By Sid Perkins, ScienceNOW
Scientists have long known that a diamond's trip from deep below Earth's surface must be quick indeed: Lab tests show that at conditions found in the crust, the gems would burn up in a matter of days, if not hours. New experiments reveal the chemical secret behind such rapid ascent. The eruptions of diamonds to Earth's surface may be driven by massive quantities of carbon dioxide fizzing from the molten rock that surrounds the gems.
Many diamonds are embedded in a dense volcanic rock called kimberlite, which gets its name from the town of Kimberley, South Africa, where several of the world's first diamond mines were discovered. It's difficult to explain how relatively heavy, crystal-rich magma becomes buoyant enough to rapidly rise through Earth's crust, so researchers have long suspected that volatile substances dissolved in the rock, such as water and carbon dioxide, play a major role in kimberlite eruptions, says Kelly Russell, a volcanologist at the University of British Columbia in Vancouver, Canada. Nevertheless, scientists have been baffled about how and why these substances begin to froth out of material in the mantle. Pressures there are typically so high that they would keep gases locked in the molten rock, just as pressure keeps carbon dioxide dissolved in a carbonated drink.
New lab tests by Russell and his colleagues provide hints about how the fizz gets started. The experiments show that in molten rock that's rich in carbonates, carbon dioxide is exceptionally soluble. But the researchers found that in molten rock that's rich in silica, carbon dioxide is only between one-fourth and one-third as soluble, regardless of the pressure. In the team's early tests, the researchers used a salt shaker to sprinkle a silica-rich mineral called orthopyroxene onto a puddle of molten, carbonate-rich rock. As the mineral dissolved into the puddle over the course of 20 minutes or so, the carbon dioxide vigorously bubbled out: "It foamed right in front of our eyes," Russell says. "It blew me away."
The lab tests mimic what goes on in the earliest phase of a kimberlite eruption deep inside Earth, the researchers speculate. First, a pocket of carbonate-rich molten rock comes into contact with silica-rich minerals somewhere in the upper mantle, where rocks contain between 15% and 27% orthopyroxene. Carbon dioxide fizzes out of the molten material, rendering the dense magma buoyant. As the magma surges upward from the upper mantle at speeds up to 14 kilometers per hour, it pummels its way into overlying rocks that contain even more silica, which accelerates the fizzing even further. At such rates, the frothy kimberlite lava could reach Earth's surface from a depth of up to 120 kilometers in between 3 and 8 hours, Russell estimates.
The chemical reaction that drives the fizzing is largely self-sustaining, Russell says. The heat needed to keep the reaction going comes from the crystallization of other minerals such as olivine, he notes.
"This is an excellent paper that really helps fill in some important parts of the kimberlite puzzle," says James Head III, a planetary geologist at Brown University. For instance, because kimberlites are readily eroded and easily altered by long-term exposure to the elements at or near Earth's surface, clues about the original chemical composition of kimberlites in their molten state are rare.
Also, he adds, the process described by Russell and his colleagues nicely complements a model of kimberlite eruptions that Head and his colleagues set forth early in 2007. In that model, due to dramatic changes in pressure as the kimberlite magma rose, the material became less buoyant and therefore slowed down as it approached Earth's surface. But the new model provides for increasing buoyancy as the eruption continues—a very important factor, Head says, that ensures diamonds survive their trip through the crust to adorn ring fingers and necklines worldwide.
This story provided by _Science_NOW, the daily online news service of the journal Science.
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