Constraints on the thermal and mixing history of lunar surface materials and comparisons with basaltic meteorites

Abstract

We have measured the induced thermoluminescence (TL) properties of mare basalts, highland rocks, glasses, regolith breccias, soils and core samples. We also performed a series of heating experiments and made cathodoluminescence (CL) observations of Apollo 16 soils. The data are readily interpreted in terms of feldspar being the dominant source of TL and CL, the known luminescence properties of feldspar and history of the samples. For example, the TL sensitivity of the mare basalts is lower than that of the highland basalts by about an order of magnitude, probably due to the differing FeO content of their feldspars. Similarly, the TL properties of regolith breccias can readily be explained in terms of thermal processes similar to those experienced by the soils and by mixing of highland and mare components. Our major new observations and interpretations include (1) that there are maturity‐dependent variations in the TL and CL properties of the core samples which reflect thermal annealing and melting during regolith working, (2) that the most “primitive” material in lunar samples in terms of their thermal histories is located in the immature lunar soils, and (3) that there is no TL evidence for widespread long‐term thermal metamorphism of lunar samples, TL being particularly sensitive to low‐level metamorphism in extraterrestrial materials. In this latter respect, lunar samples differ from basaltic meteorites which otherwise have very similar properties and histories. We argue that this reflects a greater tendency toward thick regoliths on asteroid‐sized bodies.