Experimental studies of group A1 chondrules

Abstract

Dynamic crystallization and annealing experiments have been conducted using a group Al chondrule bulk composition to examine the formation and alteration of mesostases that emit a bright yellow cathodoluminescence (CL). This type of mesostasis has only been observed in group Al and A2 chondrules in the least altered chondritic meteorites. The dynamic crystallization experiments produce yellow luminescing mesostases only in experiments cooled to their final quench temperatures at the slowest cooling rates. The yellow CL is present only where low Ca pyroxene grows from the melt so the remaining liquid is enriched in normative anorthite and diopside. This composition is strongly associated with yellow luminescence in group Al chondrule mesostases. Low temperature hydrothermal annealing experiments demonstrate that the yellow luminescence can only be destroyed and not enhanced or altered to blue luminescence by aqueous alteration. Annealing under dry conditions increased both the amount and intensity of the yellow luminescent material. Annealing experiments designed to further devitrify the mesostasis converted both yellow luminescing and nonluminescent mesostases to blue luminescing mesostases. These results confirm that yellow luminescing mesostases are the result of the thermal history group Al chondrues experienced prior to accretion, and their alteration to blue luminescing mesostases is a direct result of their metamorphic history.