Volcanic Diamonds
A possible answer to why diamonds actually rise in the magmas or kimberlites as they are known, from deep under the earth has recently emerged from research carried out with high temperature experiments.
Scientists have recently found that magmas rise to the surface by jettisoning weight. The deepest magmas, kimberlites, are the deepest of all magmas and, although they gold crystals and sometimes diamonds, rise more rapidly than other magmas.
To find out why this happens and why diamonds are not melted during the process scientists at the University of British Columbia in Vancouver have carried out some experiments.
According to Wired Science, “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.”
“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.
References:
http://news.sciencemag.org/
http://www.wired.com/wiredscience/2012/01/how-diamonds-rise/
http://www.msnbc.msn.com/id/46044094/ns/technology_and_science-science/#.Txy7-oHYWlZ