Induced Thermoluminescence Studies

of Extraterrestrial Materials


Induced thermoluminescence (TL) measurements are useful in studying the thermal and metamorphic history of a wide variety of extraterrestrial materials. In particular, TL has been used to constrain the metamorphic conditions experienced by the ordinary chondrites (Sears et al., 1980, Sears et al., 1980), CO chondrites (Sears et al., 1991), CV chondrites (Guimon et al., 1995), and the basaltic HED achondrites (Batchelor and Sears, 1991). Induced TL is produced by irradiating a sample with a known dose of ionizing radiation, usually from a beta source, although any ionizing source may be used. It is distinguished from natural TL which is produced while the sample is in space as part of a meter-sized body and thus bombarded by the ionizing effects of cosmic rays.


The parameters of interest when performing induced thermoluminescence measurements are the luminescence intensity (TL sensitivity) and the temperature at which the peak occurs, called TL peak temperature. The TL sensitivity depends primarily on the abundance of crystalline feldspar, which is the major TL phosphor in most extraterrestrial samples (Guimon et al., 1984; Batchelor and Sears, 1991) while the TL peak temperature is related to the structural state of feldspar and is often a good indicator of whether a particular sample experienced a rapid or slow cooling history. Using X-ray diffraction measurements, it has been shown (Hartmetz et al., 1986) that originally ordered terrestrial oligoclase can be transformed to the disordered state by heating to >800 C and cooling in air. This structural change is reflected by a marked increase in TL peak temperature, from ~150 C for the unheated sample to >200 C for heated samples. Lunar feldspar is enriched in Ca (usually >An70) relative to chondritic or terrestrial feldspar, but similar changes in peak temperature have been observed for lunar material when heated >800 C. These studies provide an estimate of the order-disorder transformation temperature for plagioclase feldspar and thus allow insight into the thermal conditions experienced by the samples.




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