The natural thermoluminescence of meteorites: III. lunar and basaltic meteorites

Abstract

Natural thermoluminescence (TL) data have been obtained for the lunar meteorite MacAlpine Hills 88104/5 and for 65 eucrites, howardites, diogenites, and mesosiderites in order to investigate their recent thermal and radiation histories. All these meteorites have low levels of natural TL compared to chondrites, which is primarily because they display “anomalous fading” (i.e., fading by non-classical mechanisms). However, some have especially low natural TL (<5 krad at 250°C in the glow curve) which cannot be attributed to anomalous fading or thermal fading over especially large terrestrial ages, and which must reflect heating within the last 10 5–10 6 y. In some cases, this heating may have been associated with shock (e.g., LEW85303) or regolith processes (Kapoeta), but in most cases (Bununu, Lowicz, the diogenites ALHA77256, ALHA84001, EET79002, and maybe others) solar heating at perihelia <0.8 AU may be responsible for the low TL values. The fraction of basaltic meteorites thought to have had small perihelia (about 20%) is comparable to the fraction of chondrites with low natural TL and to the fraction of observed falls and fireballs with small perihelia. This may imply ejection from the asteroid belt via similar mechanisms. Assuming plausible values for cosmic ray dose rate, and that the natural TL of MAC88104/5 was totally drained by ejection from the moon, the parameters for TL decay determined in the present study suggest that the Moon-Earth transit times for MAC88104 and MAC88105 were 2,000 and 1,800 y, respectively, compared with 19,000 and 2,500 y for Y791197 and ALHA81005, respectively. Although they are clearly not paired, the possibility that MAC88104/5 and ALHA81005 were ejected from the moon by the same event should be considered, since diverse rock types are found in close proximity on the lunar surface. The natural TL data confirm most previous published pairings among basaltic meteorites and suggest others.