Monday, November 21, 2005

Early Earth Likely Had Continents

Heterogeneous Hadean Hafnium: Evidence of Continental Crust at 4.4 to 4.5 Ga. T. M. Harrison, Janne Blichert-Toft, W. Müller, Framcis Albarede, Peter Holden, and Stephen J. Mojzsis. Published online November 17 2005; 10.1126/science.1117926 (Science Express Reports)

The long-favored paradigm for the development of continental crust is one of progressive growth beginning at ~4 Ga. To test this hypothesis, we measured initial 176Hf/177Hf values of 4.01 to 4.37 Ga detrital zircons from Jack Hills, Western Australia. Hf values show large positive and negative deviations from Bulk Earth. Negative values indicate development of a Lu/Hf reservoir consistent with formation of continental crust (Lu/Hf 0.01), perhaps as early as 4.5 Ga. Positive Hf deviations require early and likely widespread depletion of the upper mantle. These results support the view that continental crust had formed by 4.4–4.5 Ga and was rapidly recycled into the mantle.
A surprising new study by an international team of researchers has concluded Earth's continents most likely were in place soon after the planet was formed, overturning a long-held theory that the early planet was either moon-like or dominated by oceans.

The team came to the conclusion following an analysis of a rare metal element known as hafnium in ancient minerals from the Jack Hills in Western Australia, thought to be among the oldest rocks on Earth. Hafnium is found in association with zircon crystals in the Jack Hills rocks, which date to almost 4.4 billion years ago.

"These results support the view that the continental crust had formed by 4.4-4.5 billion years ago and was rapidly recycled into the mantle," the researchers wrote in Science Express. Led by Professor Mark Harrison of the Australian National University, the team also included University of Colorado Assistant Professor Stephen Mojzsis and researchers from the University of California, Los Angeles and Ecole Normale Superieure University in France.

The Kirby-Vision Goggles also see the past!

The researchers used hafnium as a "tracer" element, using isotopes to infer the existence of early continental formation on Earth dating to Hadeon Eon, which took place during the first 500 million years of Earth's history, said Mojzsis, an assistant professor of geological sciences at CU-Boulder. Mojzsis also is a member of CU-Boulder's Center for Astrobiology.

"The evidence indicates that there was substantial continental crust on Earth within its first 100 million years of existence," said Mojzsis. "It looks like the Earth started off with a bang."