Scientists find the presence of fluids derived from subducted slab in the lower mantle

In the Juína region, in the west of Brazil, a volcanic eruption brought diamonds from the interior of the Earth to the surface around 93 millions of years ago. Diamonds form perfect capsules so they retain the exact chemistry of material from the part of the Earth where they formed. Scientists are therefore studying these minerals to get information on the composition of the deep upper mantle, the transition zone and lower mantle.

Now a team led by University College Cork (Ireland) and Bayreuth Geoinstitute (Germany) has found that subducted material has penetrated into the sublithosperic mantle (below 250 km) by testing the oxidation state of several diamonds from the Juína region using the ESRF.

The oxidation state of the Earth’s mantle controls important parameters and processes, such as magma generation, speciation and mobility of fluids and melts in the Earth’s interior, deep carbon cycle, recycling of oceanic crust back into the mantle, chemical differentiation of the planet and many others.

It is generally considered that the main three layers of the Earth – its crust, mantle and core, represent profound changes in the oxidation state of iron from ferric (Fe3+) at the surface to mostly Fe2+ in the silicate minerals in the upper mantle, transition zone and the lower mantle and ultimately, to the Fe0 in the core. In short, the surface is very oxidised and the core is metallic so it is very reduced.

Mantle beneath the Amazonian Craton. Schematic illustration showing the formation of the magnesiowüstite-magnesioferrite mineral association in Juina diamonds.

The researchers came to the ESRF and used the novel synchrotron Mössbauer source at the ESRF’s nuclear resonance beamline ID18 to study inclusions in diamonds from Juína to directly test the oxidation state of the deep mantle.

The ultimate goal was to determine the oxidation state of iron in minerals that are coming from an area inaccessible for direct sampling. These mineral inclusions derive from depths between 250 and 700 kilometres and have likely formed at the boundary of the mantle transition zone and lower mantle (~660 km depths). Although through experiments and modelling the scientists know that this area of the mantle is quite reduced, studying natural samples allows to test these hypothesis. “The ESRF was pivotal for this research”, explains Kate Kiseeva, scientist at the University College Cork and corresponding author of the paper. “It is the only place where we can get such great resolution and analyse samples as small as 50 microns”, she adds. The team also carried out some experiments at PETRA III in Germany, where they confirmed the presence of ferropericlase – mineral that is common for the lower mantle.

Mössbauer source spectroscopy at ESRF‘s nuclear resonance beamline ID18 identified a very oxidised mineral within the studied inclusions that contains Fe3+ iron, common on the Earth’s surface. “This was an unexpected finding because mantle was considered very reduced at those regions”, explains Kiseeva. 

The lower mantle comprises >50% of Earth’s volume, and compositionally it is considered largely homogeneous and primitive. The research community has acknowledged, however, that modern-day subducted slabs can penetrate deep into the lower mantle, causing heterogeneities. This is the first study that confirms, by studying mineral inclusions in diamonds, that some heterogeneous and highly oxidised regions are present in the lower mantle or lower mantle – transition zone boundary.

Reference:
E. S. Kiseeva, et al, Nature Communications, volume 13, Article number: 7517 (2022), DOI: 10.1038/s41467-022-35110-x

Text by Montserrat Capellas Espuny

Top image: Georgios Aprilis, ESRF postdoc at ID18 beamline