Thermoluminescence and the shock and reheating history of meteorites: IV. The induced TL properties of type 4-6 ordinary chondrites
The induced thermoluminescence (TL) properties of 121 equilibrated ordinary chondrites have been measured. The samples were 74 H and 47 L chondrites, of which 33 H and 32 L were from Antarctica. The distribution of TL sensitivities for non-Antarctic L chondrites differs from that of non-Antarctic H chondrites, consistent with a greater proportion of the former class suffering post-metamorphic shock. Data on the effect of laboratory annealing on TL sensitivity, and step-wise Ar release measurements, enable the meteorites to be sorted into three shock-related temperature groups (<800°C, 800-1000°C,1000°C). The distribution of TL sensitivities for Antarctic meteorites suggests that only a few of the present samples have suffered intense shock. Antarctic H chondrites have TL sensitivities typically one third those of non-Antarctic H chondrites; this may reflect a greater proportion of meteorites which have suffered mild shock levels or greater degrees of weathering. On a diagram of TL peak temperature against peak width, L chondrites produce tight clusters with minimal overlap between Antarctic and non-Antarctic meteorites. Antarctic H chondrites also produce a tight cluster, but non-Antarctic chondrites plot in a band in which peak temperature increases with peak width, and there is little or no overlap between the Antarctic and non-Antarctic meteorites. Because TL peak temperature and width reflect the thermal history of the feldspar, and changes in these TL parameters can be produced by laboratory annealing experiments, this implies significant differences in the thermal (probably metamorphic) history of the Antarctic and non-Antarctic ordinary chondrites.