Apatite 4He/3He and (U-Th)/He Evidence for an Ancient Grand Canyon

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Science  29 Nov 2012:
DOI: 10.1126/science.1229390

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Grand Canyon is one of the most dramatic features on Earth, yet when and why it was carved have been controversial for more than 150 years. We present apatite 4He/3He thermochronometry data from Grand Canyon basement that tightly constrain the near-surface cooling history associated with canyon incision. 4He/3He spectra for eastern Grand Canyon apatites of differing He date, radiation damage, and U-Th zonation yield a self-consistent cooling history that substantially validates the He diffusion kinetic model applied here. Similar data for western Grand Canyon provide evidence that it was excavated to within a few hundred meters of modern depths by ~70 million years ago (Ma), in contrast to the conventional model in which the entire canyon was carved since 5 to 6 Ma.

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