In situ observation of radial pyroxene chondrule formation from levitated melts

Chondrules were crystallized from levitated melts to study the rate of crystallization during cooling. Levitation is known to reduce the heterogeneous nucleation rate considerably and thus highly supersaturated melt become stable for a long time. The levitation experiments were done by using a gas-jet levitator (Nagashima et al., 2006). MgSiO3 glass spherule (diameter < 3 mm) was set at the top of nozzle of the gas jet levitator and the melt temperature was kept at a given temperature by adjusting the power of CO2 laser power. To investigate the seeding effect, enstatite (MgSiO3) particles, ~ 5 µm in diameter, in the gas flow were forced to be impinged to the surface of the supercooled melt droplet, the nucleation process of which was observed by a high speed CCD camera. Although we had expected easy heterogeneous nucleation from the melt droplets by this seeding, this seeing experiments failed over a wide range of supercooling. Crystallization was found to be possible only at a very large supercooling temperature regime (∆T ~ 260-860K). When the ∆T is less than 260K, neither homogeneous nucleation nor heterogeneous nucleation could be detected. If ∆T is more than 860K, crystallization was impossible because of glass formation. Typical radial textures from levitated melts (fig. 1) was obtained at ∆T ~ 800K, in which a single nucleation center at surface of the melt was observed. The fact that natural radial textures in chondrule starts from the surface might suggest the impingement of a few cosmic dust particles during the cooling.