Not Forever-

But Close Enough.

The history of diamonds goes way back and way, way down : Millions of years ago, some of our planet's strangest and most volcanic outbursts dredged from deep underground diamonds that are found today in places like eastern Siberia.

Unlike the volcanoes that commonly pop up near the thinner edges continents, the furious eruptions that produced such diamonds, the blue-tinged rocks called kimberlites, came through the stable continental cores, known as cratons.

The eruptions start near the cratonic roots, at least 75 miles underground [ 120 kilometers ].

Scientists have long known that formation of kimberlites coincided roughly with the breakup of ancient supercontinents. But that relationship has been poorly understood.

A study led by Thomas Gernon, a geologist at the University of Southampton in England, says that the breakup of supercontinents like Pangaea and Rodinia caused deep disruptions in the mantle beneath Earth's crust.

But they found an odd delay of about 26 million years between the breaking up of a supercontinent and the eruptions that sent kimberlites to the surface.

And while the kimberlite formation has been associated with magma plumes, the rocks '' have no whiff of plume in their chemistry, '' Dr. Gernon said.

The team's computer modeling suggests that as continents pull apart, churning of the hot mantle heats and tugs on the root, or keel, of a continental core.

The melting keel, rich in carbonates and water, melts just enough to form a magma similar to kimberlites and drips downward.

The turbulence may help explain why many kimberlites seem to mitigate toward continental interiors over such a long period. [ Maya Wei-Haas]


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