Microscopic diamonds crack geologic mold - diamonds found in continental crust challenge geological theory

Science News, July 8, 1995 by R. Monastersky

Tiny diamond grains, discovered in rocks from southwestern Norway, are forcing geologists to rethink cherished ideas about Earth's continents.

"This is a spectacular discovery. This is a wake-up call," comments geologist Stephen E. Haggerty, a diamond expert at the University of Massachusetts in Amherst.

The diamond fragments were found by Larissa F. Dobrzhinetskaya of the Russian Academy of Science in Moscow, who collaborated with Norwegian, British, and U.S. colleagues. The team discusses its work in the July Geology.

At only 20 to 80 micrometers in size, the diamonds are too small to see without a microscope. Yet they have dazzled scientists because they formed within the continental crust, an unlikely birthplace for the world's hardest natural mineral.

According to geologic textbooks, diamonds can grow only in Earth's mantle, at depths of 120 kilometers or more. It takes the exceedingly high pressures and temperatures of the mantle--40,000 atmospheres and 900oC--to squeeze carbon into the ultracompact crystal structure of a diamond. The gems reach the surface when explosive volcanic eruptions force them up narrow conduits through the mantle and crust.

The Norwegian diamonds break the standard mold because they do not come from volcanic mantle rocks. Instead, they appear in metamorphic rocks that originally formed as ancient sedimentary deposits at Earth's surface. The layers of sediments were compacted and cooked hundreds of millions of years ago, when another continent rammed into what is now Scandinavia.

Although such continental collisions can metamorphose crustal rocks, they are considered far too docile for making diamonds. At their worst, conditions in the deep crust reach only 15,000 atmospheres and temperatures of 650oC--not nearly enough to mold carbon into diamond, says Haggerty.

Geologists have reported finding examples of diamonds in crustal rocks only twice before, in Kazakhstan in 1990 and in eastern China in 1992. While skeptical researchers may have questioned the earlier reports, the Norwegian discovery will help convince the geoscience community that diamonds can form in crustal rocks.

"This really nails it," says Haggerty.

W. Gary Ernst of Stanford University agrees. "If these are well-documented diamonds, I exult. You can't laugh it off anymore and say it's one of a kind."

To explain the diamonds' presence in crustal rocks, Dobrzhinetskaya suggests that the ancient continental collision forced pieces of the crust down to mantle depths temporarily. Carbon in the sedimentary layers then turned into diamonds before the crustal rocks rose back to the surface.

"I'm convinced that they were brought to depths of 120 km," she says.

While this theory would solve the mystery of how the diamonds formed, it raises another conundrum. Crustal rocks have a much lower density than mantle rocks; therefore, most geologists consider continental rocks too buoyant to be carried down into the mantle.

"We don't think that crustal rocks can go down and come bobbing back up, but a few of them must have," concludes Ernst.

Haggerty, however, suggests that diamonds might have formed without a trip into the mantle. Industrial researchers, he notes, have learned how to grow extremely thin diamond films at very low pressures. Because the microdiamonds from Norway, Kazakhstan, and China are so tiny, they may have formed at pressures found in the crust, Haggerty speculates.

In any case, geologists will have to rewrite some basic textbooks. "We either have a major tectonic problem, or we have an entirely new way of making diamonds," says Haggerty.