Reproduction of Chondrule from Levitated, Hypercooled Silicate Melts.

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Olivine ((Mg, Fe)_2SiO_4) and pyroxene ((Mg, Fe)SiO_3) chondrules have been successfully synthesized for the first time from levitated melt droplets. Solidification experiments have been performed by the use of a gas jet levitation system.In the case of levitated melts, the homogeneous nucleation required a large degree of supercooling (forsterite-Mg_2SiO_4 : ΔT【greater than or equal】1000 K) or was not possible at all (enstatite-MgSiO_3). In the latter case, only vitrification was observed even for long experiment times (t>1000 sec).These points out that heterogeneous nucleation centers have played a key role for the crystallization of meteoritic material during the formation of the early solar nebula. The most likely possibility for heterogeneous nucleation would have been the collision of the melt droplets with cosmic dusts, which had been universally present around chondrule formation.Although it was formerly assumed that the crystallization of chondrules was a slow process near equilibrium conditions, the present levitation data clearly demonstrates that crystallization of chondrule melts takes place in less than 10 seconds at a supercooling of approximately 700-1000 K for olivine chondrules and of 650-860 K for pyroxene chondrules.By collision with cosmic dusts, the chondrules have been crystallized from a hypercooled melt (about ΔT>500 K). In hypercooled melts, facetted crystals, like the ones present in porphyritic textures, can be formed because the hypercooled melt acts as a heat sink for the absorption of the latent heat of crystallization.A new cooling model for the chondrule formation is proposed based on the crystallization from hypercooled melts, in which the cooling rate of chondrules and the time, until the collision with cosmic dust particles, was considered. According to this model, a cooling rate larger than 100 K/s is needed for a melt droplet to realize crystallization. This is about 1000 times faster than previous models suggested.