URI scientist: Oxidized lava at subduction zones sheds light on evolution of Earth
Todd McLeish, 401-874-7892
NARRAGANSETT, R.I. – August 18, 2009 – A University of Rhode Island marine geologist used highly sensitive x-ray techniques to analyze the oxidation of lava from the Earth’s mantle and has shed light on processes involved in the evolution of the planet.
URI Assistant Professor Katherine Kelley found that material from subduction zones – sites on the seafloor where tectonic plates have collided, forcing one plate beneath the other – are more oxidized than material from mid-ocean ridges where the plates are pulling apart.
The results of Kelley’s research were published in the July 31 issue of the journal Science.
“The cycling of oxygen at the Earth’s surface is central to the life and activity that takes place at the surface, but it is equally essential in the Earth’s mantle,” said Kelley. “The availability of oxygen to the mantle is in part controlled by the oxygen at the surface.”
Kelley said that this discovery is important because the availability of oxygen to the mantle controls what minerals are found there, how certain elements behave, and what kind of gasses might be expelled from volcanoes.
Kelley and her co-author Elizabeth Cottrell of the Smithsonian National Museum of Natural History use an analogy with rust to explain the process.
At the surface, the oxidation of metals causes rust. In a similar way, the scientists said, when the oceanic crust interacts with the oxygen in seawater, the crust becomes rusted. At subduction zones, that rust is slowly carried into the Earth’s interior over millions of years in what Cottrell has described as “a rust conveyor belt.”
This discovery suggests that what takes place at the surface of the Earth probably influences what happens deep beneath the surface as well. It also contributes to a long-term debate about Earth’s evolution.
“Some scientists have argued that the availability of oxygen to the mantle hasn’t changed since the Earth was formed,” Kelley said. “But if plate tectonics carries rust into the mantle, as we have found, that process is adding oxygen to the mantle. We’ve linked plate tectonics to elevated oxidation levels in the mantle.”
To shed light on this phenomena, Kelley and Cottrell used a new analytical technique called micro x-ray absorption near-edge structure spectroscopy to determine the oxidation state of the lavas. Using this technique, they shined a narrow x-ray beam into tiny crystals from the Earth’s mantle that contain naturally-formed glass from the early histories of magmas, in which are found dissolved gases from volcanic eruptions. By analyzing the glass they determined the oxidation state of iron in lavas from different tectonic settings and related it to the dissolved gases, which are elevated in subduction zone magmas.
The next step in the scientists’ research is to expand their analysis to include lava samples from locations other than subduction zones or mid-ocean ridges, like mantle hotspots around Hawaii and Samoa.
“These are important processes to understand, but they are hard to get a clear picture of because they take place over such long periods of time,” Kelley said. “It’s one piece of the big puzzle of Earth’s evolution and how it continues to change.”
This research was funded by grants from the National Science Foundation and the U.S. Department of Energy.
Image: Mantle oxidation, a crystal sample from mid-ocean ridge lava. K.Kelley