Barrage of meteorites forged Earth's oldest rocks

Aug. 16 (UPI) -- Analysis of the world's oldest rocks suggests the silica-rich formation was forged at extremely high temperatures and surprisingly close to Earth's surface.

The findings, published this week in the journal Nature Geoscience, suggests the ancient rock formation was forged by a barrage of meteorites more than 4 billion years ago.

Earth's oldest known rocks are felsic rocks, rocks rich in silica and quartz. The ancient felsic formation is unique among the crustal rocks found in Acasta River, Canada, dominated by dark, silica-poor mafic rocks.

"Our modelling shows that the Acasta River rocks derived from the melting of pre-existing iron-rich basaltic rock, which formed the uppermost layers of crust on the primitive Earth," Tim Johnson, a researcher at Curtin University in Australia, said in a news release.

Modeling experiments showed only an extreme collision could explain the rocks' compositional transformation.

"We used phase equilibria and trace element modelling to show that the Acasta River rocks were produced by partial melting of the original mafic rocks at very low pressures," Johnson said. "It would have needed something special to produce the 900 degrees Celsius temperatures needed to generate these early felsic rocks at such low pressures, and that probably means a drastic event, most likely the intense heating caused by meteorite bombardment."

At the time of their origin, the felsic rocks would have been abundant, researchers surmise. But 4 billion years of tectonic activity has destroyed most of the ancient rocks.

"We believe that these rocks may be the only surviving remnants of a barrage of extraterrestial impacts which characterized the first 600 million years of earth history," Johnson said.

Johnson and his research partners presented their findings this week at the Goldschmidt Conference in Boston.