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Jack Hills Zircons Really Are as Old as They Seem

By February 24, 2014

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zirconThe 3-billion-year-old sandstone of the Jack Hills area, in Australia, is renowned among geologists for the microscopic grains of zircon it contains. Zircon is a highly resistant mineral that can be recycled many times, in one sedimentary rock after another, for billions of years. A few of the Jack Hills zircons are the oldest Earthly objects known, even older than the world's oldest rocks, the Acasta Gneiss from northern Canada. A paper published yesterday in Nature Geoscience features one particular zircon grain less than half a millimeter long with the laboratory name 01JH36-69. It was previously shown to be almost 4.4 billion years old, a record age.

But this paper is really about the reliability of the ages we've been determining in these extremely small and ancient objects. Crystal 01JH36-69 was painstakingly analyzed by atom-probe tomography: needles of zircon just 1 micron long and one-tenth as thick were cut out and then mapped with nanometer precision by essentially picking them apart, atom by atom. (The supplementary information to the paper has 3D files you can view to see the results.) Inside those needles are even tinier blobs in which the mix of elements differs from their surroundings. The authors show, essentially, that these blobs are too small to cast doubt on the age of the zircon, which makes it possible to confirm that Jack Hills zircons are truly the record-holding ages they appear to be. In turn, that means the first crustal rocks typical of continents, and oceans to go with them—which means conditions capable of supporting life—existed very soon indeed after our planet's fiery birth.

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