Other
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufmmr62a1050h&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #MR62A-1050
Other
3902 Creep And Deformation, 5112 Microstructure, 5114 Permeability And Porosity
Scientific paper
Previous research into the mechanism of planetary core formation has lead to the conclusion that the cores of the terrestrial planets can only develop by the settling of metallic melt through a magma ocean. This conclusion was reached by examining the equilibrium distribution of core-forming melts in a silicate matrix under hydrostatic pressure conditions. These studies verified that, under hydrostatic conditions, metallic melts are distributed in isolated pockets, that is, their dihedral angle in olivine is >60°. However, the influence of deformation (non-hydrostatic stress) on the system was not taken into account. Deformation experiments performed in our lab have demonstrated that initially isolated iron sulfide melt pockets become aligned during deformation, altering the permeability of the system. In our studies, samples composed of olivine + iron sulfide with and without MORB were deformed at 1523 K and 300 MPa at shear stresses ranging from 10 to 100 MPa. Samples were deformed either under constant load or at a constant displacement rates of 10-6}-10{-4 s-1. Shear strains ranged from 0.8 to 2.5. The microstructures of the samples were examined in reflected light and scanning electron microscopy. In the case of olivine + 5 vol% iron sulfide + 10 vol% MORB, a sample sheared to γ = 2.5 in a series of constant load steps revealed bands of high basalt concentration. These silicate melt-rich bands are aligned at ~ 20 ° to the shear plane and antithetic to the shear direction. The silicate melt fraction in the bands is ~ 0.2 while the melt fraction in the regions between the melt-rich bands is ~ 0.02. The silicate melt bands are <= 50 {μ }m wide and are space ~ 100 {μ }m apart. The average size of the iron sulfide blebs in the silicate melt-rich band is larger than the average size of blebs outside the band. Two other samples of the same composition were sheared to strains of 0.8 and 1.5 at constant displacement rates but lacked melt-rich bands. In the case of olivine + 5 vol% iron sulfide without MORB, iron sulfide melt-rich bands developed in a smaple sheared to γ=1.5 in constant load steps. The sulfide blebs in this sample coarsened as isolated blebs interconnected during deformation, leading to the development of metallic melt-rich bands. Therefore, we conclude that deformation may make it possible for core-forming melt to accumulate in and segregate from a solid silicate matrix.
Hustoft J. W.
Kohlstedt David L.
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