• DOI:10.1126/science.1229390
• Corpus ID: 32277486

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

@article{Flowers2012Apatite4A,
  title={Apatite 4He/3He and (U-Th)/He Evidence for an Ancient Grand Canyon},
  author={Rebecca M. Flowers and Kenneth A. Farley},
  journal={Science},
  year={2012},
  volume={338},
  pages={1616 - 1619}
}

• R. Flowers, K. Farley
• Published 21 December 2012
• Geology
• Science

A Grand Old Canyon In the southwestern United States, the Grand Canyon is a striking example of the power of erosion over time. Over millions of years, flowing river water carved out the canyon that today measures over 1.6 km deep and 29 km long. Most models posit that the majority of the canyon formed 5 to 6 million years ago. Using thermochronometry, Flowers and Farley (p. 1616, published online 29 November) examined the temperature-dependent diffusion of helium within mineral grains representative of the canyon basement, which cools as erosion brings crustal rocks near the surface. After validating the approach across the younger eastern canyon, the model suggests that the western canyon experienced an ancient cooling event induced by erosional processes, such that the canyon likely reached near modern depths by 70 million years ago—nearly 60 million years earlier than generally believed. The Colorado River carved the Grand Canyon to nearly its modern depth 60 million years earlier than was generally believed. The Grand Canyon is one of the most dramatic features on Earth, yet when and why it was carved have been controversial topics for more than 150 years. Here, we present apatite 4He/3He thermochronometry data from the 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 the 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.