NONEQUILIBRIUM SOLIDIFICATION AND MICROSTRUCTURES OF METAL PHASES IN THE SHOCK-INDUCED MELT OF THE YANZHUANG (H6) CHONDRITE

Dendrites in the metal-troilite spherules in both shock-induced melt v eins and a melt pocket of the Yanzhuang chondrite show zoning in their microstructures. This feature is indicative of nonequilibrium solidif ication of the metal phases. Dendrites in the melt pocket have a typic al crust-core structure consisting of martensitic interiors (7.5-8.1 w t% Ni) and Ni-rich rims (12.5-23.3 wt% Ni). In comparison, the dendrit es in melt veins have three microstructural areas: (1) core (6.4-7.3 w t% Ni); (2) martensite between the core and rim (7.4-8.5 wt.% Ni); (3) Ni-rich rim (12.8-21.4 wt% Ni). It is suggested that the difference i n cooling rates following shock-induced high temperature melting might be an important factor in producing the different dendritic microstru ctures in melt veins and melt pocket. Cooling rates deduced from measu rements of secondary dendritic arm spacings are 100-400-degrees-C/s in the melt veins and 6-30-degrees-C/s in the melt pocket, respectively, and lie in the temperature interval 950 to 1400-degrees-C.