The Rise of Continents: Megan Holycross-E2

Megan Holycross, formerly a Peter Buck Fellow and National Science Foundation Fellow at the Smithsonian’s National Museum of Natural History and now an assistant professor at Cornell University, collects X-ray absorption spectra results at the Advanced Photon Source at the U.S. Department of Energy’s Argonne National Laboratory in Illinois.

Here, Holycross maneuvers an X-ray beam onto garnet samples for analysis.

A study, published today in Science, uses laboratory experiments to show that the iron-depleted, oxidized chemistry typical of Earth’s continental crust likely did not come from crystallization of the mineral garnet, as a popular explanation proposed in 2018. The iron-poor composition of continental crust is a major reason why vast portions of the Earth’s surface stand above sea level as dry land, making terrestrial life possible today.

At Argonne National Laboratory, the research team used high-energy X-ray beams to conduct X-ray absorption spectroscopy on garnets they grew in laboratory experiments. X-ray absorption spectroscopy can tell scientists about the structure and composition of materials based on how they absorb X-rays.

The results of the X-ray spectroscopy tests revealed that the experimentally grown garnets had not incorporated enough unoxidized iron from the rock samples to account for the levels of iron-depletion and oxidation present in the magmas that are the building blocks of Earth’s continental crust.