Formation of an ilmenite-rich outer core in the moon

Details Formation of an ilmenite-rich outer core in the moon Formation of an ilmenite-rich outer core in the moon

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p31b1410d&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #P31B-1410

Physics

0545 Modeling (4255), 6250 Moon (1221)

Scientific paper

The size and composition of the lunar core are still debated. The main constraint on these parameters is the moment of inertia. However, different models can be constructed which fit the measured moment of inertia of the moon equally well. It has been suggested that a dense ilmenite-rich layer, which originally crystallized near the top of the lunar magma ocean, may have sunk to the center of the moon to form either a complete lunar core or an outer core on top of a metallic inner core. The second model seems more likely since this model combines the requirements for a lunar magnetic field, caused by an early dynamo in the lunar interior (requiring a electrically conductive core) with the fact that ilmenite is too dense to remain at shallow depths after crystallization. Using a 2-D cylindrical numerical thermo-chemical convection model, we have investigated the formation and gravitational stability of an ilmenite-rich outer core. In mantle convection models for the earth a constant gravity acceleration can be assumed. However, in the moon, gravity acceleration decreases quickly with depth due to the much smaller core mass fraction for the moon, Xc≈0.01--0.04 versus Xc=0.315 for Earth. Our convection results illustrate the importance of a depth dependent gravity distribution, showing a clear influence on the stability of an ilmenite- rich outer core. We have investigated core stability by varying two parameters, density and the internal heating rate of the ilmenite-rich layer. Varying these parameters changes the compositional and thermal buoyancy of the dense layer. These two effects counteract and are therefore studied seperately. The parameters are varied within reasonable bounds from experimental data and theoretical calculations. The density and thickness of the ilmenite-rich layer are varied, assuming a constant mass of the ilmenite, mixed with a pyroxene with a Mg# between 20 and 40. The internal heating of the layer was varied using different assumptions for the behaviour of heat producing elements during lunar mantle overturn, while the total internal heating of the model was kept constant. From the modelling results we conclude that it is possible to form a stable outer core of ilmenite-rich material. This requires a relatively thick and dense ilmenite-rich layer. For a lower density or thinner layer, the ilmenite sinks to the top of the inner core, but it does not form a stable layer. The internal heating of the ilmenite-rich layer has only a small effect on the results, since the increasing temperature of the layer results in a reduction of the density contrast of less than 10% of the compositional density contrast. The thickness of the ilmenite-rich layer, required to form a stable outer core, is of the order of 60 km and a density contrast of 11--16% 5.2--5.4 wt. ilmenite) is required. If an original deep magma ocean is assumed (~1200 km), such a layer may indeed have crystallized at shallow depth beneath the lunar crust.

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