New research by Brown University geochemists, including doctoral student Boda Liu, provides new insights on the scale at which Earth’s mantle varies in chemical composition. The findings could help scientists better understand the mixing process of mantle convection, the slow churning that drives the movement of Earth’s tectonic plates.
“We know that the mantle is heterogeneous in composition, but it’s been difficult to figure out how large or small those heterogeneities might be,” said Liu, a Ph.D. student in geology. “What we show here is that there must be heterogeneities of at least a kilometer in size to produce the chemical signature we observe in rocks derived from mantle materials.”
The research, which Liu co-authored with Yan Liang, a professor in Brown’s Department of Earth Environmental and Planetary Sciences, is published in Science Advances.
Earth’s crust is on a constantly moving conveyer belt driven by the convecting mantle. At mid-ocean ridges, the boundaries on the ocean floor where tectonic plates are pulling away from each other, new crust is created by eruption of magmas formed by the rising of the mantle materials from depth. At subductions zones, where one tectonic plate slides beneath another, old crust material, weathered by processes on the surface, is pushed back down into the mantle. This recycling can create mantle materials of different or “enriched” compositions, which geochemists refer to as “heterogeneities.” What happens to that enriched material once it’s recycled isn’t fully understood.
“This is one of the big questions in Earth science,” Liang said. “To what extent does mantle convection mix and homogenize these heterogeneities out? Or how might these heterogeneities be preserved?”
Read more of Kevin Stacey's story about the composition of the Earth's mantle.