Earth’s mantle is not uniform. Different zones display distinct physical and chemical characteristics, a fact that is central for tectonic activity. Specifically, a low-viscosity layer in Earth’s upper mantle is essential for plate tectonics; however, exactly how this low viscosity is maintained has remained somewhat of a mystery. Some have suggested that it is due to water-related defects in the mineral olivine, whereas others point to significant volumes of melted rock. However, neither model is fully supported by observations.
New research by DCO scientists, published recently in Nature, reconciles these models, implicating mantle carbon as a key piece of the puzzle . Seismic velocities and electrical conductivities provide insight regarding the physical properties of rocks constituting Earth’s interior. Geophysical surveys show that at 60 to 90 km depth, which corresponds to the transition from the lithosphere (rigid outer part of the mantle) to the asthenosphere (ductile inner part of the mantle), seismic velocities are considerably lowered and electrical conductivities are anomalously high. Fabrice Gaillard’s team, from the Institut des Sciences de la Terre d’Orléans (CNRS Orléans, France), performed high-temperature, high-pressure electrical conductivity measurements on volatile-rich (CO2 and H2O) melts, typically produced at the onset of mantle melting. They found that under such temperature and pressure conditions these melts are highly conductive, a small amount of which could explain observed electrical anomalies in the upper mantle. They further calculated electrical conductivity profiles across the asthenosphere for various tectonic plate ages. The resulting model predicts several electrical discontinuities that replicate geophysical observations in a consistent petrological and geochemical framework.
This new model will now allow researchers to assess the role of CO2 in non-oceanic regions of the asthenosphere, and ultimately understand the precise relationship between CO2 and H2O concentration, asthenosphere mechanical strength, and tectonic plate motion.
Image credit: Sifré D. (CNRS Orléans, France)