A Better Understanding of Earth’s Geological Evolution
News Jan 20, 2015
Changing conditions on the Earth’s surface can have a major influence on the composition of its overwhelmingly more massive interior.
Ken Sims, a University of Wyoming professor of geology and geophysics, has helped make that evolutionary and geological process more accurate and understandable - using new instruments to more precisely measure a uranium element carbon dating ratio of 238U/235U.
“The timing and the extent that the material is put back down into the Earth’s mantle is addressed by this ratio in a unique way that was not possible a decade ago,” Sims says. “Our ability to make these isotopic measurements is so greatly enhanced.”
Sims is a co-writer of a paper, titled “The Terrestrial Uranium Isotope Cycle.” The paper appears in the Jan. 15 issue of Nature, an international weekly journal of science that publishes the finest peer-reviewed research in all fields of science and technology.
“This paper is important because the Earth is dynamic. It is always differentiating and remixing. We see that in plate tectonics,” Sims says. “Thorium and uranium are particularly important chemical elements for understanding these processes. We’ve used them for many years to understand the time scales of these processes.”
Sims and the paper’s other writers - including Tim Elliott, a longtime colleague from the Bristol Isotope Group at the University of Bristol, United Kingdom - took a number of rock samples, collected from around the globe during the past 30 years, and used them to make very precise measurements with uranium isotopes.
These more precise measurements are obtainable because today’s mass spectrometers are so much better and can measure out a few more decimal points than previous versions of the instrument were capable of, Sims says.
“We can now routinely measure sub per million variations of the 238U/235U isotope ratio, which we used to think was constant in the earth at 137.88,” he adds.
Mass spectrometry is an analytical chemistry technique that helps identify the isotopes of chemical elements present in a sample.
“With these measurements, we are able to put a better understanding on the recycling of material back into the Earth’s mantle - the area between the Earth’s core and its crust - back into the deep earth,” Sims says. “That’s because these uranium isotopes are perturbed or changed by the surface processes that are happening. So, we can look at rocks that are coming back out of the mantle and understand this recycling as well as this differentiating of the early Earth.”
With the use of a submersible called Alvin, many of the rock samples were collected on mid-ocean ridges like the East Pacific. Mid-ocean ridges are underwater mountain systems that are composed of various mountain ranges.
These ridges typically have a mountain valley - known as a rift - that runs along its spine and is formed by plate tectonics. Mid-ocean ridges are geologically active, with new magma constantly emerging on the ocean floor and spreading away from the ridge. The crystallized magma forms a new crust of basalt, known as mid-ocean ridge basalt or MORB.
Ocean island basalts (OIBs), from places like Hawaii, Iceland, Canary Islands (La Palma), Azores (Pico and Sao Miguel islands), Society Islands and Samoa, also were analyzed. OIBs are basaltic rocks found on many volcanic islands away from tectonic plate boundaries typically associated with hot spots.
Other co-writers of the paper include researchers from the University of Bristol; Durham University, United Kingdom; the Institute of Geochemistry and Petrology at ETH Zurich, Switzerland; and Rhode Island University.
“Because of this system, we understand the evolution of Earth better,” Sims says. “That is, we can address how material that has been oxidized on the surface of the Earth has been recycled back in to the mantle.”