THERMOLUMINESCENCE AND COMPOSITIONAL ZONING IN THE MESOSTASIS OF A SEMARKONA GROUP A1 CHONDRULE AND NEW INSIGHTS INTO THE CHONDRULE-FORMINGPROCESS

A large, group A 1, porphyritic olivine chondrule in the Semarkona (LL 3.0) chondrite with induced thermoluminescence (TL) and compositional zoning in its mesostasis has been discovered. The chondrule has Ca-ric h and Fe-poor olivine (CaO, 0.36-0.40 wt%, Fa0.3-0.5) and its mesostas is is highly anorthite-normative (approximately 52.5 wt%). The chondru le shows an intense induced TL peak at approximately 300-degrees-C wit h a half-width of approximately 180-degrees-C. The induced TL in the 4 0-440-degrees-C range increases monotonically by a factor of approxima tely 6 from center to rim, while SiO2, Na2O, and MnO increase by facto rs of approximately 1. 1, approximately 3.6, and approximately 6, resp ectively. The spectrum of the induced TL over the 200-350-degrees-C ra nge (i.e., a dominant peak at approximately 570 nm with a half-width o f about 100 nm) and the Mn-TL correlation suggest Mn-activated plagioc lase is an important constituent of the refractory mesostases in group A1 chondrules. The zoning may reflect fractional crystallization, Sor et diffusion, transport of volatiles into the chondrule by aqueous alt eration, a zoned precursor, reduction of precursor dust aggregate, or recondensation of volatiles lost during chondrule formation. The first four possibilities seem unlikely explanations for the zoning of the m esostasis, but both reduction of precursor dust aggregate and reconden sation of volatiles seem to have played significant roles to the forma tion of the zoning. Reducing conditions at high temperatures are sugge sted to have prevailed during the formation of group A1 chondrules.