Reproduction of chondrules from levitated, hypercooled melts

摘要

Chondrules, small silicate spherules that are a major component of primitive meteorites and which consist mainly Of Mg2SiO4 and MgSiO3, have been synthesized from strongly undercooled (hypercooled) melts. The pathway of nucleation and the importance of heterogeneous nucleation for the formation of chondrules have been identified and demonstrated. Solidification experiments have been performed using a gas jet levitation system. Those results are compared to those of conventional experiments, in which a melt was in contact with sample holders. In the case of levitated melts, homogeneous nucleation required a large degree of supercooling (forsterite-Mg2SiO4: Delta T >= 1000 K) or was not possible at all (enstatite-MgSiO3). In the latter case, only vitrification was observed, even for long experimental cooling times (t > 1000 s). In experiments using sample holders, the holders act as nucleation centers, which do not represent the original conditions of chondrule formation. Experiments under microgravity were also performed. Those results showed that, nucleation is even more suppressed under reduced gravity. Taken together, these results clarify that only a containerless growth arrangement can simulate the formation of chondrules and early cosmic materials. Furthermore, the onset of crystallization was triggered by heterogeneous nucleation, as in that which occurs during the collision with cosmic dust. (c) 2006 Elsevier B.V. All rights reserved.