Thermal and Compositional Stratification of the Inner Core

Wednesday, 17 December 2014
Stephane Labrosse, Ecole Normale Supérieure Lyon, Lyon, France
The improvements on the knowledge of the seismic structure of the
inner core and the complexities thereby revealed ask for a dynamical
origin. Sub-solidus convection was one of the early suggestions to
explain the seismic anisotropy but requires an unstable density
gradient either from thermal or compositional origin, or
both. Temperature and composition profiles in the inner core are
computed using a unidimensional model of core evolution including
diffusion in the inner core and fractional crystallisation at the
the inner core boundary (ICB). The thermal conductivity of the core
has been recently revised upwardly and, moreover, found increasing
with depth. Values of the heat flow across the core mantle boundary
(CMB) sufficient to maintain convection in the whole outer core are
not sufficient to make the temperature in the inner core
super-isentropic and therefore prone to thermal instability. An
unreasonably high CMB heat flow is necessary to this end. The
compositional stratification results from a competition of the
increase of the concentration in O and S in the outer core with
inner core growth, which makes the inner core concentration also
increase, and of the decrease of the liquidus which makes the
partition coefficient decrease as well as the concentration of light
elements in the solid. While the latter (destabilizing) effect
dominates at small inner core sizes, the former takes over for a
large inner core. The turnover point is encountered for an inner
core about half its current size in the case of S but much larger
for the case of O. The combined thermal and compositional buoyancy
is stabilizing and solid-state convection in the inner core appears
unlikely, unless an early double-diffusive instability can set in.