Physics – Fluid Dynamics
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29r.472h&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 472-473
Physics
Fluid Dynamics
1
Asteroids, Chemical Composition, Chondrites, Cores, Fluid Dynamics, Melts (Crystal Growth), Interfacial Tension, Metals, Photomicrography, Silicates, Temperature Effects
Scientific paper
The question of how metallic cores form reduces to the fluid dynamic problem of understanding the surface tension between metallic melts and silicates. This problem was addressed by performing experiments to determine the surface tensions between metallic melts with variable S contents and silicate phases; olivine, for which data were presented last year; and orthopyroxene, for which we now have complete data. Experiments were conducted in a piston-cylinder apparatus at P = 1 GPa and T = 1250 - 1450 C. Textural and chemical equilibration was confirmed in several ways. The dihedral 'wetting' angles (Theta) were measured from high-resolution photomicrographs using a 10x optical protractor; 100-400 measurements were made for each experiment. The dihedral angle is related to the ratio of interfacial energies. The extent to which a melt is interconnected along grain boundaries, and hence able to flow and segregate, depends on the value of (Theta) and the fraction of melt present. In the experiments where dense metallic melt drained away, the disconnect (Theta) values match the theoretical predictions. Since data exist for the pertinent solid-solid energies, the liquid interfacial energies can be computed from measured (Theta) values. Moreover, similar data exist on the systems: olivine, orthopyroxene, and clinopyroxene in contact with basalt liquid. Ratios of the three solid-solid energies can be computed and the ratios are in good agreement with our data. This indicates a negligible effect due to gross difference in melt composition. The clinopyroxene-metallic melt systems can be extrapolated to acquire a complete set of data for the major silicate phases expected in planetary interiors. Ordinary chondrites typically contain about 4 vol% FeS. For this volume percentage, the system will interconnect at (Theta) values of 75 deg, very close to that predicted for eutectic melts in contact with silicates at 1000 C. Element distributions between mantle and core should therefore be computed on the basis of metallic melt-solid silicate in most cases.
Herpfer Marc A.
Larimer John W.
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