A new technique developed by Carnegie's Yingwei Fei and Lin Wang provides fresh insight into the process by which the materials that formed Earth's core descended into the depths of our planet, leaving behind geochemical traces that have long mystified scientists. Their work is published by Science Advances.
Earth accreted from the disk of dust and gas that surrounded our Sun in its youth. As Earth grew from smaller objects over time, the densest material sank inward, separating the planet into distinct layers -- including the iron-rich metal core and silicate mantle.
"The segregation of the core and mantle is the most important event in the geologic history of Earth," explained Fei. "Convection in the outer core powers the Earth's magnetic field, shielding us from cosmic rays. Without it, life as we know it could not exist."
Each of our planet's layers has its own composition. Although the core is predominantly iron, seismic data indicates that some lighter elements, like oxygen, sulfur, silicon and carbon, were dissolved into it and brought along for the ride into the planet's center. Likewise, the mantle is predominately silicate, but its concentrations of so-called "iron-loving," or siderophile, elements have mystified scientists for decades.
"Understanding the mechanisms by which materials migrated through these layers, and identifying any remnants of this process, will improve our knowledge of the various ways Earth's core and mantle have interacted throughout its history," Wang added.
In the lab, Carnegie scientists use heavy hydraulic presses, like the ones used to make synthetic diamonds, to bring samples of material to high pressures, mimicking the conditions found in Earth's interior. This enables them to recreate Earth's differentiation process in miniature and to probe different possible ways by which the core was formed.
Using these tools, Wang and Fei developed a new method of tracing the movement of the core-forming liquid metal in their sample as it migrated inward. They showed that much like water filtering through coffee grounds, under the dynamic conditions found on early Earth, iron melts could have passed through the cracks between a layer of solid silicate crystals -- called a grain boundary -- and exchanged chemical elements.
Wang and Fei suggest that the violent environment of early Earth would have actually created the circumstances that would turn the mantle into a giant "pour over" coffee apparatus, allowing percolation of liquid metal through an interconnected network. They analyzed the chemical exchanges during this percolation process. Their results would account for iron-loving elements being left behind in the mantle, shedding light on a longstanding geochemistry question.
Looking ahead, Wang and Fei believe their new technique is generally applicable to studying other rocky planets and can help answer more questions about the core and mantle interactions occurring deep in their interiors.
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