Physics
Scientific paper
May 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agusm.p41a..01s&link_type=abstract
American Geophysical Union, Spring Meeting 2004, abstract #P41A-01
Physics
5430 Interiors (8147), 8125 Evolution Of The Earth
Scientific paper
The initial composition of a planetary core is the legacy of formation (the T and P paths of the constituent materials and the extent to which chemical equilibrium with the mantle phase is possible along those paths). It is conventional to consider the subsequent evolution as "closed", with the only changes arising through the redistribution across the inner core-outer core boundary as central freezing proceeds. This is reasonable if one thought that transport across the CMB were limited by solid state diffusion, since this process is inefficient even on billion year time scales. However, there are three reasons to question "core closure": (1) As the core cools, it is likely to become supersaturated in the least soluble mantle constituents, probably MgO and perhaps a high pressure phase of silica or magnesium perovskite. This material will sediment upwards to the underside of the CMB, helping to drive core convection and possibly providing an energy source for the geodynamo. If a wet adiabat develops (analogous to earth's troposphere), it may change the convective and even seismic properties of the outermost outer core. The outer core need not be compositionally uniform vertically in this picture (but still must have horizontal uniformity of density), despite vigorous convection. (2) Seismic evidence suggests that the lowermost mantle is partially molten. One possible aid to this melting is the presence of excess hydrogen fugacity in the core relative to the partially degassed mantle. Hydrogen is particularly interesting as the only chemically active element that also may have fast solid state diffusivity. In addition, the presence of liquid pathways may provide much higher chemical interaction because of the much higher diffusivity in liquid coupled with circulation or transport of the melt. (3) Independent of this, metasomatism of the topographic relief (~ a kilometer) at the CMB can arise to the extent that core fluid develops permeable pathways in the mantle rock (a property that depends on unknown surface tension properties).
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