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Tiny diamonds yield new geological insights


Tiny diamonds yield new geological insights

(February 26, 2001)

Diamonds, long the ultimate symbol of love between two people, are now giving geologists new insights about the interactions between the Earth's crust and interior.

The story is told inside so-called microdiamonds - minerals so small they can't be seen without the aid of a specialized electron microscope.

Writing in the March issue of Geology, scientists from the University of California, Riverside and Los Alamos National Laboratory report their findings that microdiamonds - known to occur in only five locations around the world - form in conditions very different from those of traditional gem-quality diamonds. If their model of microdiamond formation is correct, geologists will need to rework one of the major tenets of plate tectonics, the theory that describes the behavior of the Earth's thin outer crust and its interior.

Diamonds of gem quality and size are believed by scientists to form in the Earth's mantle, at depths more than about 75 miles below the surface. Comprised of a single element, carbon, which crystallizes into a very strong, stable cubic structure, diamonds have been transported to the surface during volcanic eruptions through narrow channels called kimberlite pipes.

Since they were discovered in the mid-1970s in Kazakhstan, and later in areas of Norway, China, Germany and Indonesia, microdiamonds have puzzled geologists, according to lead author Larissa Dobrzhinetskaya, an adjunct professor of earth sciences and geophysicist in UCR's Institute of Geophysics and Planetary Physics. Microdiamonds are found associated with rocks on the Earth's surface in areas of ancient continental collisions, not unlike the present-day collision between India and Asia that continues to uplift the Himalaya mountain chain.

According to the theory of plate tectonics, the Earth's outer "lithosphere" is composed of about a dozen large plates that carry the continents and ocean basins and which sit atop the Earth's mantle - a layer that extends between about 24 miles below the surface to more than 1,800 miles. When two plates collide, the heavier of the two is thrust, or subducted, beneath the other, back into the mantle where temperatures and pressures are high enough to form diamonds.

"The microdiamonds were discovered within crustal rocks, which until recently was considered not possible because the laws of physics hold that they are too light to be subducted," Dobrzhinetskaya said. "So we are dealing with a different phenomenon."

Dobrzhinetskaya and her colleagues - UCR Professor of Geophysics Harry W. Green II and scientists Terrence E. Mitchell and Robert M. Dickerson of the Center for Materials Science at Los Alamos National Laboratory - examined mineral inclusions within the microdiamonds. The inclusions serve as an imprint of the environmental conditions under which microdiamonds were formed. They found the minerals had an imperfect crystalline arrangement, resembling a skeletal or rose-like structure.

That suggests the microdiamonds formed in an impure environmental, "meaning drops of dense hydrous fluid oversaturated with carbon crystallized diamonds in an environment laden with other minerals," Dobrzhinetskaya said. Among those other minerals were silicates, oxides and carbonates typical of crustal rocks, mixed with chromium oxide minerals from the mantle, she said.

Dobrzhinetskaya and her colleagues have proposed a scenario in which surface sedimentary rocks such as shale and sandstone, as well as organic matter rich in carbon, were enveloped in denser rocks and subducted to a depth of about 75 miles. There, temperatures and pressures are high enough to stabilize the carbon in a diamond form. The relatively light rocks then rose back to the surface of the Earth. The entire process occurred over the course of about 15 million years.

If the diamond growth model proposed by Dobrzhinetskaya and her colleagues proves valid, it may provide a model for the growth of other diamonds - such as those included in rocks called eclogites - that are thought to have grown in subducted material.

The University of California, Riverside (www.ucr.edu) is a doctoral research university, a living laboratory for groundbreaking exploration of issues critical to Inland Southern California, the state and communities around the world. Reflecting California's diverse culture, UCR's enrollment has exceeded 21,000 students. The campus opened a medical school in 2013 and has reached the heart of the Coachella Valley by way of the UCR Palm Desert Center. The campus has an annual statewide economic impact of more than $1 billion.

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