Earth’s deep mantle was drier from the start

July 12 () –

The deep part of the ancient mantle closest to the Earth’s core began as substantially drier than the one closest to the surface of the young planet.

This is what a researcher from the University of Washington suggests, who publishes a study on the subject in ‘Proceedings of the National Academy of Sciences (PNAS)’.

The Earth’s mantle is the thick layer of silicate rock that lies between the Earth’s crust and its molten core, making up about 84% of our planet’s volume. The mantle is predominantly solid but, on geological time scales, behaves like a viscous fluid.

By analyzing noble gas isotope data, Rita Parai, associate professor of Earth and Planetary Sciences in Arts and Sciences, determined that the plume’s ancient mantle (the deep part) had a concentration of water that it was a factor of 4 to 250 times lower compared to the upper mantle water concentration.

The resulting viscosity contrast could have prevented mixing within the mantle, helping to explain long-standing mysteries about Earth’s formation and evolution.

“A primordial viscosity contrast could explain why the giant impacts that triggered magma oceans throughout the mantle did not homogenize the growing planet,” he says. it’s a statement Parai, a faculty member at the university’s McDonnell Center for Space Sciences–. It could also explain why the plume’s mantle has undergone less processing by partial melting throughout Earth’s history.”

Parai’s research challenges an assumption once widely held in his field: that the Earth’s mantle was uniform from the beginning. When the solar system settled into its current arrangement about 4.5 billion years ago, Earth formed when gravity pulled gas and dust that swirled to become the third planet from the sun.

Volatiles such as water, carbon, nitrogen and noble gases reached Earth during its formation, but the Parai study suggests that the material that accumulated earlier was a drier type of rock than that which accumulated later.

The researcher found that the isotopes of helium, neon and xenon (Xe) in the mantle require that the plume mantle have lower concentrations of volatiles such as Xe and water at the end of that accretion period, compared to the upper mantle. The upper mantle may have benefited from a higher mass contribution from volatile-rich materials, similar to a class of meteorites called carbonaceous chondrites.

Parai takes a multipronged approach to figuring out a planet’s life history. This study presents a model developed by her, but Parai also does her own experimental work with rock samples in her high-temperature isotope geochemistry laboratory at the University of Washington.

She studies the isotopes of the noble gases -especially those of Xe- in volcanic rocks to understand the evolution of the composition of the Earth’s mantle and in terrestrial rocks on the Earth’s surface to see the evolution of the atmosphere.

“In my lab,” Parai explains, “we take samples of natural rocks – mostly modern volcanic rocks, but also some ancient rocks – and try to understand different things about the history of the Earth. Specifically, we want to know how the Earth got its atmosphere, oceans, and other features related to habitability“, he assures.