January 10, 2013
Earth’s Mantle Magma Melts Hotter And Deeper Than Previously Thought
April Flowers for redOrbit.com - Your Universe Online
According to a new study by researchers at Rice University, the Earth's mantle magma melts far hotter and deeper in the Earth's core than previously thought, a discovery that will have lasting implications for our understanding of the planet's geophysical and geochemical properties.
The research team, led by Rajdeep Dasgupta, put small amounts of peridotite under large pressures in a laboratory to determine that rock can and does liquefy, at least in small amounts, as deep as 155 miles beneath the ocean floor. Dasgupta claims that his findings, published in the journal Nature, will explain several puzzles that have been irking scientists.
The Earth's middle layer, the mantle, is a buffer of rock between the crust, which is the top five miles or so, and the core. The mantle would look like a roiling mass of rising and falling material if it were possible to compress millions of years of observation down to mere minutes. Materials are brought from deep within the planet to the surface by the slow but constant process of mantle convection, a phenomenon which also thought to be linked with Earth's geomagnetic fields. Occasionally, such materials are brought higher than the surface through volcanoes.
Because it is where the Earth's crust is created and, as Dasgupta says, where "the connection between the interior and surface world is established," the Rice team focused on the mantle beneath the ocean. Silicate magma rises with the convective currents to cool and spread out to form the ocean crust. Scientists have long believed that the starting point for this melt is around 45 miles beneath the surface.
Dasgupta, an assistant professor at Rice, said that this depth has confounded geologists who suspected but could not prove the existence of deeper silicate magma.
The density of the Earth's mantle is determined by measuring the speed of a seismic wave after an earthquake from its origin to other points on the planet. Such waves travel faster through solids than liquids. Geologists have been surprised to detect waves that are slowing down through what should be the mantle's "express lane."
"Seismologists have observed anomalies in their velocity data as deep as 200 kilometers beneath the ocean floor," Dasgupta said. "Based on our work, we show that trace amounts of magma are generated at this depth, which would potentially explain that."
The findings offer tantalizing new clues about the electrical conductivity of the oceanic mantle, as well. "The magma at such depths has a high enough amount of dissolved carbon dioxide that its conductivity is very high," Dasgupta said. "As a consequence, we can explain the conductivity of the mantle, which we knew was very high but always struggled to explain."
Though some are trying, humans have not yet dug deep enough to sample the mantle directly. This means that researchers must currently extrapolate data on rocks carried up to the surface. Melting in the Earth's deep upper mantle is caused by the presence of carbon dioxide, Dasgupta determined in an earlier study. The current work shows that carbons not only lead to making the silicate magma at significant depths.
Non-carbonated rocks melt at significantly higher temperatures than carbonated rocks. "This deep melting makes the silicate differentiation of the planet much more efficient than previously thought," Dasgupta explained. "Not only that, this deep magma is the main agent to bring all the key ingredients for life — water and carbon — to the surface of the Earth."
The Rice research team crushed tiny rock samples containing carbon dioxide to determine the depth of the magma's formation.
"Our field of research is called experimental petrology," he said. "We have all the necessary tools to simulate very high pressures (up to nearly 750,000 pounds per square inch for these experiments) and temperatures. We can subject small amounts of rock samples to these conditions and see what happens."
The team employed powerful hydraulic presses to partially melt "rocks of interest" containing tiny amounts of carbon to simulate what they believe is happening under equivalent pressures in the mantle.
"When rocks come from deep in the mantle to shallower depths, they cross a certain boundary called the solidus, where rocks begin to undergo partial melting and produce magmas," Dasgupta said.
"Scientists knew the effect of a trace amount of carbon dioxide or water would be to lower this boundary, but our new estimation made it 150-180 kilometers deeper from the known depth of 70 kilometers," he said.
"What we are now saying is that with just a trace of carbon dioxide in the mantle, melting can begin as deep as around 200 kilometers. And when we incorporate the effect of trace water, the magma generation depth becomes at least 250 kilometers. This does not generate a large amount, but we show the extent of magma generation is larger than previously thought and, as a consequence, it has the capacity to affect geophysical and geochemical properties of the planet as a whole."