NUCLEATION AND GROWTH OF EARTH'S INNER CORE (Invited Presentation)

Earth’s inner core is thought to have crystallized during slow cooling from an initial fully molten core. The traditional view is that growth commenced immediately upon reaching the liquidus temperature at Earth’s center, and continued at a rate governed by the loss of heat from the core to the mantle. However, this mode of growth tacitly assumes that the thermodynamic barrier to nucleation of the inner core is negligible. In fact, the homogenous nucleation barrier for liquid metals and alloys is very large, requiring supercooling ~30% below the melting temperature to spontaneously produce crystal nuclei - for the inner core, this implies a critical supercooing on the order of 1000 K below the liquidus. A nucleation barrier of this magnitude is implausible for Earth’s inner core; if it had been reached the inner core would have grown rapidly to a size far larger than it is at present. The nucleation barrier must have been depressed substantially, most likely by the presence of a solid substrate that has low interfacial inergy with the solid iron-nickel alloy of the inner core. The heterogeneous nucleation barrier on such a substrate, and the timing of its introduction to the central portion of the core, are critical to the timing and early growth rate of the inner core.

If delivery of a nucleation substrate happens after cooling to the liquidus, or if a substantial energy barrier exists for heterogeneous nucleation on the substrate, there will be a delay in nucleation and an initial phase of rapid inner core growth. Such rapid growth, where the heat sinks are local, may lead to crystal texturing that is distinct from that produced during slow, steady growth, which is governed by (anisotropic) heat loss to the outer core. More work needs to be done to understand fully the textures that would be produced by rapid growth from a supercooled state, but the presence or absence of such texturing could ultimately provide constraints on the supercooling at inner core nucleation. Rapid growth after nucleation would also produce a spike in chemical and thermal buoyancy that could have a significant transient effect on the geomagnetic field – another potential constraint on the nucleation process.