Within Earth’s roughly 3,000-kilometer-thick mantle lies one of geoscience’s mysteries. Magma emanating from the lower mantle has a different chemical composition than magma from the upper mantle, indicating there’s a separation between the two. What keeps these two layers apart? This question has puzzled experts, as geophysical models predict that on very long timescales, the mantle should be a churning fluid, with all parts mixing.
With less water, the lower mantle is stickier and harder to stir. “That’s how you can keep these two reservoirs from mixing throughout Earth’s history.”
A recent publication by geochemist Rita Parai from Washington University in St. Louis holds a possible answer: Earth’s lower mantle contains a lower concentration of volatile compounds, such as xenon and water, than the upper mantle. With less water, the lower mantle is stickier and harder to stir. “That’s how you can keep these two reservoirs from mixing throughout Earth’s history,” Parai said.
Geochemistry and Geophysics Are Telling Different Stories
Parai first encountered the question of why mantle layers segregate about 15 years ago, as an undergraduate student at Harvard University, but it wasn’t until graduate school that her own research brushed up against the problem. Parai was analyzing the isotopic signature of the upper mantle using samples of magma that had bubbled to the ocean floor in spots where tectonic plates had pulled apart. Some isotopes were present at very different concentrations in the upper mantle samples than in lower mantle samples collected after volcanic eruptions. When Parai looked at the ratio of isotopes that are produced on Earth today compared with those produced about 4 billion years ago, she saw a pattern that’s become familiar to geochemists: The lower mantle seems to have lost less of its original gas over time than the upper mantle.
Initially, this seems sensible—the deep mantle is farther from Earth’s surface, so it’s logical that it’s more protected. Were it not for earthquakes, scientists never may have suspected that this wasn’t the whole story. When Earth’s outermost layer moves, waves of energy ripple through the mantle, then bounce back to the surface, where scientists can measure them. How they reverberate gives scientists information about the composition of Earth’s interior. Using this method, scientists have seen that tectonic plates can get pushed all the way from Earth’s surface into the lower mantle, showing that mixing between the layers is possible. So why do the isotopic compositions of the two mantle layers remain so different?
To solve the puzzle, Parai first questioned the assumption that the lower mantle formed with a chemical composition similar to the upper mantle. Rather than losing a high proportion of its original gas over time, the lower mantle may have had a lower quantity to begin with. “We were sort of mixing up the interpretation,” Parai said.
According to Parai, water may have been lacking when the lower mantle formed and could explain why the two layers stay relatively separated.
Not All Meteorites Are Created Equal
The conclusion that Earth’s lower mantle is dry is well supported and in line with other recent research, said geochemist Michael Broadley from Université de Lorraine in Nancy, France, who was not involved in the new research. But “you have to come up with a reason for why that could be.”
“There’s a lot left to do to figure out what this means, fundamentally, for global mantle models.”
Parai has theorized that as Earth formed, bombarding meteorites that built up much of the planet’s mass may have switched from ice-poor meteorites that came from close to the Sun to ice-rich meteorites that came from the cold outer reaches of the solar system. This idea could explain why the lower and upper mantles developed different water concentrations. Future research could analyze drier meteorites that fall to Earth today to determine whether they have isotopic compositions similar to Earth’s lower mantle and, conversely, whether wetter meteorites are more similar to the upper mantle.
Although Parai hopes these analyses will take place in the future, analyzing more samples to make her model more robust could be the next step, Broadley said. Finding additional samples from the lower mantle could strengthen her conclusions or maybe reveal even more complexities in the mantle’s composition. “There’s a lot left to do to figure out what this means, fundamentally, for global mantle models,” Broadley said.
—Saima May Sidik (@saimamaysidik), Science Writer