Geophysics and Tectonics Seminar fall-2020

Xiyuan Bao: Are hotspots hotter than ridges? The temperature and composition of the mantle domains sourcing hotspots and mid-ocean ridges are fundamental to understanding their origin. Are hotspots sourced from compositionally distinct reservoirs than ridges through active, therefore hotter upwellings deep in the mantle? Geochemical signals seem to indicate so. For example, primordial 3He/4He signals as high as 50 times the present atmospheric ratio (Ra) can be found in ocean island basalts (OIBs, erupted at hotspots) , and these signals are correlated with higher buoyancy flux (plume strength) and lower seismic shear-wave velocity in both the upper mantle and lowermost mantle. In contrast, mid-ocean ridge basalts (MORBs) have lower 3He/4He values around 8 Ra, which is consistent with a different source reservoir shallower than that of OIBs. Olivine geothermometry suggests 100-300 °C of excess potential temperature at hotspots compared to ridges. Here we re-examine the temperature of oceanic hotspots and ridges from seismic tomographic models, using a self-consistent velocity to temperature conversion. Assuming the upper mantle and plume source are compositionally homogeneous and consist of depleted MORB mantle (DMM), we find that up to 60% oceanic hotspots are not resolvably hotter than ridges regardless of their proximity to each other. However, hotspots with high 3He/4He and high buoyancy flux are significantly hotter than ridges. These conclusions are robust even if there are compositional differences between plume sources and DMM. /////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////// Leslie Insixiengmay: Earth’s Early Magnetic Field Powered by Exsolution of Silicates from Liquid Iron The Earth’s magnetic field is generated by the geodynamo: the process in which the rotation and convection of liquid iron in the outer core generates a magnetic field. Convection in the liquid outer core is driven by a combination of release of latent heat and gravitational energy from the inner core. Paleomagnetic observations show that the Earth’s magnetic field dates back at least 3.45 billion years. However, thermal evolution models suggest that the Earth’s inner core began to crystallize only one billion years ago. While we have a convincing explanation for what has powered the magnetosphere for the last billion years, it is not clear what powered it prior to the solidification of the inner core. Recent studies suggest that the reaction of mantle and core material from the Moon-forming giant impact could play a key role in understanding the formation of the Earth’s early magnetic field. Here, we focus on understanding silicate exsolution from liquid iron by running first-principles molecular dynamics (MD) simulations. The calculations implement density functional theory (DFT) through the Vienna ab initio Simulation Package (VASP), allowing us to observe how the system behaves at an atomic scale. We compute chemical compositions, reaction rates, reaction mechanisms, solubility, and the energy released by exsolution of silicate components from liquid iron.