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Nothing Says "Early Earth Was Cool" Like World's Oldest Diamonds

The zircon in imitation diamonds proves the best way to preserve more than four-billion-year-old versions of the real thing
diamonds



©ISTOCKPHOTO.COM/HENRY CHAPLIN

Earth is roughly 4.5 billion years old, but her early eons were tempestuous. Not even rock survives from the first 500 million years of her life—an eon known as the Hadean—because geologists speculate the planet's surface boiled and bubbled with molten lava under a steady bombardment of comets and meteorites. But tiny diamonds discovered in antediluvian zircon crystals sprinkled in three-billion-year old rocks from Australia hint that the planet's surface fire might have ceased much earlier than previously believed.

Mineralogy graduate student Martina Menneken of the Westfälische Wilhelms–University of Münster in Germany and her colleagues probed 1,000 of these ancient zircon crystals for inclusions—tiny outcroppings of other minerals hidden in the unusually stable lattice. They discovered diamonds of different shapes and sizes in 45 of the old crystals by using a laser technique called Raman spectroscopy.

"The biggest [diamond] we found was about 60 microns [(roughly 0.002 inches, or 35 times smaller than the head of a pin)] but some only were about seven microns," Menneken says. But their sizes or shapes notwithstanding, all of the diamonds are unique, she adds, because they come from zircon grains that can be dated (via the decay of uranium impurities into lead) to as long ago as 4.25 billion years, a scant 250 million years after Earth formed.

Zircon crystals can form in a number of ways—on the moon, for example, the mineral crystallizes in the wake of a meteorite impact. But some geologists suspect, based on minerals and oxygen isotope levels in the current grains, that they crystallized in an ancient crust that formed from cooling granite magmas. Yet diamonds only form when high pressure squeezes graphite into exquisite clarity—more than 45,000 bars of pressure only found at depths of 100 kilometers (62 miles) or more below the surface.

"Options are shock, bolide [meteorite] impact or burial" for how the diamonds formed, says geologist Ian Williams of The Australian National University in Canberra. "The key question is whether the zircon grew around the diamonds or the diamonds grew in the zircons."

Because zircon crystals can survive even the dissolution of their host rock by weathering, it is possible that the grains formed in the surface rock, which then disappeared below newer surface rock before resurfacing as their latter hosts' outer layers wore away. The researchers are searching for other signs of the history of these grains, such as coesite, a dense form of silicon dioxide created under similar pressure. "We might have had coesite that is totally transformed to quartz now," which is relatively common in the grains, Menneken says. Because coesite is "not stable at surface conditions."

"According to our findings, probably the early Earth might have been a quiet, cool, habitable place," Menneken says. But more research is needed to say for sure, she notes, such as studies of the carbon isotopes and nitrogen content of the diamonds to determine how long the little gems experienced extreme pressures and temperatures.

"A likely explanation is that the diamonds were formed from graphite inclusions in the zircons during an event when the zircons were buried. If that was so, then the burial must have postdated the youngest zircon in which a diamond was found, about 3.1 [billion years ago]," Williams says. But "if the zircons grew around diamonds as early as 4.2 billion years ago, this implies that there was a very thick crust on the Earth by that time." And that means the early Earth may have cooled sufficiently for a host of other adornments, including life.

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