Module 1 - The basic principles of luminescence dating
)
Luminescence dating determines the last time minerals (typically quartz or potassium-rich (K) feldspar) were exposed to sunlight or heat (Aitken, 1998). In the case of sunlight, luminescence ages tell us how long a deposit or artefact has been buried.
After quartz and feldspar minerals are buried, they are exposed to ionizing radiation emitted from the surrounding sediments and cosmic rays from outer space that penetrate the ground surface. At the molecular scale, this radiation re-mobilizes electrons, some of which accumulate within defects (so-called “traps”) inside the crystal lattice. These defects exist in the form of structural imperfections or impurities. The longer a mineral is buried, the more electrons (i.e., charge) accumulate within the traps (Fig. 1).
)
Figure 1. Schematic showing accumulation and bleaching of minerals. Inherited electrons (charge) within traps in quartz and feldspar minerals (Phase I) are depleted as they are sun-exposed during transport (Phase II). Charge accumulates once again after deposition and burial (Phase III).
If a buried mineral is collected, brought into lab with controlled light conditions and stimulated with a light and/or a heat source, the electrons become evicted from their traps and immediately recombine into other areas of net positive charge (i.e., electron vacancies) also known as “holes”. Recombination into some of these holes leads to the release of photons (luminescence) (Fig. 2). The intensity of the luminescence (i.e., photon counts per second) is proportional to the amount of radiation absorbed by the mineral during burial.
)
Figure 2. A) Schematic showing luminescence signal growing during burial as a result of exposure to alpha (ɑ), beta (β), gamma (Ɣ) and cosmic (c) radiation. The sample is collected and stimulated with a light source in the laboratory to evict trapped electrons, which emits photons (luminescence). Modified from Mahan et al. (2022). B) Luminescence signal intensity plotted against stimulation time.
The brightness of the resulting luminescence is measured and used to calculate the total amount of radiation the minerals had been exposed to during their period of burial in the environment (also known as the absorbed dose and measured in units of Gray). To obtain an age, this value is divided by the amount of radiation that the sample receives from its environment in a year (i.e., the dose rate measured in units of Gray per year).
The total radiation the minerals were exposed to during burial (Gray)
the amount of radiation the minerals receive in a year (Gray per year)