Critical cooling rates for glass formation in levitated Mg_2SiO_4-MgSiO_3 chondrule melts

摘要

Using a gas-jet levitator, Mg-silicate glasses containing forsterite (Mg_2SiO_4) or enstatite (MgSiO_3) are obtained, with a particle diameter of 2 mm. The levitator aids in floating the melt droplets using a gas-jet. It can transform the levitated droplets into crystals or glass during cooling. The critical cooling rate for glass formation (R_c) for levitated Mg-silicate melts ( 《 10~(-1)-10~2 K/s) is considerably lower than that for non-levitated melts ( > 10~3 K/s) because of the inhibition of crystallization by the sample holder in the former. Furthermore, the crystallization temperature of the levitated Mg-silicate melts is very low (～ 900 ℃). Chondrules in chondrites are assumed to have been formed from the melt droplets in such a levitation environment. Several researchers have concluded that R_c for a chondrule melt held by a platinum wire is equal to the upper limit of the cooling rate for the chondrule melt, estimated to be ～ 1000 K/h. However, in a levitation environment, SiO_2-rich melts do not crystallize at this cooling rate. We conclude that the previously proposed maximum cooling rate for chondrule melts has been greatly overestimated. We infer that spontaneous crystallization of a completely melted chondrule precursor is extremely difficult, and almost all chondrules turn into glass, despite a low cooling rate. For crystallization of chondrule melts, it is necessary that the precursor of the chondrules should be partially melted or the completely melted precursor should be in contact with nebular dust.