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Kerlingarfjoll Volcano in central Iceland

Professor Calvin Miller of Vanderbilt University at the Kerlingarfjoll Volcano in central Iceland. Some geologists have proposed that early Earth may have resembled regions like this.

More about this image Conditions on Earth for the first 500 million years after it formed -- a period called the Hadean -- may have been surprisingly similar to present day, complete with oceans, continents and active crustal plates. This theory has gained substantial support based on a study by Miller and a team of researchers in which they performed the first detailed comparison of zircon crystals that formed more than 4 billion years ago with those formed contemporaneously in Iceland.

"We reasoned that the only concrete evidence for what the Hadean was like came from the only known survivors -- zircon crystals. And yet no one had investigated Icelandic zircon to compare their telltale compositions to those that are more than 4 billion years old, or with zircon from other modern environments," said Miller.

Also working with Miller was Tamara Carley, a Vanderbilt doctoral student at the time, who collected samples from volcanoes and from sand, derived from erosion of Icelandic volcanoes. Carley separated thousands of zircon crystals from the samples, which covered the islands regional diversity and represent its 18 million-year history, then analyzed about 1,000 zircon crystals for their age and elemental and isotopic compositions. She then searched literature for all comparable analyses of zircon from the Hadean period and for representative analyses of zircon from other modern environments.

"We discovered that Icelandic zircons are quite distinctive from crystals formed in other locations on modern Earth. We also found that they formed in magmas that are remarkably different from those in which the Hadean zircons grew," said Carley.

But most important was their analysis, which found that Icelandic zircons grew from much hotter magmas than Hadean zircons. Although surface water played an important role in the generation of both Icelandic and Hadean crystals, in the Icelandic case the water was extremely hot when it interacted with the source rocks while the Hadean water-rock interactions were at significantly lower temperatures.

"Our conclusion is counterintuitive," said Miller. "Hadean zircons grew from magmas rather similar to those formed in modern subduction zones, but apparently even 'cooler' and 'wetter' than those being produced today."

This research was supported in part by National Science Foundation grants (NSF) EAR 1220523, EAR CAREER 0844772 and DGE 0909667.

To learn more about this research, see the NSF News From the Field story Early Earth less hellish than previously thought. (Date of Image: September 2014)

Credit: Tamara Carley, Earth and Environmental Studies, Vanderbilt University

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