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USGS Luminescence Dating Laboratory

Luminescence Dating—Introduction and Overview of the Technique

Luminescence laboratory showing the darkened state during analyses.

USGS Luminescence Laboratory lighting during analyses.

A brief introduction and overview of luminescence dating is presented.

What is Luminescence Dating?

Luminescence dating is a form of geochronology that measures the energy of photons being released. In natural settings, ionizing radiation (U, Th, Rb, & K ) is absorbed and stored by sediments in the crystal lattice. This stored radiation dose can be evicted with stimulation and released as luminescence. The calculated age is the time since the last exposure to sunlight or intense heat. The sunlight bleaches away the luminescence signal and resets the time 'clock'. As time passes, the luminescence signal increases through exposure to the ionizing radiation and cosmic rays. Luminescence dating is based on quantifying both the radiation dose received by a sample since its zeroing event, and the dose rate which it has experienced during the accumulation period (See the Luminescence Age Equation). The principal minerals used in luminescence dating are quartz and potassium feldspar.

Graph showing stored energy vs. Time (LA-UR-01-6064)
Image used with permission of Dr. K. Lepper, North Dakota State University

Applications of Luminescence

Applications and Environments of Luminescence
Applications Comments Contexts
Table used with permission of E.J. Rhodes, doi: 10.1146/annurev-earth-040610-133425.
Tectonic and paleoseismic Relatively recently applied Marine, fluvial, lacustrine, slope deposits
Paleoclimatic and paleoenvironmental Now widely used Wide range of sediments
Geomorphic and Quaternary Now widely used Wide range of sediments
Environmental and process studies Relatively recently applied Soils, fluvial, potential for wider range
Archaeological and paleoanthropological Now widely used Fluvial, colluvial, soils, heated/fired materials
Environments Comments Contexts
Eolian (wind transported) Usually successful Sand dunes, loess
Fluvial, alluvial, lacustrine (water-lain) Zeroing can be problematic River terraces, alluvial fans, flood plains, lakes
Marine: coastal and offshore Zeroing issue in deep water Raised beaches, beach ridges, deeper water
Glacigenic Zeroing and characteristics can be problematic Moraine and till, outwash, glaciomarine
Slope deposits Zeroing and characteristics can be problematic Colluvium, rockfall, debris flow
Caves (karstic) Zeroing can be problematic Shallow cave sediments, sands, and silts
Anthropomorphic Zeroing can be problematic Artificial fill, anthropomorphic soils
Volcanic rocks, precipitation Characteristics are often problematic Opal, biogenic opal, phenocrysts, xenocrysts
Soils Grain movement studies Modern, compound and buried soils
Heated materials Usually successful Ceramics, wild fires, thermochronology
Strained and shocked materials Little research to date Impact craters, fault movement
Photo of layered flood sediments. Layered Flood Sediments in a Boxelder Creek Cave, Black Hills, SD.

Types of Luminescence Dating Techniques

Luminescence Age Equation, Equivalent Dose, and Dose Rate

Controlling Assumptions - TL and OSL Method

The Luminescence Age Equation

Age = DE / DR

DE is measured in grays (absorbed dose) and commonly known as the Equivalent Dose or paleodose

DR is measured in grays/ka and commonly known as the Dose Rate. Comprised of K , U, Th, Rb, and cosmic ray components

Determination of Equivalent Dose (DE)

Determination of Dose Rate (DR)

Contributions to Sediments

The application of luminescence to dating archaeological or geological materials relies on determining two quantities. The first is the amount of radiation absorbed by the sample during the period since the event being dated, measured as De. To determine the age of the sample in years, De has to be divided by the radiation dose received by the sample each year – the dose rate.

There are four types of environmental radiation: alpha particles (α), beta particles (β), gamma rays (γ) and cosmic rays. The first three originate from naturally occurring elements in the sample itself and its surroundings. The most important of these sources are radioactive isotopes of uranium (U), thorium (Th) and potassium (K).

Once the concentrations of these three elements are known, conversion factors enable the calculation of the radiation dose rate (Adamiec and Aitken 1998). For example, 1% potassium in sediment will produce a gamma radiation dose rate of 0.243Gy per thousand years (Gy/ka), a beta dose rate of 0.782Gy/ka, but no alpha dose rate, as the decay of 40K does not result in the emission of alpha particles. Adding together the alpha, beta and gamma dose rates gives the total radiation dose rate.

(For further reading on the dose rate: Duller, G.A.T., 2008, Luminescence Dating: Guidelines on using luminescence dating in archaeology: English Heritage Publishing, 43 p. [PDF file, 1.3 MB].)

  Coarse-Grained Fine-Grained
Alpha (α) 20-24% 20%
Beta (β) 45-51% 48%
Gamma (γ) 25-30% 26%
Cosmic Rays 3-6% 3-6%
Potassium (K) ≥21% no α ≥53%
Thorium (Th) 20% no α 37%
Uranium (U) 25% no α 39%

Sampling

Photo shows scientists augering into trench wall. Photo shows scients collectige sample with black cloth cover.

Excavation Process

  • Excavate Back ∼20 cm
  • Expose a fresh face
  • Soft Sediment - auger/push in PVC or aluminum tubes
  • Hard Sediment - carve out block
  • Moisture and light tight, black cloth cover during collection

Sample Amount Requirements

  • Need 100 mg silt (4-11 microns) and 1 gram of fine sand size (90-125 microns or dominant grain size) for OSL (corresponds to 10-100 grams initial sample)
  • Usually an extra bulk sample is needed (about 400-600 grams) for moisture

Additional information on sampling is located under the "For Prospective Users" section.

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