From
Science:
Flowers and Kenneth Farley, a geologist at the California Institute of Technology in Pasadena, are employing yet another approach: using concentrations of the noble gas helium in calcium phosphate minerals called apatites found in the canyon's rocks. Helium in these minerals can fluctuate in several ways over time: For instance, concentrations increase due to the radioactive decay of uranium and thorium in the apatites. As long as the apatites remain deep within Earthat a temperature above 70Cthe helium can escape from the minerals through diffusion, Flowers explains. But as the minerals rise toward the Earth's surfaceor as erosion carves a canyon downwardthe rocks cool, trapping the helium within the apatites so that it begins to accumulate. So, helium concentrations in apatite can help scientists estimate when the rocks cooled.
In the new study, published online today in Science, Flowers and Farley analyzed four rock samples from the western portions of the Grand Canyon and four from the eastern reaches of the gorge. The pattern of helium concentrations in the samples suggests that substantial parts of the western portion of the Grand Canyon were already carved to within a few hundred meters of their current depth by about 70 million years ago and that erosion hasn't increased dramatically in recent eras, the researchers report. That's a far cry from the 5-million-to 6-million-year-old age suggested by previous research, and is about quadruple the oldest previous estimate from other teams for the canyon's age.
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Nevertheless, not everyone is convinced by the team's evidence. Karl Karlstrom, a structural geologist at the University of New Mexico, Albuquerque, describes the findings as "out in left field." His team has also analyzed helium concentrations in apatites that were collected just a couple of kilometers downstream from where Flowers and Farley collected their samples in the western Grand Canyon. And their preliminary results, Karlstrom says, bolster the notion of a young gorge. Those soon-to-be-published results suggest that those rocks were still between 50 and 60Cimplying that they were well over 1 kilometer below the surface of Earth's crustbetween 15 million and 20 million years ago.