The Earth’s mantle is a thick layer of silicate rock that lies between the crust and the molten core, accounting for approximately 84 percent of our planet’s volume. On geologic time scales, the mantle is predominantly solid, but it behaves as a viscous fluid—as difficult to stir and mix as a pot of caramel.
But, if you must make a candy comparison, consider malt balls rather than gooey caramels. According to a study from Washington University in St. Louis, the deep part of the ancient mantle closest to the Earth’s core began significantly drier than the part of the mantle closest to the young planet’s surface.
Rita Parai, assistant professor of earth and planetary sciences in Arts & Sciences, discovered that the water concentration in the ancient plume mantle (the deep part) was 4 to 250 times lower than in the upper mantle by analyzing noble gas isotope data.
The resulting viscosity contrast may have prevented mixing within the mantle, which may help to explain some long-standing mysteries about Earth’s formation and evolution. The study will be published in the Proceedings of the National Academy of Sciences the week of July 11th (PNAS).
“A primordial viscosity contrast may explain why the giant impacts that triggered whole-mantle magma oceans did not homogenise the growing planet,” said Parai, a McDonnell Center for the Space Sciences faculty fellow. “It could also explain why the plume mantle has undergone less processing by partial melting throughout Earth’s history.”
Parai’s research calls into question a widely held belief in her field: that the Earth’s mantle was uniform from the beginning. When the solar system settled into its current configuration about 4.5 billion years ago, gravity drew swirling gas and dust in to form the third planet from the sun. Volatiles such as water, carbon, nitrogen, and noble gases were delivered to Earth as it formed, but Parai’s research suggests that the earlier material accreted was a drier type of rock than the later material.
She discovered that mantle helium, neon, and xenon (Xe) isotopes require low concentrations of volatiles like Xe and water at the end of that period of accretion when compared to the upper mantle. The upper mantle may have benefited from a greater mass contribution from volatile-rich materials, similar to a type of meteorite known as a carbonaceous chondrite.
Parai employs a multi-pronged approach to determining a planet’s life history. This PNAS study presents a model she created, but Parai also conducts her own experiments with rock samples in her high-temperature isotope geochemistry laboratory at Washington University. She investigates noble gas isotopes, particularly those derived from Xe, in volcanic rocks to better understand the evolution of Earth’s mantle composition and in terrestrial rocks at the Earth’s surface to better understand the evolution of the atmosphere.
“In my lab, we take natural rock samples—mostly modern volcanic rocks, but also some ancient rocks—and try to understand various aspects of Earth history,” Parai explained. We specifically want to know how Earth got its atmosphere, oceans, and other habitable features.”