The natural thermoluminescence of meteorites II. Meteorite orbits and orbital evolution

Natural thermoluminescence (TL) data for 26 meteorites for which orbital elements have been estimated are reported. Calculations of equilibrium natural TL level in ordinary chondrites suggest that TL should be a sensitive indicator of perihelion. Meteorites with perihelia <0.85 AU should have very low levels of natural TL (<5 krad at 250°C in the glow curve), while meteorites with perihelia >0.85 AU should show a range of natural TL (>5 krad) with significant scatter as a result of slight variations in dose rate (shielding) and albedo. The data presented here generally agree with the theory and suggest an effective dose rate of ≈3 rad/year for most meteorites. Comparison with cosmic ray exposure ages indicates that natural TL level is also partly related to exposure age. Meteorites with high (>20 Ma) exposure ages generally have a lower and more constrained (10-30 krad) range of natural TL than meteorites with ages <20 Ma ages (10->90 krad). We suggest that this reflects orbital evolution, since mature meteorite orbits evolve to lower perihelia. The data presented here confirm the earlier observations that only a very small proportion of meteorites have experienced orbits with low perihelia within the last 10 ⁵ years. The data suggest that <15% of ordinary chondrites have been in recent orbits with perihelia <0.85 AU. The reheating of these meteorites is independently confirmed by their generally low ³He/ ²¹Ne ratios.