Deep-Earth Chemistry of Core Formation : DECore

European Research Council

Scientist in charge: James Badro

Core formation and mantle crystallisation represent the major chemical differentiation events on the terrestrial planets, involving the separation of a metallic liquid that forms the core from the silicate that subsequently solidifies and evolves into the current mantle and crust. Beyond the importance of the geological event itself, the broader impacts go as far as defining planetary building blocks and habitability; indeed, the generation of the Earth’s magnetic field is ultimately tied to the segregation and crystallisation of the core, and is an important factor in establishing planetary habitability.

Despite their fundamental importance, the processes that control core segregation and Magma Ocean crystallisation, the depths at which the processes took place, the crystallisation of the inner core and appearance of the magnetic field, the primordial chemical stratification of the mantle, all remain poorly understood.

The object of this project is to study those processes experimentally, by reproducing the very high pressures and temperatures of the deep primitive Earth in the laboratory.

Specifically, the density of the core is lower than would be expected for pure iron, indicating that a light component must be present. Similarly, the Earth’s mantle is richer in siderophile elements than would be expected based upon low-pressure metal-silicate partitioning data. The chemical stratification of the mantle is very poorly constrained, to the extent that current models do not consider it; the models for the Primitive Upper Mantle and Bulk Silicate Earth are based on upper-mantle chemistry and phase relations. Last, the onset of inner core crystallisation and the composition of the inner core are unconstrained.

Solutions to these problems are hampered by the pressure range of existing experimental data, ≤ 25 GPa, equivalent to ~700 km in the Earth. We propose to extend the accessible range of pressures and temperatures by developing and applying protocols that link the laser-heated diamond anvil cell (LHDAC) with analytical techniques such as (i) nanoscale secondary ion mass spectrometry (nanoSIMS), (ii) the focused ion beam device (FIB), (iii) and scanning and transmission electron microscopy (SEM & TEM), allowing us to obtain quantitative data on element partitioning and chemical composition at extreme conditions relevant to the Earth’s lower mantle and core. The technical motivation follows from the fact that the real limitation on trace element partitioning studies at ultra high-pressure has been the grain size of the phases produced at high P-T, relative to the spatial resolution of the analytical methods available to probe the experiments; we can bridge the gap by combining state-of-the-art laser heating experiments with new nano-scale analytical techniques.

IPGP Participants: James Badro, Angèle Ricolleau, Julien Siebert, Daniele Antonangeli, Guillaume Morard, Alexandre Côté

External Participants: Rick Ryerson (LLNL, USA), John Brodholt (UCL, UK)