Scientists know little about the Hadean eon, the tumultuous first 600 million years of Earth’s history, when its molten surface was bombarded with chunks of planetary debris. Now, the materials at the surface have long been recycled into new rocks through the actions of plate tectonics. But tiny capsules from that time period remain today in the form of hardy crystals of zircon and olivine, which can reveal details of the early years of Earth’s evolution.
In a new project called Monitoring Earth Evolution Through Time (MEET), DCO members will use bits of once-melted rocks trapped inside those ancient crystals, called melt inclusions, to reconstruct Earth’s history all the way back to the Hadean. Reservoirs and Fluxes Community members Alexander Sobolev (University Grenoble-Alpes, France), Stephan Sobolev (German Research Center for Geosciences (GFZ), Germany), and John Valley (University of Wisconsin, Madison, USA) have teamed up to explore the processes that shaped Earth’s chemical composition from 4.4 billion years ago until the present. The team received a European Research Council Synergy grant of about $14.2 million over the next six years to support the research.
“The most sensitive tracers of evolution – volatiles and mobile elements - are not accessible in old rocks,” said Alexander Sobolev, because these elements, like carbon, don’t stay put in the minerals. To compensate for loss of these tracers in rocks, the researchers will look specifically at the compositions of melt inclusions from the crust and the mantle. These tiny pockets of material are just 15 microns across – much smaller than the thickness of a human hair – but contain small amounts of water and other compounds that offer a window into Earth’s surface environment millions of years ago.
Using the secondary ion mass spectrometer located at the University of Wisconsin and complementary instruments at the Université Grenoble-Alpes, the researchers will analyze the composition of the melt inclusions, measure their isotopic signatures, and see how they have changed over time. Then, they will put all the data together, and with the help of GFZ geodynamic modelers, develop a geophysical framework that explains how plate tectonics and other geological processes have altered Earth’s composition.
“All together, this approach will give us full coverage of the Earth’s history from the very beginning to now,” said Sobolev.
The project builds on two recent pilot studies. One used similar approaches to those proposed in MEET to find evidence that subduction or an earlier system for recycling ocean crust was already in place by 3.3 billion years ago, while the second found that surface processes, like erosion, helped kickstart plate tectonics.
Besides offering insights into how the global carbon and water cycles evolved, the new research will address the question of when plate tectonics began. “Some people think that it started very early, from the beginning of the Hadean. Some think it started only 1 billion years ago. Many people think it started somewhere around 3 billion years,” said Sobolev, “but nobody knows for sure.”
Main image: An artist's rendition of young Earth being bombarded by asteroids. Credit: NASA's Goddard Space Flight Center Conceptual Image Lab