Iron Oxide’s Resilience in the Earth’s Depths Against Intense Temperatures and Pressures

While direct travel to the Earth’s core remains beyond reach for scientists, they employ a fascinating indirect method to unveil the secrets hidden beneath the planet’s surface—studying earthquakes. By measuring seismic waves, researchers can discern valuable insights about the composition below. Seismic waves exhibit varying speeds depending on the materials through which they travel, enabling scientists to make inferences about the deep layers of the Earth through seismic signatures. This process is akin to the way ultrasound utilizes sound waves to create images of the internal structures within the human body.

Emerging studies reveal a nuanced understanding of the Earth’s mantle base, challenging prior assumptions of uniformity. Notably, the presence of intricate features has been identified, resembling mountainous regions where seismic waves exhibit enigmatic deceleration. Coined as ultralow velocity zones (ULVZs) and originally observed by Don Helmberger at Caltech, these sizable formations, measuring several kilometers in thickness, reside approximately 3,000 kilometers below the Earth’s surface.

Jennifer Jackson, the William E. Leonhard Professor of Mineral Physics, underscores the challenges in directly exploring the core-mantle boundary (CMB) and obtaining measurements from this crucial region in the Earth’s evolution. Questions persist about the existence and composition of ultralow velocity zones (ULVZs). These mysteries prompt inquiries into the broader understanding of Earth’s evolution and the specific role played by this region in Earth’s dynamics. Fundamental questions revolve around the nature of these ULVZs—whether they are solid or molten under the extreme conditions found at the CMB. The inability to directly access this region prompts ongoing scientific exploration to unravel these enigmas.

Back in 2010, Jennifer Jackson and her research team proposed that these peculiar blobs might harbor a higher concentration of iron oxide compared to the surrounding mantle. The hypothesis rested on the notion that solid iron oxide could potentially account for the observed slowdown of seismic waves as they traverse these regions. However, a lingering question persisted: Could iron oxide maintain a solid state under the extreme temperatures and pressures characteristic of the core-mantle boundary (CMB)?

In a recent investigation conducted by Jackson’s research laboratory, comprehensive measurements were undertaken to scrutinize the behavior of iron oxide under conditions akin to the extreme temperatures and pressures found at the core-mantle boundary (CMB). The resulting phase diagram, which charts the material’s behavior, challenges prior theories by demonstrating that iron oxide remains in a solid state even under exceptionally high temperatures. This compelling evidence strongly supports the hypothesis that solid iron-rich regions could indeed account for ultralow velocity zones (ULVZs), potentially exerting a significant influence on the generation of deep-seated plumes. These findings not only reshape our understanding of ULVZs but also prompt further exploration into solid iron-rich materials to enhance our comprehension of the Earth’s profound interior.

The findings from this research were published in the journal Nature Communications on November 13th, elucidating the insights gleaned from the study.

On an atomic scale, solid iron oxide is characterized by a meticulous arrangement of iron and oxygen atoms in repetitive, orderly patterns. The transition from solid to liquid occurs as these atoms shift from a rigidly ordered structure to a more fluid, mobile state. Spearheaded by Vasilije Dobrosavljevic, a former Caltech graduate student who earned his PhD in 2022, the recent study sought to empirically ascertain the specific temperatures and pressures at which this transformative transition occurs.

For decades, achieving extreme temperatures and pressures in experiments has been feasible, albeit with the constraint of using minuscule samples, smaller than the average width of a human hair. The challenge lies in accurately pinpointing the temperature at which a material undergoes the solid-to-liquid transition when working with such diminutive samples. Over the course of more than a decade, Jackson and her collaborators have dedicated efforts to refine a technique for detecting melting at high pressures. In the current study, this meticulous approach, known as Mössbauer spectroscopy, was employed to scrutinize the dynamic arrangement of iron atoms during the transition process.

Dobrosavljevic explains that Mössbauer spectroscopy is employed to gain insights into the dynamic behavior of iron atoms. Within a brief timeframe of approximately 100 nanoseconds, the goal is to discern whether the atoms exhibit minimal movement, characteristic of a solid, or significant mobility, indicative of a liquid state. The recent study enhances Mössbauer spectroscopy by incorporating an additional, independent method—X-ray diffraction. This complementary technique allows researchers to observe the positions of all atoms within the sample, providing a comprehensive understanding of the material’s structural dynamics.

Conducting numerous experiments spanning a spectrum of temperatures and pressures, the research team made a significant revelation. At the pressure conditions characteristic of Earth’s core-mantle boundary (CMB), iron oxide was found to undergo melting at higher temperatures than previously thought—exceeding 4,000 Kelvins, equivalent to approximately 6,700 degrees Fahrenheit.

Additionally, the study produced an unforeseen finding concerning atomic defects within iron materials.

Scientific understanding has acknowledged that, under sea-level pressure, every iron oxide sample exhibits minuscule, regularly spaced defects in its atomic arrangement. This entails a deficiency of approximately five iron atoms for every 100 oxygen atoms. The broader implications of these atomic-level defects, such as their influence on the material’s conductivity of electricity and heat, deformation under pressure, and other relevant factors, have been subjects of debate among researchers. These aspects are crucial for comprehending planetary interiors, where phenomena like heat flow and material deformation steer planetary dynamics. Yet, the behavior of these defects under the high pressures and temperatures akin to those found at the core-mantle boundary (CMB) remained unknown until the recent study.

Dobrosavljevic and his research team made a noteworthy discovery: at temperatures several hundred Kelvins below the melting point of iron oxide, the minute atomic defects exhibited a transition, shifting within the solid material and entering a state of “disorder.” This observation offers a potential explanation for prior experiments that indicated iron oxide was melting at lower temperatures. It appears that these earlier experiments might have detected alterations in the defects rather than the complete melting of the crystal structure.

Dobrosavljevic outlines a key observation: preceding the transformation of the solid crystal into a liquid state, there is a discernible shift in the defect structure from an ordered to a disordered state. The focus now shifts to understanding the implications of this newfound transition on the physical characteristics of iron-rich areas, such as the ultralow velocity zones (ULVZs). Questions arise about how these defects influence heat transport and what significance this holds for the development and emergence of upwelling plumes reaching the Earth’s surface. These inquiries will serve as guiding factors for future research endeavors in this domain.

Resources

1. ^NEWSPAPER Dajose, L. & California Institute of Technology. (2023, November 17). Deep within the Earth, iron oxide withstands extreme temperatures and pressures. Phys.org. [Phys.org]
2. ^JOURNAL Dobrosavljevic, V. V., Zhang, D., Sturhahn, W., Chariton, S., Prakapenka, V. B., Zhao, J., Toellner, T. S., Pardo, O. S., & Jackson, J. M. (2023). Melting and defect transitions in FeO up to pressures of Earth’s core-mantle boundary. Nature Communications, 14(1). [Nature Communications]