For example, when scientists try to determine the mantle's density, they do so by measuring the speed of a seismic wave after an earthquake, from its origin to other points on the planet.
Because such waves travel faster through solids (e.g., crust) than through liquids (e.g., magma), geologists had been surprised to detect waves slowing down, as though passing through liquid, in a zone that should be the mantle's faster "express lane."
"Seismologists have observed anomalies in velocity data as deep as 200 kilometers beneath the ocean floor," Dasgupta said.
"It turns out that trace amounts of magma are generated at this depth, which would potentially explain that" slower velocity.
The research also offers clues to the electrical conductivity of the oceanic mantle.
"The magma at such depths has a high enough concentration of dissolved carbon dioxide that its conductivity is very high," Dasgupta said.
But, because scientists have not yet been able to sample the mantle directly, researchers have had to extrapolate from the properties of rocks carried up to the surface.
So, in a previous study, Dasgupta determined that melting in Earth's deep upper mantle is caused by the presence of carbon dioxide.
The present study shows that carbon helps to make silicate magma at significant depths. And, the researchers also found that carbonated rock melts at significantly lower temperatures than non-carbonated rock.
"This deep melting makes the silicate differentiation [changes in silicate distribution that range from the dense metallic core, to the less-dense silicate-rich mantle, to the thinner crust] of the planet much more efficient than previously thought," Dasgupta said.
"Deep magma is the main agent that brings all the key ingredients for life--water and carbon--to the
|Contact: Cheryl Dybas|
National Science Foundation