NSFGEO-NERC:Integrated Experimental and Dynamical Modeling of Top-down Crystallization in Terrestrial Cores:Implications for Core Cooling in the Earth

Earth's magnetic field is generated almost 3000 km below our feet in the liquid iron core by a process known as the geodynamo. The field protects the surface environment and low-orbiting satellites from solar radiation; its existence for at least the last 3.5 billion years therefore has broad implications for the presence of life and the operation of modern global communications. The standard model describing the origin of the geodynamo posits that the field is maintained by slow cooling of the liquid core below a solid mantle and gradual bottom-up freezing of the solid inner core. This model is no longer tenable following the first calculations of the thermal conductivity of iron alloys at core conditions, which predict rapid cooling, a young inner core and pervasive melting of the lower mantle early in Earth's history. In this scenario it is presently unclear how the geodynamo was powered before the inner core formed some 0.5-1 billion years ago.

Recent studies have argued that the ancient core could have crystallized from the top down. The central aim of this joint experimental-theoretical project is to understand if and how top-down crystallization generates magnetic fields and influences the thermochemical evolution of Earth's core. The project consists of two major interlinked components: experiments on core analogues and theoretical models of core evolution. Phase equilibria experiments will be carried out at pressure up to 30 GPa and temperature up to 2200 C in the multi-anvil apparatus at UCSD-SIO using NSF-COMPRES assemblies. We will consider the Fe-S-Mg(-O) and Fe-S-O(-Si) systems, building on our recent experimental work in the Fe-S-O system. Chemical analyses of quenched products will be used to determine the chemistry of phases, the liquidus curve and the eutectic temperature for the investigated systems. Results will be applied to the Earth's pressure and temperature conditions using rigorous thermodynamic extrapolation and will also be directly applicable to small terrestrial planets. In parallel we will develop a new theoretical model that describes the thermal and chemical evolution of two-phase regions at the top of Earth's core using techniques that were recently employed to study the Martian core. The model will predict the properties of the two-phase region and the evolution of the magnetic field, which can be tested using a variety of observations, and will therefore provide a coherent description of Earth's core evolution over the past 3.5 billion years.

A novel aspect of this proposal is the constant interactions between experiments and theoretical models. Laboratory-based chemistry will be used to refine the models, and numerical results will then be used to motivate new experiments at specific compositions. The proposed study will significantly improve the current understanding of core crystallization in the Earth and also in other planets such as Mercury and Mars.

NSF Broader Impacts:

1. Advance discovery and understanding while promoting teaching, training, and learning:
The research and outreach activities will contribute to the training and mentoring of one undergraduate student, one graduate student, and two postdoc researchers both in the US and the UK. A significant amount of the PI's time will be dedicated to mentoring. Results will be incorporated into teaching material with educational purposes, such as courses,
lectures or talks given by PI Pommier at UCSD and by PI Davies at the University of Leeds.

2. Broaden participation of underrepresented groups:
This project involves the training of female graduate student Ishita Pal at UCSD-SIO, who arrived directly from India in 2017 to work in PI Pommier's lab and female undergraduate student Tu Tran who joined the lab in 2018. Both UCSD and University of Leeds are equal opportunity employers.