Transport of heat from the Earth’s interior causes convection in the mantle. This includes the deformation of solid rocks over billions of years. Significant progress has been made in recent years in studying assemblies representative of the lower mantle under the relevant pressure and temperature conditions.

In a new study, Caltech scientists modeled the surprising behavior of minerals deep in the planet’s interior over millions of years. Their model shows that the processes run in the opposite direction to what was previously theorized.

The lower mantle consists mainly of bridgmanite, a magnesium silicate. However, it also contains small but significant amounts of periclase, a magnesium oxide, and trace amounts of other minerals. Periclase has been found to be weaker and more easily deformable than bridgmanite in laboratory tests; however, these tests did not take into account how minerals behave over millions of years.

Scientists discovered that periclase grains are stronger than the bridgmanite that surrounds them after incorporating these time scales into a sophisticated computer model.

Jennifer M. Jackson, William E. Leonhard Professor of Mineral Physics, said: “We can use the analogy of boudinage in the rock record [image at right]where boudins, which is French for sausage, develops into a stiff, ‘stronger’ layer of rock between less competent, ‘weaker’ rock.

“As another analogy, think of chunky peanut butter. For decades, we thought that periclase was the “oil” in peanut butter, acting as the lubricant between the harder grains of bridgmanite. Based on this new study, it appears that periclase granules act as the ‘nuts’ in thick peanut butter.”

“Periclase granules go with the flow, but do not affect the viscous behavior, except when the granules are highly concentrated. We show that mobility under pressure is much slower in Periklase compared to bridgmanite. There is a reversal of behavior: periclase hardly deforms, while the main phase, bridgmanite, controls the deformation in the deep mantle of the Earth.”

For realistic four-dimensional simulations of our planet and to learn more about other planets, it is crucial to understand these extreme processes that take place far below our feet. Since there are already thousands of confirmed exoplanets, learning more about mineral physics in extreme conditions can help us understand how planets very different from our own have evolved.

Magazine reference:

  1. Cordier, P., Gouriet, K., Weidner, T. et al. Periclase deforms more slowly than bridgmanite under mantle conditions. Nature 613, 303-307 (2023). DOI: 10.1038/s41586-022-05410-9