Samples of four type 3 chondrites have been annealed at 400–850°C and 0.77–1 kbar for 10–500 h in the presence of various amounts of water (0–10 wt.%) and sodium disilicate (0–2 molal) and their thermoluminescence properties measured. After annealing for >20 h at temperatures >600°C, the TL sensitivity of the samples increased by factors of up to 40. After annealing at <600°C for 10–500 h, or relatively short periods at high temperatures ( e.g., ≤20 h at 850°C), the TL sensitivity of the samples decreased by up to 2 orders of magnitude (depending on the original value). The TL peak temperatures observed in the present experiments are consistent with a low form of feldspar (the TL phosphor) being produced at <800°C and a high form being produced at >800°C. When both high and low forms were present originally, the low-form was destroyed preferentially. We suggest that these data are consistent with the TL-metamorphism trends observed in type >3.2 chondrites, being due to the formation of feldspar (with structural details being dependent on formation temperature) by the devitrification of chondrule glass during metamorphism. For types <3.2, the TL data are equally consistent with these types experiencing lower levels of metamorphism than the higher types, or with type 3.0 being produced from higher types by aqueous alteration. The presence of water with non-terrestrial D/H ratios, and petrographic evidence for aqueous alteration in Semarkona, lead us to favour the aqueous alteration hypothesis.
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