Physics
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufm.p12b1068h&link_type=abstract
American Geophysical Union, Fall Meeting 2003, abstract #P12B-1068
Physics
3902 Creep And Deformation, 5114 Permeability And Porosity
Scientific paper
Core formation was a significant event in Earth's early history, affecting both the geophysical and geochemical properties of our planet. Understanding the mechanics of metal-silicate segregation is important for interpreting the observed properties of the mantle. Previous hydrostatic experiments on metallic melts of core composition in silicate matrices demonstrate that the dihedral angle between an Fe-Ni melt and olivine is relatively high (i.e., > 60°), hence, the melt would be trapped in a solid silicate, unless the critical melt fraction for interconnection were exceeded. Even then, draining of metallic melt would leave a higher melt fraction stranded after the pinch-off boundary was crossed than is observed in the mantle. The results of these hydrostatic annealing experiments left the magma ocean hypothesis as the only viable method for core formation. It is unlikely, however, that the state of stress in an accreting terrestrial planet was hydrostatic. To understand the effect of deformation on the distribution of a metallic melt in dunite, simple shear experiments on olivine + 1-9 vol% Fe-S were performed at a confining pressure of 300 MPa and a temperature of 1473-1523 K in our gas-pressure medium deformation apparatus. Previous simple shear experiments on olivine + 5 vol% Fe-S revealed the development of Fe-S melt rich bands oriented at 20° to the shear plane and antithetic to the shear direction. The melt fraction in these bands is 0.20-0.25, a value that is well above the critical melt fraction for interconnection, while between the bands the melt fraction is 0.02-0.03. Electrical conductivity tests by Yoshino et al. (2003) on olivine containing an Fe-S melt revealed a dramatic increase in conductivity at a melt fraction between 0.03 and 0.06, a range that these authors interpreted as the critical melt fraction for porous flow for this metallic melt - silicate matrix system. Therefore, our previous experiments on olivine + 5 vol% Fe-S were near or at the critical melt fraction, and porous flow of metallic melt might then be expected. Porous flow of metallic melt is necessary for the development of bands, but the ranges of melt fractions and strains required to segregate the melt are unknown. Experiments are now in progress to determine the minimum melt fraction at which melt segregation will occur in metallic melt - olivine rocks.
Hustoft J. W.
Kohlstedt David L.
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