Results: “Sinking” Metal
Following the experiments, each source metal was found in the sink layer, proving that the metals could in fact find a pathway through the mantle rock that is believed to be impenetrable by some scientists.
Hayden and Watson hypothesize that the metal atoms move along the surfaces formed between adjacent grains of the mantle rock. Like a sugar cube, mantle rocks are comprised of individual crystals squeezed tightly together into a larger structure. The atoms of the core metals are too large to diffuse through the structured arrangement of atoms that make up an individual crystal or grain of rock. But, the boundary between each grain is less crowded with atoms, according to the researchers, and could be a fast pathway for metals to migrate between the mantle and core.
“[In our experiments], some of the metals moved through grain boundaries at surprisingly fast rates about as fast as sodium ions move through water,” Hayden said. “This shows that metals can in fact travel over great distances through mantle materials. Over geologic time, this diffusion of metals could have a significant impact on their distribution in the Earth.” Their experiments revealed that some elements could move up to 100 kilometers through the Earth’s mantle in a billion years.
The findings have implications for the field, but also for broader economic reasons, Hayden explains. If these metals are able to move out of core and into the mantle as their findings suggest, they would enter the geologic upwelling of mantle convection and could be gradually moved toward Earth’s surface, potentially leading to valuable deposits. “As we learn more about the movement of precious and base metals through the Earth, we could at some point find out how they are deposited, where, and why,” she said.
The research was funded by the National Science Foundation.
Photo credit: RPI/Leslie Hayden