The discovery of two new iron oxides in experiments conducted using particle accelerators at the Deutsches Elektronen-Synchrotron (DESY) research center in Germany suggests the presence of a large, previously undiscovered source of oxygen hidden in Earth’s lower mantle.
The experiments, which are detailed in the latest edition of the journal Nature Communications, were conducted using a special high-pressure chamber at DESY’s X-ray light source PETRA III and other facilities, Dr. Elena Bykova of the University of Bayreuth and colleagues from France and the US explained earlier this week in a statement.
A model of Fe5O7. Credit: Elena Bykova, Universität Bayreuth
According to Dr. Bykova, naturally-occurring iron oxides come in a number of forms, with the most common beaing hematite (Fe2O3), the product that remains after several different kinds of geological processes. While hematite is the primary source of iron, scientists have in recent years discovered several other types of iron oxides that form at high pressures and temperatures.
While using a special pressure chamber known as a diamond anvil cell, the research team took a closer look at the behavior of hematite and magnetite (Fe3O4) and found that the decomposition of both iron oxides at pressures above 60 gigapascals and temperatures over 2,000 Kelvin led to the crystallization of unusual Fe5O7 and Fe25O32 phases and the release of oxygen.
A model of Fe25O32. Credit: Elena Bykova, Universität Bayreuth
Oxygen source could significantly impact geological processes
Using the diamond anvil cell, “a minute sample can be compressed between two diamonds to several hundred thousand times the atmospheric pressure, while a meticulously aligned laser can also heat the sample… to several thousand degrees Celsius,“ said Dr. Hanns-Peter Liermann, the head of the measuring station at DESY and co-author of the newly published paper.
These pressure and temperature conditions correspond to the environment that exists at depths of at least 1,500 kilometers below the Earth’s surface, the research team said. Fe5O7 resulted when they applied pressure of more than 67 GPa and heated it to temperatures of 2400 degrees Celsius, while Fe25O32 required pressures of more than 70 GPa, they added.
The structural changes to the samples were monitored using the small, extremely bright beam of the PETRA III storage-ring-based X-ray radiation source, the study authors noted. Measurements were also taken at the European Synchrotron Radiation Source (ESRF) in Grenoble, France, and at the Advanced Photon Source (APS) at the Argonne National Laboratory in Illinois.
Based on their observations, when exposed to conditions corresponding to the middle of the Earth’s lower mantle, both hematite and magnetite release oxygen-rich fluids as they decompose, Dr. Bykova said that she and her colleagues “estimate that this source so far provided an amount of oxygen equivalent to eight to ten times the mass of oxygen in the atmosphere.”
She said that it is “not quite clear” what happens to the oxygen there. It could locally oxidize the surrounding materials, or pass either to the transition zone or the upper mantle. Study co-author Dr. Maxim Bykov of the University of Bayreuth said that this “remains to be explored” and that for now, all they can say is that a large source of oxygen in the mantle could “significantly affect geochemical processes by changing oxidation states and mobilizing trace elements.”
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