5th Swiss Geoscience Meeting, Geneva 2007
Earth’s core formation aided by flow channelling
induced by Rayleigh-Taylor instabilities
Golabek Gregor*, Tackley Paul*, Schmeling Harro**
*Institut für Geophysik, Schafmattstrasse 30, CH-8093 Zürich
(gregor.golabek@erdw.ethz.ch)
**Institut für Geowissenschaften, Facheinheit Geophysik, Altenhöferallee 1, D-60438
Frankfurt am Main
The core formation process remains poorly known. Isotopic constraints by Hf/W
systematics indicate a fast process which was largely completed within 33 Ma
for the Earth. An unstable gravitational configuration of a dense molten metallic
layer overlying a colder chondritic protocore is predicted by most studies for the
time a planetary embryo reaches Mars-size [e.g. Höink et al. (2006)]. This leads
to the formation of a Rayleigh-Taylor instability. We propose the application of
Stevenson's (1989) stress-induced melt channelling mechanism in the region
surrounding an incipient iron diapir. We therefore perform numerical
experiments solving the two-phase, two compositions flow equations within a
2D rectangular box or 3D cuboid with symmetrical boundary conditions. We
apply the Compaction Boussinesq Approximation (CBA) [see Schmeling (2000)]
and include a depth-dependent gravity. For simplicity we use a constant
viscosity for the solid phase and melt-fraction dependent rheology for the
partially molten region around the diapir.
A systematic investigation of the physical conditions under which the melt
channels can form is being performed in 2D and 3D, and results are being
compared to the isotopic time scale of core formation and applied to the early
Earth.
As a result, for sufficiently small retention numbers iron-rich melt channels
develop within a region of approximately twice the diapir's size. This could lead
to effective draining of the surrounding region and might initiate cascading
daughter diapirs. The region of the protocore drained by this cascading
mechanism is expected to significantly increase with depth, and thus proposes
an effective mechanism to extract iron melt also from deeper parts of the initially
chondritic protocore.
This mechanism could effectively enhance melt accumulation in the Earth's
protocore, accelerate the process of core formation and affect the metal-silicate
equilibration in the deep planetary interior prior the Moon-forming giant impact.
Therefore the channelling mechanism could also be interesting for planets like
Mars, which never experienced complete melting.
REFERENCES
Höink, T., Schmalzl, J. & Hansen U. 2006: Dynamics of metal-silicate
separation in a terrestrial magma ocean, Geochem. Geophys. Geosyst. 7,
Q09008, doi:10.1029/2006GC001268.
Schmeling, H. 2000: Partial melting and melt segregation in a convecting
mantle, in: Physics and chemistry of partially molten rocks (Ed. by
Bagdassarov, N., Laporte, D. & Thompson, A.B.). Kluwer Academic Press, 141178.
5th Swiss Geoscience Meeting, Geneva 2007
Stevenson, D.J. 1989: Spontaneous small-scale melt segregation in partial
melts undergoing deformation, Geophys. Res. Lett. 16, 1067-1070.