THERMOLUMINESCENCE Technique

Thermoluminescence (or TL) is a geochronometric technique used for sediment. The technique has an age range of 1,000 to 500,000 years. The technique is used on sediment grains with defects and impurities, which function as natural radiation dosimeters when buried. Part of the radioactive decay from K, U, Th, and Rb in the soil, as well as contributions from cosmic rays, are trapped over time in sediments. The longer the burial, the more absorbed dose is stored in sediment; the dose is proportional to a glow curve of light obtained in response when the sample is heated or exposed to light from LEDs. Greater light doses indicate an older age. Sample grain zeroing is usually obtained by exposure to sunlight, so analyses are carried out in a darkroom. Each soil naturally has its own particular dose rate, so the in situ Dose Rate is obtained with a portable gamma ray spectrometer (read as Grays/ka). Material for analysis is collected in light-tight conditions. Common grain size used is fine silt (4-11 microns). The sample is treated with various acids to remove carbonate and organics. The sample is irradiated with a B source to artificially age the sample; the sample is preheated and finally heated to 5000C in a vacuum oven with a nitrogen atmosphere under a photomultiplier tube. The tube measures light emitted by the sample (in Grays), providing an Equivalent Dose calculation. Age = Equivalent Dose/Dose Rate Thermoluminescence is used in conjunction with U-series, 14C, stratigraphy, and associated biological processes whenever possible. Applications (numerical ages are obtained): Loess and silt deposits Sand dunes or sheets A and C soil horizons (rarely B horizons) Fissure fills Volcanic ash and glass Colluvial and alluvial materials Fluvial deposits--floodplain, deltaic, lacustrine, coastal Paleodischarge deposits--tufa mounds, with windblown eolian additions Rock shelters, paleo-Indian mounds, cave floors Bog, peat, or marsh deposits   Yucca Mountain Project Fault trenches (time of movement on faults) Cinder cone development (sand sheets beneath cones) Paleodischarge sites (dating the time of active discharge) Norman, Oklahoma Toxic Landfill Project Evolution of deltaic and floodplain sediments (scouring history) Associated loess formation (timing of glacial episodes) Las Vegas Valley Project Paleodischarge sites A-horizons of soils Chemehuevi Formation near Lake Mojave and Lake Mead (tracing large eddy currents and time of miximum fluvial deposition) Albuquerque, New Mexico Geologic Mapping Rio Rancho fault displacement Isleta Reservation trench (Hubble Fault movement) Yakima, Washington--Yakima Indian Nation Fault trenches (dating the time of movement on faults) Greater Yellowstone Project Loess deposits (timing of glacial episodes) Fault trenches (dating the time of movement on faults) A- and B-horizons of soils (correlation of soils between trenches)

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URL http://geology.cr.usgs.gov/capabilities/gronemtrac/geochron/thermo/tech.html