Induced Thermoluminescence Studies
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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. |
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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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