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New contender for Earth's oldest rocks

Northern Quebec may host intact rocks as old as 4.28 billion years

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12:52pm, September 25, 2008
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Scientists may have found the world’s oldest intact rocks in a 10-square-kilometer patch of bedrock on the eastern shore of Canada’s Hudson Bay. Geochemical analyses suggest the rocks are around 4.28 billion years old, which would mean they solidified less than 300 million years after Earth formed.

If the dating holds true, the new oldest rocks could be a trove of information about geological processes during Earth’s earliest history, the researchers report in the Sept. 26 Science.

The rocks have the same chemical composition as volcanic deposits, says Jonathan O’Neil, a geochemist at McGillUniversity in Montreal and coauthor of the new study. He and his colleagues measured the ratio of two rare chemical isotopes — neodymium-142 and samarium-146 — to come up with an age estimate for the rocks. The previous oldest known rocks formed about 4.03 billion years ago and were found in what is now Canada’s Northwest Territories.

Scientists have discovered zircon crystals that are about 4.4 billion years old. However, those individual mineral grains, which are now part of much-younger sedimentary rocks found in Western Australia, originated in rocks that eroded long ago (SN: 8/2/08, p. 13). The rocks from Hudson Bay, which have been heated and squeezed deep within Earth at least once since they formed, may be the world’s oldest rocks that remain intact, O’Neil and his colleagues speculate.

Geochemical analyses of the Hudson Bay rocks are the first to show an unusually low proportion of neodymium-142 to another isotope, neodymium-144, says O’Neil. Scientists have long been looking for this signal, which indicates that the outer mantle — the layer just below Earth’s crust — had, before 4.1 billion years ago, begun to segregate into zones having different chemical compositions.

“I’m very excited about this work,” says Vickie C. Bennett, a geochemist at AustralianNationalUniversity in Canberra. “Now we’re beginning to see into the first 500 million years of Earth’s history,” she adds. Scientists can now start to assess how the geophysical processes early in Earth’s history have influenced those occurring today, she notes.

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