Reproduction of Chondrule from Levitated, Hypercooled Silicate Melts.

首次成功地从悬浮熔滴中合成了橄榄石（（Mg，Fe）_2SiO_4）和辉石（（Mg，Fe）SiO_3）球粒。利用气体射流悬浮系统进行了凝固实验，发现在悬浮熔体的情况下，均匀形核需要很大程度的过冷（镁橄榄石-Mg 2SiO 4：ΔT ≥ 1000 K）或根本不可能（顽火辉石-MgSiO 3）。在后一种情况下，即使实验时间很长（t>1000 sec），也只观察到玻璃化现象，这表明在早期太阳星云形成过程中，非均匀成核中心对陨石物质的结晶起着关键作用。非均质成核最有可能的原因是熔融液滴与宇宙尘埃的碰撞，这种碰撞在球粒形成过程中普遍存在。虽然以前认为球粒的结晶在接近平衡状态时是一个缓慢的过程，目前的悬浮数据清楚地表明，球粒熔体的结晶发生在不到10秒的过冷度约为橄榄石球粒的结晶温度为700-1000 K，辉石球粒的结晶温度为650-860 K，球粒与宇宙尘埃碰撞后，从过冷熔体中结晶出来（ΔT>500 K）。在过冷熔体中，由于过冷熔体作为热沉吸收结晶潜热，可以形成类似于斑状结构中的小面晶体，基于过冷熔体的结晶过程，提出了一个新的球粒形成冷却模型，该模型考虑了球粒的冷却速率以及球粒与宇宙尘埃粒子碰撞的时间.根据该模型，需要大于100 K/s的冷却速率的熔体液滴实现结晶。这比以前的模型快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.