Steven J. K. Symes

The induced thermoluminescence and thermal history of plagioclase feldspars

Abstract Feldspars are a common component in igneous and metamorphic rocks. Most feldspars exhibit luminescence, and this has proved useful in a number of mineralogical applications. In this paper, we concentrate on the thermoluminescence (TL) properties of feldspar, or the luminescence produced when a sample of feldspar is heated. We determined the induced TL properties of four feldspars of various compositions in their natural states, and after heating, and we compared the TL data with structural changes as determined by X-ray diffraction. The major TL peak at 120-240 °C in the TL glow curve, a plot of light intensity against temperature, varies significantly among feldsparbearing samples. Meteorites and lunar samples with slow cooling histories of ∼10 °C/My as determined by independent methods, have induced TL peak temperatures of ∼120 °C, while samples with fast cooling histories (∼100 °C/My) have induced TL peak temperatures of ∼220 °C. This variation in TL peak temperature can be reproduced by heating the present feldspar samples, meteorites and lunar samples prior to the TL measurement. Most of the present samples in their natural state had TL peak temperatures of ∼120 °C. Heating below 750 °C in the laboratory caused no change in TL peak temperatures or the structural disorder of the feldspar, while heating >750 °C caused TL peak temperatures to move to ∼220 °C and disordered the feldspar structure. We suggest that induced TL peak temperature in feldspar is influenced by the degree of Al-Si ordering in the feldspar. Thus, induced TL peak temperature can be used as an indicator of cooling rate for igneous and metamorphic rocks.

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

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.