Flawed, Precious Stones Carry Clues From Deep in the Mantle

Earth’s oxygen-rich atmosphere creates extremely oxidizing conditions at the surface. Anyone who has seen rusty metal has observed oxidation in action: the oxygen steals electrons from the metal, creating crusty, red, iron oxide. Within Earth’s core, however, low levels of oxygen create much more reduced conditions. Intuitively, many researchers have supposed that Earth’s layers become less oxidized with depth, but few samples have emerged from the deep Earth to support or disprove this idea.

In a new paper in Nature Geoscience [1], DCO researchers report the surprising news that iron in the mantle may become more oxidized at deeper levels. DCO members Kate Kiseeva (University of Oxford, UK), Thomas Stachel (University of Alberta, Canada), Aleksandr Chumakov, Valerio Cerantola (both at European Synchrotron Radiation Facility, France), Jeff Harris (University of Glasgow, UK), Catherine McCammon, and Leonid Dubrovinsky (both at Bayreuth University, Germany), analyzed the oxidation state of iron from garnets. The garnets formed in the mantle between 240 and at least 500 kilometers deep and traveled to the surface as inclusions within diamonds. The researchers suspect that the increasing oxidation of iron in garnets with depth is due to oxidized carbonate compounds that react with iron and form diamonds.

Inclusions within diamonds offer the rare opportunity to peek into conditions in the mantle where diamonds formed. “The majority of samples that scientists recover from the mantle are coming from the upper mantle, at depths shallower than 200 kilometers,” said Kiseeva. “Because diamonds may also originate at greater depths, including the transition zone (410 to 660 kilometers) and lower mantle (660 to 2,900 kilometers), sometimes they encapsulate inclusions from different materials that reside at those very deep levels.” Diamonds containing inclusions are rare, however, comprising just 2% of all diamonds mined, and diamonds from the transition zone are even rarer.

The current study began when Kiseeva met Dubrovinsky at a conference and he told her about a new technique that could analyze the oxidation state of iron present in an inclusion, even tiny ones measuring just 200 to 300 microns in diameter. Iron is the only abundant element in the mantle with multiple oxidation states, so it can act as an indicator for different parts of the mantle.

At the European Synchrotron Radiation Facility, Kiseeva and colleagues used synchrotron Mössbauer source spectroscopy to analyze garnet inclusions in a set of diamonds from the Jagersfontein Mine in South Africa. This technique measures minuscule changes in the energy levels of iron atoms as they absorb gamma rays, thus revealing the oxidation state of iron atoms from garnet inclusions. The researchers also used X-ray diffraction, which measures how atoms within a crystal scatter a beam of X-rays, to verify that all of the inclusions were an ultra-high pressure form of garnet called majorite.

Counterintuitively, the garnets contained higher percentages of the most oxidized form of iron the deeper they formed in the mantle. Garnets from 500 kilometers, for example, had twice as much oxidized iron as garnets from the shallow mantle.

The researchers speculate that the carbon compounds in fluids or melted rock may be responsible for this surprisingly high level of oxidation deep in the mantle. “Carbon plays an important role in oxidizing iron in the mantle transition zone,” said Kiseeva. “The reaction has oxidized iron, while simultaneous reducing carbon from carbonate into diamonds.”

Kiseeva cautions that the scarcity of these minerals hampers investigations into the lower parts of the mantle. “We still have only about 100 crystals from the mantle transition zone,” she said. “That’s really nothing.” She looks forward to the discovery and analysis of new samples. Kiseeva plans to continue working with colleagues at the European Synchrotron Radiation Facility, who have obtained a new set of iron-containing garnet inclusions that come from the Argyle diamond mine in Western Australia.

 

Diamonds that form in the mantle sometimes contain garnet inclusions. These majoritic garnets have a maximum diameter of 0.2 millimeters. Credit: Jeff Harris

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