Core Exsolution: A Likely Consequence of Giant Impacts and a Likely Energy Source for the Geodynamo

Exsolution (mantle underplating by material leaving the core) is an old idea but has been difficult to pin down, primarily because of our ignorance of the relevant phase diagrams. If exsolution occurs, it is roughly an order of magnitude more important per unit mass of exsolved material than the growth of the inner core and can therefore play a major role in the geodynamo even when only 0.5 percent of the core mass is exsolved. Some general principles can be identified even though many of the details remain obscure: (1) Hf/W data suggest that a substantial amount of core/mantle equilibration took place in the high T and high P core formation events that follow immediately after a giant impact, e.g., the Moon-forming impact. This mandates a significant amount of solution of mantle constituents into the core, including (very importantly) components that would not normally be viewed as very soluble, such as MgO. (2) It is precisely those low solubility (yet otherwise major) Earth constituents that are most likely to exsolve upon dilution and cooling in the core because of the Arrhenius form of the solubility law and the large enthalpy of solution. Of course, Gibbs phase rules preclude simply following the MgO, but simplified three component models suggest this is the most important factor. Extrapolation of experimental data together with core cooling models suggest (but does not prove) that the likely solubilities and subsequent exsolution of MgO are important for the geodynamo. (3) The post-giant impact model of a magma ocean at the base of the mantle (either through being very hot or through downward percolation of dense mantle melt) allows additional extraction of mantle constituents into the core through double diffusive convection that does work against gravity using the early primordial heat reservoir. This is an energy storage device useful for later core convection and geodynamo generation. (4) The consequences for the geochemistry we can sample (e.g., OIBs) is likely to be subdued or non-existent because the lowermost mantle is stably stratified and lacks efficient mixing with the shallow mantle. (5) The core composition (density deficit relative to pure iron) is a constraint on these models since it places a limit on the amount of equilibrated material subjected to severe T and P. The core would be even less dense than observed if this amount is too great. Thus, the core composition (and complementary mantle siderophiles) should be viewed as being on a mixing line between high T,P and lower T,P reservoirs at time of last equilibration. (5) This is an attractive alternative to K-40 for aiding the core geodynamo because it is complementary to inner core growth, i.e., acts early to mid-history when the inner core growth is possibly not present. It may still be mildly significant even now. (6) It can also help maintain the Mercury dynamo.