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  Geotimes - February 2008 - Still mushy after all these years

Still mushy after all these years

Two years ago, scientists discovered that contrary to long-held thought, Earth’s chemical composition differs from the composition of chondrites, meteorites thought to represent the original makeup of the solar system. To explain those differences, they suggested that the missing elements may in fact be tucked away deep inside Earth’s interior, in the crystalline remnants of what was once a magma ocean at the base of the mantle. Now, new models suggest that the proposed magma ocean may have lingered longer than previously suspected, slowing Earth’s cooling and potentially delaying the formation of Earth’s magnetic field.

Exactly how and where the lost elements might be locked away is still up for debate. One hypothesis is that Earth was largely molten during its formation, rather than solid, as previously thought, and that the ancient magma ocean crystallized rapidly, forming chemically distinct layers within its first 30 million years, and burying the missing elements in a deep reservoir. Such a basal magma ocean “is the ideal place” to store those missing elements, says Stéphane Labrosse, a geophysicist at the Université de Lyon in France.

That idea opened up new questions and new avenues of research into exactly how the magma ocean would have crystallized, why melt appears to remain at the bottom of the mantle today and what it all means for the thermal evolution of Earth, Labrosse says.

Labrosse was particularly interested in possible regions of melt near the core-mantle boundary, where seismic waves appear to slow as though passing through liquid, and how that melt might have affected heat being transferred out of the core throughout Earth’s history. Using thermal evolution models for Earth that incorporate estimates of the missing elements possibly present in the basal magma ocean, he and his team assessed how the deep magma ocean would ultimately cool and begin to form crystals. They found that the deep magma ocean — rich in radioactive elements and surrounded by solid mantle above and the equally hot core below — would have lost heat very slowly. That suggests a much longer-lived ocean than previously thought, they reported Dec. 6 in Nature. As it did form crystals, the ocean would have become even more concentrated in heavy iron, they suggest, forming a dense, slushy layer at the base of the mantle that would eventually be too heavy for the churning mantle to drag upward into its convective cycles. As a result, the magma “ends up puddling at the base of mantle,” and locking in elements never sampled at the surface, Labrosse says.

The remnants of such a magma ocean at the base of the mantle have been considered as the possible source of “superplumes,” vast regions of melt within the solid mantle that appeared to have swelled up from near the core-mantle boundary. Labrosse says these are also accounted for in his basal ocean model. In addition, one implication of a longer-lived, very hot magma ocean is that it may have delayed the onset of Earth’s “geodynamo,” the electromagnetic churning in the core that powers the planet’s magnetic field. If the core and base of the mantle are equally hot, there’s no heat flow to drive the churning in the core that generates the geodynamo.

The concept of a dense magma ocean at the base of the mantle — which might be feeding superplumes — has been around for some time, says Dave Stevenson, a planetary geologist at Caltech in Pasadena, Calif. However, Labrosse’s team’s research “brings together several ideas that can now be better constrained than in the past,” including the differentiation of early Earth and a possible explanation for the missing elements, Stevenson says. As for whether the ocean was longer-lived than previously thought and thus may have delayed the onset of the geodynamo, as Labrosse’s team suggests, Stevenson says, “I think this is possible.”

Labrosse acknowledges that his is only one model of many new ideas scientists are generating to answer the questions raised by the idea that the missing elements may be buried in Earth’s interior. “It’s a very nice idea, but it has to be tested,” he says. “We’re changing the view we had on the deep Earth, and so we have to explore all the different implications of this model to see if it resists comparison with all types of observations. Clearly [we] have to work this out and see if the model will stand the test of time.”

Carolyn Gramling

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