipelago of the Azores. Their goal: to collect new information about the compositional development of the mantle, which is located at a depth between 30 and 2,900 kilometers. Using sophisticated analysis techniques, they found that the composition of the mantle below the Azores differs from that previously thought, suggesting that large parts contain surprisingly few so-called incompatible elements. These are chemical elements that accumulate through the constant melting of the mantle in the earth's crust, the outermost solid layer of the earth.

The researchers conclude that more Earth's mantle is present in the course of Earth's history – and eventually formed the Earth's crust – than previously thought. "In order to maintain the material budget between the Earth's mantle and the Earth's crust, the mass fluxes between the surface and the Earth's interior must have had a stronger effect," says Prof. Andreas Stracke from the University of Münster, who heads the study.

The Azores rise from the depth of the mantle – and unexpectedly resemble most of their upper part -. The composition of the entire mantle may differ from the current way of thinking. "Our results have opened up a new perspective," says Andreas Stracke, "because we now have to reassess the composition of most of the Earth – after all, the Earth's mantle accounts for over 80 percent of Earth's volume." The study was published in the journal Nature Geoscience published.

Background and Method:

In their study, geochemists investigated the mineral olivine and its melt inclusions, ie magma encapsulated during the crystallization of olivine before the lava broke out. The researchers isolated these melt inclusions, which were only a few micrometers in size, dissolved them chemically and separated certain chemical elements. These elements change through radioactive decay during their lifetime and during ascent from the Earth's interior over thousands of kilometers for hundreds or even thousands of millions of years.

The researchers analyzed the isotopic composition of the melts using high-sensitivity mass spectrometers. Such methods allow the measurement of the relative abundance of different atoms in one element – so-called isotopes. "Thanks to the high efficiency of our measurements, we were able to analyze the isotopic composition of one billionth of a gram of the element," says co-author Dr. Felix Genske from the Institute of Mineralogy of the University of Münster, who has carried out most of the analytical work. In this way, the researchers indirectly obtained information about the composition of the material in the mantle: The isotope analyzes showed that it contains far fewer rare earth elements such as samarium and neodymium, but also chemically similar elements such as thorium and uranium.

"Due to similar geochemical data in volcanic rocks from different regions, eg Hawaii, other parts of the mantle can also contain a higher proportion of material that is heavily depleted of incompatible elements," says Andreas Stracke. The researchers suspect that this global deficit can be offset by a higher recycling rate of the incompatible, element-rich earth crust into the Earth's mantle. With their ongoing studies, the researchers want to confirm their working hypothesis by studying samples from other volcanic islands around the world.

Funding:

The study was funded by the German Research Foundation and by the international research network "Europlanet 2020 RI", which is funded by the Horizon 2020 program of the European Union.

Reference:

"Ubiquitous Ultra-Depleted Domains in the Earth's Mantle" by Andreas Stracke, Felix Genske, Jasper Berndt and Janne M. Koornneef, September 16, 2019, Nature Geoscience .
DOI: 10.1038 / s41561-019-0446-z


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