Molten rock materials, also known as silicate melts, are found deep inside rocky planets like Earth. They play a crucial role in the formation and evolution of these planets, as well as in the generation of magnetic fields. One of the key factors that influences the properties of silicate melts is the spin state of iron, which is a quantum property of the electrons in each iron atom that drives their magnetic behavior and reactivity in chemical reactions. Changes in the spin state can affect whether iron prefers to be in the molten rock or in solid form and how well the molten rock conducts electricity.
However, measuring the spin state of iron in silicate melts is very challenging, because it requires recreating the extreme conditions of temperature and pressure that exist deep inside planets. Until now, scientists have only been able to observe patches of melt at a similar depth, but not its global extent and its part in plate tectonics.
A new method to probe molten rock materials
A team of researchers from the Department of Energy’s SLAC National Accelerator Laboratory, Stanford University, Universite ́ Grenoble Alpes, Laboratoire pour l’Utilisation des Lasers Intenses (LULI), and Arizona State University has developed a new method to probe molten rock materials under extreme conditions. They used powerful lasers and ultrafast X-rays to recreate and measure the spin state of iron in silicate melts at pressures up to 1.4 million times higher than atmospheric pressure and temperatures up to 4,000 degrees Celsius.
The researchers reported their findings on Friday in Science Advances. They showed that at these extreme conditions, the iron in silicate melts mostly has a low-spin state, meaning its electrons stay closer to the center and pair up in their energy levels, making the iron less magnetic and more stable.
Implications for understanding Earth’s history and habitability
The results support the idea that certain types of molten rock might be stable deep inside Earth and other rocky planets, potentially lending a hand in the creation of magnetic fields. The research has implications for understanding Earth’s evolution, interpreting seismic signals, and even the study of exoplanets for insights into habitability.
“In terms of exploring Earth’s history, we’re investigating processes that took place over 4 billion years ago,” said collaborator Dan Shim, a researcher at Arizona State. “The only way to study this is by using modern technology that operates in femtoseconds. The contrast between these immense time scales is both eloquent and startling: it’s akin to the idea of a time machine.”
The researchers also found that the spin state of iron does not appear to notably influence the flow of mantle rocks, which is driven by the convection of heat and rock in the mantle. This means that computer models of plate tectonics can simplify their calculations by ignoring the effect of melt on mantle viscosity.
“When we think about something melting, we intuitively think that the melt must play a big role in the material’s viscosity,” said lead author Junlin Hua, a postdoctoral fellow at UT’s Jackson School of Geosciences who led the research. “But what we found is that even where the melt fraction is quite high, its effect on mantle flow is very minor.”
The researchers plan to continue their experiments with different compositions of silicate melts and different wavelengths of X-rays to further explore the behavior of iron under extreme conditions.
