What do pallasite meteorites tell us about core formation and the evolution of Earth’s mantle?

A sample of the pallasite meteorite Conception Junction from the University of Leeds pallasite collection

Project synopsis: It is now widely believed that chemical heterogeneity in the core-mantle boundary region profoundly influences the dynamics and evolution of Earth’s core and mantle, and the behaviour of the geomagnetic field. However, we presently don’t know much about what chemical species are responsible for this heterogeneity, where exactly they are located and what their size might be. For example, tantalizing hints of geochemical heterogeneity in the outermost core are given by anomalous seismic signals at the core-mantle boundary, but this region of our planet is by no means accessible for direct geochemical investigation. Pallasite meteorites may be derived from differentiated planetesimals, subsequently smashed into fragments by collisions with other planetesimals early in the history of the solar system. These meteorites may provide indirect evidence of processes that affected the early Earth. This project will use a combination of geochemical investigations of pallasite meteorites and geophysical modelling of the composition of the Earth’s outermost core to investigate the nature and origin of pallasite meteorites and to determine what information they contain regarding differentiation processes in planetary bodies, specifically, Earth.
Pallasite meteorites comprise roughly equal proportions of olivine and iron, similar to that found in fractionated iron meteorites. Historically, they have been thought to represent remnants of the core-mantle boundary of planetesimals that were assembling at the same time as the formation of Earth. These planetesimals grew to a sufficient size for the segregation of an iron-rich core to take place but were subsequently broken into meteorite-sized chunks by collisions with other planetesimals. However, it is possible that the metal component comes, not from the differentiated body itself but, from the impactor. Pallasite meteorites are rare (only 61 found to date, and only 4 of those being observed falls; we will have samples from 12 main group pallasites available for this project) but they may offer insights into planetary differentiation, specifically related to Earth.

The project: The unique nature of pallasite meteorites means that many different hypotheses relating to the deep Earth can be investigated. The first part of the project would entail detailed characterization of minor phases, for example sulphides and phosphates, which may prove useful for geochronology, i.e. for dating the solidification of the core of a differentiated body. Once characterized, there are several potentially high impact hypotheses that could be explored related to the timing of core crystallisation, and the impacts that destroyed the pallasite parent body (or bodies). Combined with geophysical modelling to better understand the record of planet(esimal) cooling, this project has the potential to unlock the secrets of some of the most enigmatic features of our planet’s evolution

Requirements: The willingness and potential to undertake extensive delicate chemical analyses under clean laboratory conditions is essential for this project. In addition, a high degree of numeracy will be advantageous, as will a desire to build and test geophysical models relating to the chemical and physical properties of the Earth’s deepest geochemical reservoirs.

Training: Thorough training in cutting edge geochemical methods and geophysical modelling will be an integral part of this project and will be tailored to the successful student as required – we realize that these are not necessarily off-the-shelf skills, but the right candidate will acquire them!

Related opportunities: At the University of Leeds we have active and vibrant research groups who focus on the deep Earth and high temperature geochemistry. The Deep Earth Research Group is one of the largest groups of scientists studying the structure and dynamics of Earth’s core and mantle in the world while the Rocks, Melts and Fluids Research Group offers outstanding experimental and analytical facilities with which the elemental and isotopic characteristic of Earth and planetary materials can be determined in world class clean laboratories with state of the art instrumentation. Please contact any of the supervisors to discuss further PhD opportunities.