Physics – Fluid Dynamics
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.533l&link_type=abstract
Meteoritics, vol. 30, no. 5, page 533
Physics
Fluid Dynamics
1
Asteroids, Cores, Metal, Meteorites, Iron, Partial Melting
Scientific paper
Iron meteorites are generally thought to be fragments of asteroid cores. The mechanism by which cores might form via melting and gravitational segregation in internally heated chondritic parent bodies has been modeled. Physical and chemical data drawn from experiments have been used in the modeling. Experimentally determined dihedral angles between silicates and metallic melts of appropriate composition are used to understand the fluid dynamics of the process. Experimentally measured distribution coefficients are used to track the variation in composition as the cores evolve. In bodies with FeS and FeNi metal contents and S/Fe ratios similar to those observed in E, H and L chondrites, segregation will begin when the temperature at the center reaches the Fe-FeS eutectic, about 1000 degrees C. In a body with less total FeS + metal, such the LL body, or in a body with a S/Fe ratio substantially lower than chondritic, somewhat higher temperatures will be required to initiate segregation because the volume fraction of melt will be too small (< 5%) to allow interconnection. When the melt becomes interconnected, the segregation process can be described by using Darcy's Law [1]. The rate of melt segregation as inferred from either theoretical estimates or experimental observations is sufficiently rapid that segregation will proceed as rapidly as melt forms. The rate controlling factor will thus be the rate at which the 1000 degrees C isotherm migrates upward through the body. As the 1000 degrees C isotherm migrates, each increment of new melt will have the composition of the eutectic. At greater depths, where the temperature is now higher than the eutectic, larger fractions of FeNi metal will melt. Although more metal in the melt increases the surface tension, this effect is offset by a temperature effect which simultaneously decreases the surface tension. In addition, since the volume fraction of melt with the eutectic composition increases as a function of R^3, the effective melting point of the metal + sulfide inside the 1000 degrees C isotherm will approach the eutectic, where the surface tension effects are minimized. The region of the body inside this 1000 degrees C isotherm therefore will be swept clean of its metal as well as its sulfide. References: [1] Turcott D. L. and Schubert G. (1982) Geodynamics.
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