Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.455w&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 455
Other
1
Asteroid Cores, Core Crystallization, Iron Meteorites, Magmatic Iron Meteorites
Scientific paper
Large fractionations (factors of 2000-6000) in Ir/Ni and other ratios demonstrate that the magmatic groups of iron meteorites formed by fractional crystallization, and thus that the residual liquid remained well stirred during core crystallization. Past models have relied on solidification at the base or the top of the core, but body crystallization offers an attractive alternative. The simplest of the earlier models involved convective maxing induced by the liberation of heat and light elements (especially S) during upward crystallization from the center of the core. Other models involving downward crystallization from the core-mantle interface are based on the fact that temperatures at this location are slightly lower than those at the center; no whole-core stirring mechanism is provided by these models. Haack and Scott recently published a variant of the downward crystallization model involving the growth of giant (kilometer-scale) dendrites. Because crystallization creates a boundary layer enriched in S that does not participate in the convection, these models require several K of supercooling to induce crystallization (this undercooling is much greater than the temperature difference between the center of the core and the core-mantle interface). Buoyant forces will occasionally remove droplets of the basal boundary fluid; thus it was thinner and its degree of undercooling less than in that at the ceiling of the magma chamber. Homogeneous nucleation of metals is difficult to achieve; generally 200-300 K of undercooling is required, much more than could possibly occur in an asteroidal core. Crystals could, however, nucleate in the magma body on chromite, probably the first liquidus phase (A. Kracher, personal communication, notes that this is required to explain why Cr behaved like a compatible element despite having a solid/liquid D < 1). In addition, some tiny, submillimeter dendrites that formed at the top of the core must have pinched off and fallen into the magma. Such seeds settle as a result of buoyant forces (thus stirring the magma) and, as a result, achieve very thin boundary layers and require low degrees of undercooling in order to crystallize. The rate of core crystallization is limited by the rate of heat transport across the core-mantle interface. If sufficient nuclei are available at different sites, the bulk of the crystallization occurs where undercooling is least. It is possible that a larger fraction of the total crystallization occurred in the body of the magma than at its base or ceiling.
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