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.