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

A large, group A 1, porphyritic olivine chondrule in the Semarkona (LL3.0) chondrite with induced thermoluminescence (TL) and compositional zoning in its mesostasis has been discovered. The chondrule has Ca-rich and Fe-poor olivine (CaO, 0.36-0.40 wt%, Fa0.3-0.5) and its mesostasis is highly anorthite-normative (approximately 52.5 wt%). The chondrule shows an intense induced TL peak at approximately 300-degrees-C with a half-width of approximately 180-degrees-C. The induced TL in the 40-440-degrees-C range increases monotonically by a factor of approximately 6 from center to rim, while SiO2, Na2O, and MnO increase by factors of approximately 1. 1, approximately 3.6, and approximately 6, respectively. The spectrum of the induced TL over the 200-350-degrees-C range (i.e., a dominant peak at approximately 570 nm with a half-width of about 100 nm) and the Mn-TL correlation suggest Mn-activated plagioclase is an important constituent of the refractory mesostases in group A1 chondrules. The zoning may reflect fractional crystallization, Soret diffusion, transport of volatiles into the chondrule by aqueous alteration, 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 mesostasis, but both reduction of precursor dust aggregate and recondensation of volatiles seem to have played significant roles to the formation of the zoning. Reducing conditions at high temperatures are suggested to have prevailed during the formation of group A1 chondrules.