Speaker – David Gubbins (University of Leeds), D.Gubbins@leeds.ac.uk, David Gubbins, (hosted by the MEAS Seminar Committee/S. Bishop)

Seminar Title – What First Principles Calculations Tell Us About Earth Evolution

Abstract – The Earth formed 4,500 million years ago and rapidly differentiated into rocky crust, mantle, and iron core. It has subsequently cooled slowly by solid state convection of the mantle and freezing of the liquid core from the bottom up. Seismology, the study of elastic waves from large earthquakes, has revealed the basic divisions of crust (depth 0-50 km), upper mantle (olivine-spinel, 50-700 km), lower mantle (perovskite, 700-2890 km), liquid outer core (iron, 2890-5100 km) and solid inner core (5100-6371 km). The Earth has always had a magnetic field generated by a dynamo driven by fluid convection in the liquid outer core, which proves the Earth has been cooling at a slow but measurable rate throughout its existence. Early calculations on thermal evolution of the Earth were very simple and relied on ideal solution theory. They showed the thermal budget for maintaining the geomagnetic field was rather tight but lack of accurate numbers for the properties of silicates and iron mixtures at high temperatures and pressures prevented further progress. The accurate calculation of properties of minerals and iron mixtures at extreme pressures and temperatures has opened up whole new areas of research that were unimaginable a decade ago. Recent work on the deep mantle involves fitting its seismic properties (density and compressibility) to those calculated for candidate mineral compositions (Mg-Al-Fe silicates) at the high temperatures and pressures of the deep Earth. An important recent discovery is that of a phase change (to post-perovskite, now called Bridgemanite) near the bottom of the mantle. It was identified by seismic discontinuities, first principles calculations, and experiments in a laser-heated diamond anvil cell. This lies in the lower boundary layer for mantle convection, an important region for understanding heat loss from the core. The geodynamo is driven by a combination of heat and differentiation of light elements, notably O, Si and S, that are expelled from the liquid core on freezing to form the solid inner core. First principles calculations showed that O would not enter the solid lattice but Si and S would, making oxygen the light element driving the dynamo. This enabled more calculations on physical properties of the outer core, matching the density with the seismological estimates, melting temperatures and, most importantly, the thermal and electrical conductivities. These had been extrapolated from experiments and calculations done at much lower pressures and temperatures than pertain in the core. The new calculations gave values some 2-3 times higher, meaning the Earth was cooling more rapidly and the solid inner core freezing more rapidly than previously thought. The inner core is now known to be very young, perhaps forming 500 million years ago rather than just after Earth formation. This raises the question: what drove the geodynamo before that date? One idea is separation of other phases from the outer core mix. The current view of Earth evolution is more complicated, and more interesting, than it was 10 years ago. The mantle has many minor phase changes at different depths. Studies are possible of a time in the past when part of the lower mantle was molten, and a layer today of partial melt around the inner core. Temperature variations at the base of the mantle influence the form of the geomagnetic field today and in the past.