Reproduction of Chondrule from Levitated, Hypercooled Silicate Melts.

首次成功地用悬浮熔融液滴合成了橄榄石（(Mg, Fe)_2SiO_4）和辉石（(Mg, Fe)SiO_3）球粒。利用气体喷射悬浮系统进行了凝固实验。在悬浮熔体的情况下，均匀形核需要很大程度的过冷（forsterite-Mg_2SiO_4： ΔT【大于或等于】1000 K）或根本不可能（enstatite-MgSiO_3）。在后一种情况下，即使在很长的实验时间内（100 - 1000秒），也只观察到玻璃化。指出在早期太阳星云形成过程中，非均相成核中心对陨石物质的结晶起了关键作用。非均质成核最有可能是熔融液滴与宇宙尘埃的碰撞，宇宙尘埃普遍存在于球粒形成周围。虽然以前假设球粒的结晶是一个接近平衡条件的缓慢过程，但目前的悬浮数据清楚地表明，在橄榄石球粒约700-1000 K和辉石球粒约650-860 K的过冷条件下，球粒熔体的结晶在不到10秒的时间内发生。通过与宇宙尘埃的碰撞，球粒从过冷熔体（约ΔT> 500k）中结晶。在过冷熔体中，由于过冷熔体作为吸收结晶潜热的散热器，可以形成像斑状结构中存在的切面晶体。基于过冷熔体的结晶，提出了一种新的球粒冷却模型，该模型考虑了球粒的冷却速率和与宇宙尘埃粒子碰撞前的时间。根据该模型，熔滴要实现结晶，需要大于100k /s的冷却速率。这比以前的模型快了大约1000倍。

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.