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Earth's plate boundaries may nurture diamond formation

High-pressure, high-temperature experiment supports idea about gems’ origins

DEEP DEVELOPMENT  Diamonds (dark grey) begin to form from minerals containing iron and magnesium (white) and from carbon-oxygen compounds called carbonates (light grey) in a scanning electron microscope image. 

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Boundaries between tectonic plates may make ideal diamond nurseries, according to an experiment that mimics conditions deep in the Earth.

Diamonds form only at temperatures and pressures far greater than those on Earth's surface. Scientists have long thought diamonds could form in subduction zones, where one tectonic plate plunges under another and sinks hundreds of kilometers into the mantle.

To simulate this extreme environment, scientists led by Yuri Palyanov of the Russian Academy of Sciences in Novosibirsk put minerals that are common in Earth's crust into a chamber where the researchers could apply intense pressures and temperatures. The team found that at around 74,000 times atmospheric pressure and above 1,200° Celsius, diamonds crystallized.

The exact makeup of the diamonds depended on which mineral the carbon came from. Diamonds forming from minerals in which carbon bound to both oxygen and metals had around 10 times as many nitrogen impurities as ones forming from minerals in which carbon simply bound to metals.

The team reports its findings December 2 in the Proceedings of the National Academy of Sciences.

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