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Putting Iron Oxide Under Pressure to Peer into the Deep Earth

A graph showing data points on a sloping line, an illustration of two diamonds with a sample of material between them, and an image taken at the APS beamline of a sample under pressure.

In a real way, the depths of the Earth are more mysterious than the depths of space. While our instruments can peer almost to the very edges of the universe, and our space probes have ventured millions of miles into the galaxy, the deep interior of our own planet remains ever impenetrable and inaccessible. But seismic waves can still provide a detailed if indirect view into the Earth, moving at different speeds through various regions and materials, and revealing new mysteries. One of these concerns the region called the core-mantle boundary (CMB).

A transitional area between the base layer of the rocky mantle and the Earth's iron core, the CMB is a heterogeneous region whose exact nature and composition are still the subject of debate, especially the puzzling regions known as ultralow velocity zones (ULVZs). When seismic waves traverse these areas, their speed is sharply reduced for unknown reasons. Formerly thought to consist of partially melted material, a more recent theory holds that the ULVZs are areas enriched in iron oxide (FeO), also known as the mineral wüstite. 

It was unclear, however, whether FeO could exist in a solid phase under the extreme heat and pressures of the CMB. To find out, a team of investigators from Caltech, University of Hawai’i, University of Chicago, and Argonne National Laboratory explored the phase diagram of FeO up to CMB conditions for the first time. Their work was published in Nature Communications.

Previous studies of the behavior of FeO at such high pressures have mainly relied on extrapolations to CMB conditions from lower pressures and indirect proxy methods to approximate the melting point. These studies have thus yielded conflicting and inconclusive results. Iron defects have also complicated the determination of precise melting temperatures under high pressures. In the current work, the researchers used a new multimodal in situ approach which combines synchrotron X-ray diffraction (XRD) and synchrotron Mössbauer spectroscopy (SMS) in the laser heated diamond anvil cell. 

Samples of FeO were studied from pressures of 30 to 140 GPa and temperatures of 300 to 4500 K. Researchers used beamlines at the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science user facility at DOE’s Argonne National Laboratory. Approximately 1000 XRD measurements were collected at the 13-ID-D beamline (the University of Chicago’s GeoSoilEnviroCARS) and about 200 SMS measurements at the 3-ID-B beamline. 

The XRD studies, which show atomic positions, reveal that the iron defects exhibit long-range ordering that becomes enhanced as temperature increases. At high enough temperatures, the defects transition to a disordered state in the solid sample several hundred degrees below melting. At a CMB pressure of 136 GPa, melting finally occurs at a temperature of approximately 4140 K.

The Mössbauer experiments, sensitive to iron atom dynamics, produced independent unambiguous observations of melting and revealed changes in iron oxidation states during heating. The melting temperatures found by the SMS and XRD measurements are in strong agreement, supporting the interpretation of the two different transitions that were observed. 

The investigators note that some previous studies have yielded an FeO melting curve considerably lower than the present work due to differences in experimental techniques that apparently led to possible misinterpretation of the observed phenomena. Other work shows good agreement with the current measurements. The research team notes, however, that none of the earlier studies investigated FeO under as wide a range of simultaneous high pressures and temperatures as these experiments, which have now allowed determination of the complete FeO phase diagram up to CMB conditions.

The FeO melting temperature of ~4140 K found in these experiments is considerably higher than recent estimates of the CMB temperature, which suggests major implications for the nature of the core-mantle boundary and ultralow velocity zones. Primarily, the results give the strongest evidence to date for the viability of solid FeO-rich ULVZs in the lowermost mantle. Such structures could also promote conditions for the generation of plumes of material into upper Earth layers. The presence of an order-disorder defect transition in FeO under the CMB environment could further influence the viscosity, electrical conductivity, and thermal properties of the ULVZs. 

The findings may also provide new insights into Earth's geophysical evolution, including the nutation of the poles, the evolution of the magnetic field, and our planet's deep history. – Mark Wolverton


See: V. V. Dobrosavljevic1,2,, D. Zhang3, W. Sturhahn1, S. Chariton4, V. B. Prakapenka4, J. Zhao5, T. S. Toellner5, O. S. Pardo1, J. M. Jackson1, “Melting and defect transitions in FeO up to pressures of Earth’s core-mantle boundary,” Nat Commun 14 7336 (2023)

Author affiliations: 1California Institute of Technology; 2Carnegie Institution for Science; 3University of Hawai’i; 4University of Chicago; 5Argonne National Laboratory

We thank June Wicks for sample synthesis. We thank Paul Asimow, Michael Gurnis, and Zhongwen Zhan for valuable discussions. We are grateful to the National Science Foundation for supporting this work under EAR-1727020 and EAR-CSEDI—2009935 (J.M.J.). We acknowledge the JPL Strategic Research & Technology Development Program, “Venus Science Into The Next Decade”. GeoSoilEnviroCARS and Sector 3 operations are partially supported by COMPRES (NSF-EAR-1661511). GeoSoilEnviroCARS is supported by the National Science Foundation—Earth Sciences (NSF-EAR-1634415) and Department of Energy- GeoSciences (DE-FG02-94ER14466). Use of APS is supported by the U.S. DOE, Office of Science (DE-AC02-06CH11357). SMS data collected during hybrid mode of the APS used a dual, fast-shutter spectrometer built by T.S.T. and supported by Laboratory Directed Research and Development (LDRD) funding from Argonne National Laboratory, provided by the Director, Office of Science, of the U.S. DOE under Contract No. DE-AC02-06CH11357.

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