Luminescence & Radiometric Dating


Luminescence is a phenomenon exhibited by many crystals, such as diamond, quartz, feldspars and calcite. Energy absorbed from ionizing radiation (alpha, beta, gamma, cosmic rays) frees electrons to move through the crystal lattice and some are trapped at imperfections in the lattice. Subsequent heating of the crystal, or stimulation by absorption of light can release some of these trapped electrons with an associated emission of light - thermoluminescence (TL) or optically stimulated luminescence (OSL) respectively. This is the technology used for dosimetry badges in areas where radiation safety is a concern. The time over which the badge has been exposed is well known, and the total radiation does controls the final luminescence. The badges are heated (TL), luminescence recorded, and total dose derived. Since we know the time period of exposure and total does, we know the average dose per unit time. Now turn the process around; if you know the average dose per unit time, and the total dose from the luminescence, then you know the time period of exposure. This is the fundamental process behind luminescence dating (TL and OSL), as well as electron spin resonance (ESR) dating, which uses a different technique to achieve the same result.

OSL and TL dating techniques don't get as much press in the creation/evoluton game, because they cannot be used to measure the billions of years time period characterized by the age of the Earth. But they can be used to measure significant periods nonetheless, especially in the range between about 40,000 to 50,000 years where radiocarbon dating cuts off, and the 1,000,000 years or so required for most radiometric techniques to become reliable. The idea here is that all materials carry extremely low concentrations of radiogenic isotopes, line Uranium, which in turn expose the material to extremely low doses of radiation over a long time. That radiation frees electrons that get trapped in crystal defects, just like dosimetry badges. The total population of trapped electrons in turn determines the total dose. If you know the average dose per unit time, by studying the geology of the site, you can then use the ratio of total dose over average dose, and get the time period.

Sunlight on a crystal will evict the trapped electrons much faster than background radiation puts them in. So once the crystal is buried, the "clock" starts. Dig up the crystal, measure its luminescence (either optically or thermally stimulated), and you know the total dose. Compare with the average dose per unit time, and you know how long the crystal has been buried. This is a favorite means for dating buried sediments that are often rich in quartz and feldspar. For other materials, notably non translucent material, electrons become trapped in defects where the lattice potential is too deep and the electrons cannot be stimulated to come out. In those cases, electron spin resonance (ESR), which is much more complicated that luminescence techniques, can be used to count the number of trapped electrons by using a combination of microwaves and a variable magnetic field. The disadvantage of ESR is that it is much more complicated, and has larger uncertainties than luminescence techniques. The advantage of ESR is that, unlike luminescence, the electrons are not evicted from their traps, so the measurement can be repeated as desired on the same sample.

One of the key tests of reliability for any dating technique is the ability to intercompare with other techniques; they should all give the same age for the same sample, within the bounds of the usual experimental uncertainties. There is a lot of literature available that demonstrates intercomparison between these luminescence techniques and radiometric dating. But here is one recent, and very good example.

Australia's oldest human remains: age of the Lake Mungo 3 skeleton

Alan Thorne, Rainer Grün, Graham Mortimer, Nigel A. Spooner, John J. Simpson, Malcolm McCulloch, Lois Taylor, Darren Curnoe

Journal of Human Evolution, v 36, n 6, June, 1999, p591-612
(ID jhev.1999.0305)

Abstract:
We have carried out a comprehensive ESR and U-series dating study on the Lake Mungo 3 (LM3) human skeleton. The isotopic Th/U and Pa/U ratios indicate that some minor uranium mobilization may have occurred in the past. Taking such effects into account, the best age estimate for the human skeleton is obtained through the combination of U-series and ESR analyses yielding 62,000±6000 years. This age is in close agreement with OSL age estimates on the sediment into which the skeleton was buried of 61,000±2000 years. Furthermore, we obtained a U-series age of 81,000±21,000 years for the calcitic matrix that was precipitated on the bones after burial. All age results are considerably older than the previously assumed age of LM3 and demonstrate the necessity for directly dating hominid remains. We conclude that the Lake Mungo 3 burial documents the earliest known human presence on the Australian continent. The age implies that people who were skeletally within the range of the present Australian indigenous population colonized the continent during or before oxygen isotope stage 4 (57,000-71,000 years)

The authors performed the following dating measurements, with the indicated resulting ages expressed in thousands of years.

  • Mass spectrometer U-series (Th/U) on 4 bone shavings
    69.8 ± 2.1
    58.3 ± 1.2
    50.7 ± 0.9
    54.5 ± 0.7

  • Gamma spectrometer U-series (Th/U and Pa/U) on the skull cap
    69.5 ± 2.9 (Th/U) 74 ± 7 (Pa/U)
    64.1 ± 3.7 (Th/U) 60 ± 5 (Pa/U)

  • Electron Spin Resonance (ESR) on tooth enamel
    63 ± 6 (EU) [closed system, early U uptake]
    78 ± 7 (LU) [open system, late U uptake]

  • Mass spectrometer U-series (Th/U) on calcitic matrix recovered from the original skeletal material
    81 ± 21 (isochron)

  • Optically Stimulated Luminescence (OSL) on sediment recovered directly under the soil horizon
    59 ± 3
    63 ± 4
    61 ± 2 [weighted mean of previous 2 ages]
  • Some discussion of the results is in order. The authors have a long discussion of the 4 Th/U dates on bone shavings, which cover a fairly wide spread. Bones, especially teeth, do not normally lose uranium, but take it in ("uranium uptake"), which would make the radiometric ages appear younger than the true age. However, the geological setting of the skeleton, in a rapidly eroding area, can indicate that uranium in this case was lost due to oxidation, which would make the ages appear too old. The authors conclude that uranium mobility has affected these ages, but they cannot be sure in which direction (whether or not net uranium was lost or gained). The OSL age should represent the maximum age of the specimen, since sediment deposit occurred before burial. Since uranium mobility is more of an issue for the bones (which are permeable) that for the crystals (which are not), it is not surprising that the OSL ages agree with each other fairly well, and lie in the range of bone Th/U ages, which indicates that different bone samples experienced different uranium mobility. Using the mass spectrometer to study other isotope ratios, and model uranium mobility, the authors decided that the "late uptake" model was not applicable here, an idea bolstered by the discordant ESR date if one assume late uptake. They conclude a weighted mean age 62 ± 6 as indicated in the abstract.

    The paper includes lengthy and detailed discussions of techniques, assumptions, and tests of assumptions that lead to their final conclusion. You really need to read the paper to get a good feel for how carefully the work was done. My point here is that the OSL and ESR dates are consistent across the board with the radiometric dates. Radiometric dating is an obvious target for young-Earth creationists, but criticisms are more effective when oriented towards geologic assumptions (like isotope abundances or mobility, or "excess argon), than towards fundamental concepts (like variable decay rates). But this kind of concordance between extremely different techniques is common, and illustrated by far more papers than this one. My own Radiometric Dating Resource List includes a link to an EOS paper, "Breakthrough Made in Dating of the Geological Record", which shows concordance between radiometric dates and astronomical dates via the Milankovitch cycles.

    This is a real fundamental problem for the continued assertion that there is scientific validity to the argument that the Earth is only about 10,000 years old . This is not just concordance between different radiometric techniques, this is a wide concordance between techniques that share no common fundamental conception beyond physics itself. If they are so wrong, why are they so consistent? This question has to be addressed if there is any hope for the scientific revival of a "young Earth".


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