Cosmogenic effects in shergottites

Abstract

Heavy nuclei tracks, induced radioactivity and thermoluminescence have been studied in Shergotty. ALHA 77005 and EETA 79001 achondrites with a view to understanding their thermal and cosmic ray exposure history. The track data indicate low ablation, on the average 1 to 3 cm. for all three, indicative of low geocentric velocities and small meteoroid masses (1-25 kg). The ²⁶A1 activity of 91 dpm/kg in Shergotty, when compared with the expected production rates, suggest a small (effective radius ~12 cm) size of the meteoroid in space, consistent with the track data. Nuclear track, ²⁶A1 and other cosmogenic effects in the three shergottites are consistent with their single stage exposure in space as small objects. The TL sensitivity of the shergottites is low, about 10^-2 to 10^-4 of the value measured in Dhajala (H3) chondrite. This is probably an effect of severe shock suffered by the shergottites. The TL data indicate low equivalent dose (30 krads) at low glow curve temperature (T^∗ ≈ 250°C) for all the shergottites, whereas for ALHA 77005 even the high glow curve temperature (T^∗ ~ 380#176;C) equivalent dose is low, 35 krads compared to ~ 100 krads found in Shergotty, EETA 79001 and also in ordinary chondrites. Two plausible causes for the unusually low value of the equivalent dose include anomalous TL characteristics of shock-induced phases and solar heating in meteoroid orbit with small perihelion distance (0.70-0.85 A.U.).