Oxygen in the earth’s mantle is revealing clues to the history of tectonic plates, says new UO research. A study from UO geoscientist Ilya Bindeman’s lab suggests that plate tectonics as we know it today began around 2.5 to three billion years ago.

Bindeman and his colleagues published their findings July 4 in Nature Communications.

Earth’s crust and its rocky upper mantle are broken up into chunky plates, like a crumbled cookie. The plates sit on top of the softer lower mantle, which allows them to drift and bump against each other over time. The geological processes that formed the plates have left chemical signatures that can be read by scientists to piece together their history.  

For instance, like many elements, oxygen comes in heavier or lighter varieties, called isotopes. Because the relative prevalence of particular isotopes depends on factors like temperature and pressure, the levels of these different isotopes in rocks can be a clue to past conditions on Earth. Bindeman and his lab use these different oxygen isotopes to understand changes in the Earth’s composition over time.

Here, the team analyzed oxygen isotopes in olivine, a mineral that makes up much of the rocky part of the mantle. They measured the levels of different oxygen isotopes in olivine samples that formed at different times. They found a gradual decrease in the heavier oxygen isotope over time, starting around three billion years ago.

Fluids with lower levels of the heavier oxygen isotope could have infiltrated the upper mantle after plate tectonics began, Bindeman suggests. Over time, that lowered the proportion of heavy oxygen compared to lighter oxygen in the mantle. The findings could also help explain why heavy oxygen has progressively accumulated in the crust over the past three billion years.