Geochemists measure new composition of Earth’s mantle


Share post:

What is the chemical composition of the Earth’s interior? Because it is impossible to drill more than about ten kilometres deep into the Earth, volcanic rocks formed by melting Earth’s deep interior often provide such information. Geochemists at the Universities of Munster (Germany) and Amsterdam (Netherlands) have investigated the volcanic rocks that build up the Portuguese island group of the Azores. Their goal: gather new information about the compositional evolution of the Earth’s mantle, which is the layer roughly between 30 and 2,900 kilometres deep inside the Earth. Using sophisticated analytical techniques, they discovered that the composition of the mantle below the Azores is different than previously thought -suggesting that large parts of it contain surprisingly few so-called incompatible elements. These are chemical elements which, as a result of the constant melting of the Earth’s mantle, accumulate in the Earth’s crust, which is Earth’s outermost solid layer.

Geochemists measure new composition of Earth's mantle
The mineral olivine contains melt inclusions (black dots), just a few micrometers in size. The geochemists
isolated these inclusions and investigated the isotopic composition with mass spectrometers
[Credit: Munster University – Felix Genske]

The researchers conclude that, over Earth’s history, a larger amount of Earth’s mantle has melted – and ultimately formed the Earth’s crust – than previously thought. “To sustain the material budget between Earth’s mantle and crust, mass fluxes between the surface and Earth’s interior must have operated at a higher rate,” says Munster University’s Prof. Andreas Stracke, who is heading the study.

As the material below the Azores rises from very deep within Earth’s mantle – and is unexpectedly similar to most of its upper part – the composition of Earth’s entire mantle may differ from current thinking. “Our results have opened up a new perspective,” says Andreas Stracke, “because we will now have to reassess the composition of the largest part of the Earth – after all, Earth’s mantle accounts for over 80 percent of Earth’s volume.” The study has been published in the journal Nature Geoscience.

Background and method

In their study, the geochemists examined the mineral olivine and its melt inclusions, i.e. magma encapsulated during the crystallisation of olivine before the lavas erupted. The researchers isolated these melt inclusions, just a few micrometers in size, dissolved them chemically and separated certain chemical elements. These elements are altered by radioactive decay during their lifetime and ascent from Earth’s interior – travelling over thousands of kilometres for hundreds or even thousands of millions of years.

The researchers analysed the isotopic composition of the melts with highly sensitive mass spectrometers. Such methods allow measurement of the relative abundance of different atoms in an element – so-called isotopes. “Owing to the high efficiency of our measurements, we were able to analyse the isotopic composition of one billionth of a gram of the element,” says co-author Dr. Felix Genske from the University of Munster’s Institute of Mineralogy, who carried out most of the analytical work. In this way, the researchers indirectly obtained information on the composition of the material in the Earth’s mantle: the isotope analyses showed that it contains far fewer rare Earth elements such as samarium and neodymium, but also of chemically similar elements such as thorium and uranium.

“On the basis of similar geochemical data in volcanic rocks from different regions, e.g. Hawaii, other parts of the Earth’s mantle may also contain a higher proportion of material that is strongly depleted in incompatible elements,” says Andreas Stracke. The researchers presume that this global deficit may be compensated by a higher rate of recycling Earth’s incompatible element-rich crust back into Earth’s mantle. With their continuing studies the researchers want to confirm their working hypothesis by investigating samples from other volcanic islands across the globe.

Source: University of Munster [September 16, 2019]



Related articles

The origin of the Andes unravelled

Why do the Andes exist? Why is it not a place of lowlands or narrow seas? Wouter Schellart,...

Gentle giraffes threatened with ‘silent extinction’

For most of his life as a Samburu warrior, Lesaiton Lengoloni thought nothing of hunting giraffes, the graceful...

Rise in Southeast Asia forest clearance increasing greenhouse gases

Forest clearance in Southeast Asia is accelerating, leading to unprecedented increases in carbon emissions, according to new research.Forests...

Study of Patagonian Glacier’s rise and fall adds to understanding of global climate change

Glaciers play a vital role in Earth’s climate system, and it’s critical to understand what contributes to their...

Climate change does not bode well for picky eaters

In a part of the world that is experiencing the most dramatic increase in temperature and climate change,...

Methane-eating microbes may reduce release of gases as Antarctic ice sheets melt

Lurking in a lake half a mile beneath Antarctica's icy surface, methane-eating microbes may mitigate the release of...

Hawaiian islands are dissolving from within

Most of us think of soil erosion as the primary force that levels mountains, however geologists have found...

Why there are no kangaroos in Bali (and no tigers in Australia)

If you travel to Bali, you won't see a cockatoo, but if you go to the neighbouring island...