Deep inside a rocky planet like Earth, the behavior of iron greatly affects the properties of the molten material. New research exploring the quantum properties of elements in extreme environments has important implications for our understanding of Earth's history, explaining unique seismic activity, and studying exoplanets to gain insights into their potential to support life.
Deep inside a rocky planet like Earth, the behavior of iron greatly affects the properties of the molten material: properties that influence how the planet formed and evolved. Scientists used powerful lasers and ultrafast X-rays to recreate the extreme conditions in these molten materials, known as silicate melts, and measure the iron's properties. Source: Greg Stewart/SLAC National Accelerator Laboratory
These properties played a crucial role in the formation and evolution of the Earth. Our planet's evolution may have been driven in large part by the microscopic quantum states of iron atoms. Iron's "spin state" is a quantum property of its electrons that affects its magnetic behavior and chemical reactivity. Changes in spin state affect the molten or solid form of iron and its conductivity.
Until now, it has been a challenge to reproduce the extreme conditions in these molten materials, known as silicate melts, to measure iron's spin state. An international team of researchers from the U.S. Department of Energy's SLAC National Accelerator Laboratory, Stanford University, Université Grenoble Alpes, the Laboratory for the Utilization of Intense Lasers (LULI), and Arizona State University overcame this challenge using powerful lasers and ultrafast X-rays. They found that at extremely high pressures and temperatures, the iron in silicate melts was mostly in a low-spin state, meaning its electrons were closer to the center and paired up in their energy levels, making the iron less magnetic and more stable.
The results, published in Science Advances, support the idea that certain types of lava may be stable deep within Earth and other rocky planets, potentially playing a role in the creation of magnetic fields. The research has potential implications for understanding Earth's evolution, interpreting seismic signals, and even the study of exoplanets.
"In terms of exploring Earth's history, we are looking at processes that occurred more than 4 billion years ago," said co-author Dan Shim, a researcher at Arizona State University. "The only way to study Earth's history is to use modern technology measured in femtoseconds. The contrast between these vast time scales is both eloquent and shocking: it's akin to the concept of a time machine."
Asteroid bombardment and magma oceans
About 4.3 billion to 4.5 billion years ago, the early Earth experienced a strong impact and was hit by an asteroid as large as a city. These impacts generated so much heat that they may have completely melted the Earth's outer layers, creating a deep ocean of lava.
"There are theories that under the intense pressure of these impacts, lava could become denser than solid rock," said co-author Arianna Gleason, a scientist at SLAC. This denser magma sinks toward the Earth's core, capturing the chemical signature of that era. Some believe remnants of this magma layer may still exist today, holding clues to 4.5 billion years ago. Volcanoes in places like Hawaii may be releasing these ancient chemical signatures, giving us a glimpse into Earth's distant past. "
At shallow levels, molten rock takes up more space than the same material in its solid state. But as depth and pressure increase, this difference decreases. The composition of iron, especially its spin state, plays an important role in determining these properties. Previous studies have shown mixed results for iron's spin state under similar conditions: Some studies found rapid changes in iron's spin state at high pressure, while others found slower, more gradual changes.
The new study is the first direct observation of how iron behaves in real lava under extreme conditions.
"While we can gain a lot of information from the study of rocks and fossils, some aspects of Earth's early history have been lost because there are few records from the time. That's what makes this study unique," Shim said. "The formation of the Earth was a turbulent process involving strong impacts and the creation of a global lava layer. The pressure in the lava layer was enormous. We studied this through laboratory experiments simulating the conditions at the time."
In the Materials in Extreme Conditions (MEC) chamber at SLAC's Liner Coherent Light Source (LCLS), the team blasted the sample with powerful lasers, converting solid material into silicate melt in nanoseconds, recreating the extreme pressures found in Earth's early magma oceans. The scientists then used femtosecond X-ray pulses from LCLS to study the electronic structure of elements such as iron under these extreme conditions, gaining insight into how electron configurations change under different conditions and revealing that molten magma indeed becomes denser than solids under certain conditions.
"By understanding the Earth's internal dynamics, we can improve models of tectonic movements and other geological phenomena," Gleason said. "In addition, because Earth's layers are interconnected, these findings have implications for climate science."
Learn about our planet
In this study, the team focused on melts with low iron content. But as material pours toward the center of the Earth, it theoretically absorbs more iron, making it denser. To follow up the study, the team plans to study melts with higher iron content. They also hope to conduct experiments on melts containing some water to learn more about Earth's water cycle and climate.
The research could also reveal special seismic velocities deep in Earth's mantle. These anomalies have puzzled scientists for decades. Some theories suggest these areas may be remnants of magma from 4.5 billion years ago, while others believe they are the result of tectonic plates sinking into the Earth's interior, spreading low-melting-point materials. By comparing different hypotheses using seismic imaging techniques, the team aimed to determine the origin of these areas and distinguish between ancient and recent material.
"As technology advances, we are at the forefront of solving grand challenges from mineralogy to climate science, bridging research fields together," said SLAC scientist and collaborator Roberto Alonso-Mori. "The vast amount of information we're able to gather changes our capabilities. It's a game changer. It's exciting to work with such a diverse team to develop new technologies and apply them to solve pressing problems."