The Chemical Architecture of the Deep Earth

The kinetic energy of accretion coupled with radiogenic heating led to the repeated and near total melting of the Earth during the first few per cent of its lifespan. The resulting magma oceans, that may have extended all the way to the core mantle boundary, solidified within a few tens of millions of years at most. The manner in which this process proceeded has substantial implications for the evolution of Earth’s unique characteristics, including its habitability. Recent theories have suggested that the large low shear velocity provinces (LLSVP) and ultra-low velocity zones (ULVZ) in the lowermost mantle may represent the final vestiges of a crystallising magma ocean. It has been further suggested, on the basis of the Earth’s Nd isotope systematics that these structures may represent an enriched chemical reservoir complementary to the accessible mantle that itself appears depleted relative to the canonical building blocks of Earth, the chondritic meteorites.

To determine the veracity of these claims, I want to answer two key questions: Can magma ocean crystallisation lead to structures like those we observe in the mantle today, and is it possible to create an enriched reservoir with the correct chemistry to explain the depleted mantle we observe?

I propose to do this with by combining novel laser-heated diamond anvil cell experiments on fully encapsulated samples with cutting edge nano-scale electron and X-ray beam techniques for chemical analysis and ab initio molecular dynamics simulations. The goal is to determine the crystallisation sequence in a chondritic magma ocean as well as the major and trace element partitioning behaviour between the coexisting solid and liquid phases, and their physical properties, including density and viscosity.

Like a newborn child, the young Earth developed fast. During its violent birth it suffered repeated giant impacts, culminating in the cataclysm from which the moon formed. The huge kinetic energies involved led to the formation of magma oceans that may have extended throughout the entire planet and the segregation, under gravity, of iron metal to Earth’s centre, creating the core. While Earth was still less than a few per cent of its current age, these magma oceans had solidified almost totally. I want to understand the details of how this happened, because they have big implications for how the Earth developed and will help to explain some of its most enigmatic features.
It is often assumed that the Earth’s building blocks are represented by a specific class of meteorites called chondrites, from which geochemists have estimated Earth’s overall composition. But there is a problem: the same geochemists, by analysing rocks exposed or erupted at Earth’s surface, have determined that the composition of Earth rocks accessible to us don’t match the chondrites. Many elements, including radioactive, heat-producing uranium (U) and the aptly named rare Earth elements (REE), such as neodymium (Nd) are depleted. What’s more, by looking at the isotopic ratio of Nd in these rocks, geochemists can show that whatever caused the depletions must have occurred within the first few per cent of Earth’s lifespan. Various theories have been proposed to explain this: perhaps the Earth wasn’t built from chondrites; perhaps these elements were partitioned into an early crust that formed on the magma ocean and were ejected into space by impacts in Earth’s infancy; or perhaps they are locked away in a ‘hidden’ reservoir deep in Earth’s mantle. This last theory has been boosted by geophysicists using seismology: they have found two vast, dense structures in the deepest mantle, one below the Pacific and one below Africa called large low shear-wave velocity provinces as well as smaller, even denser, possibly molten patches of material right at the boundary between the core and the mantle called ultra-low velocity zones. Might these structures have formed early enough in Earth’s history, as the magma ocean solidified, and then survived 4.6 billion years of vigorous convection, to become the site of the hidden reservoir?

That possibility leads to the two central questions that I want to answer: firstly, if you start with a molten, chondritic mantle, will these structures form and could they survive to the present day? Secondly, would they be sufficiently enriched in the trace elements, such as U and Nd, to explain the relative depletions we see in the rocks at the surface?

To tackle these questions, I will recreate the extreme pressures (1.3 million atmospheres) and temperatures (4000 K) encountered in the magma ocean as it cooled and crystallised in two ways: firstly in the laboratory, by laser heating samples in a device called a diamond anvil cell and then chemically analysing them at the nano-scale using cutting edge electron and X-ray beam techniques, and secondly, inside the UK supercomputer, Archer, using quantum mechanics simulations.
I’m excited about tackling these questions, because the answers have big implications for our understanding of why and how our planet developed its key features: plate tectonics, volcanism, a protective magnetic field, a clement climate and ultimately, how it became a habitable home for us.