OSL. Sampling procedure for rock dating

Necessary tools:

1. Plastic (if the deposits are very dry and dense, then metallic) opaque pipe having the external diameter of about 50 mm, wall thickness of about 3 mm, and length of about 300 mm (Fig. 1). For example, an adhesive PVC pressure-water pipe of 50x3.7 mm. The edge of one end of the pipe should be sharpened at an angle of 45° for easier penetration into the rock.

Fig.1. The edge of one end of the pipe should be sharpened at an angle of 45o

2. Hammer

3. Steel nozzle, the expanded part of which corresponds to the outer diameter of the pipe and is put on ledgeless to prevent the daylight enter the pipe. There should be a small hole in the nozzle wall with a diameter of not more than 1 mm to allow air to escape when the pipe is driven into the rock. The reverse side of the nozzle serves as a handle to hold it and at the end has a thickening for working with a hammer on it. For lack of a special nozzle, improvised means can be used, for example, a brake caliper piston from a car (Fig. 3). Old pistons can be found in car services.

Fig. 2. Special nozzle for driving pipes into the rock


Fig. 3 Instead of the nozzle, it is possible to use a brake caliper piston from a car

4. Elastic adhesive tape of about 30-50 mm wide. The ideal option is a white Folsen tape for sealing windows (Fig. 4). Scotch tape is not a good option, as it does not ensure the tightness of the sample.

Fig. 4. Folsen adhesive tape is ideal for waterproofing samples

5. Opaque U-shaped caps (Fig. 1) for closing the ends of the pipe from the outside. For example, an external decorative black plastic d51 plug.

6. Rags.

7. Oil or permanent marker.

8. Hand trowel.

9. Shovel.


Sampling:

The main principle of sampling is to prevent the samples from being exposed to daylight. For sampling to determine the age by the optically stimulated luminescence method, quartz-bearing and/or feldspar-bearing sands are suitable. Sampling is carried out from a subvertical well-cleaned wall without signs of recent gravitational shifts. It is advisable to avoid areas of strong secondary rock alteration, including evidence of post-sedimentary groundwater migration. It is necessary to exclude any light from entering the sample, with the exception of those parts of the sample that end up in the ends of the pipe.

Fig. 5. On the cleared wall, it is necessary to mark the sampling location, sample number, photograph it and measure the height

On the cleared wall, it is necessary to mark the sampling location, sample number, photograph it and measure the height (Fig. 5). For OSL dating, it is also necessary to know the depth of sampling relative to the day surface or the landform surface in the area of sampling. For example, it can be the distance from the edge of the cliff or from the surface of the alluvial terrace to the place of sampling in the outcrop wall. These data are necessary when calculating the influence of cosmic radiation, the effect of which on the rock weakens with depth.

Fig. 6. The pipe is clogged perpendicular to the outcrop wall

The hollow pipe is clogged perpendicular to the outcrop wall (Fig. 6). The upper layer of the rock is removed for about 2 to 5 cm immediately before sampling. Then, a nozzle is put on the pipe end far from the wall and with the help of a hammer the pipe is completely clogged into the wall. The pipe should be clogged 3 or 4 cm deeper than its length to compress the sand in it. Ideally, the rock in the pipe does not crumble, but enters the pipe as a solid core. When clogging the pipe into the outcrop wall, safety regulations should be strictly observed. Do not take samples under walls with a negative angle of incidence and in other dangerous places.
After the pipe is clogged, holding the nozzle and using the trowel, it is necessary to completely remove the sand around the outer end of the pipe. After that, you can remove the nozzle and immediately put the cap on the pipe, gently knocking it with a hammer for tight fixation. Then the pipe is dug up, the sand is removed with a rag from the open end of the pipe and the pipe is tightly closed with the second lid. To make sure that the sampling was successful, it is necessary to inspect the place where the pipe was removed from. The hole should not have cracks or traces of post-sedimentary migration of water. Covers must be tightly wrapped with waterproofing tape to preserve natural moisture in the sample (Figs. 7.8).

Fig. 7. Covers are wrapped with waterproof tape

On the pipe, it is necessary to write a sample number with a permanent marker at least in two places.
To obtain the minimum statistical sampling, it is necessary to take no less than three samples from one dating target. In this case, the sampling points are located one above the other, evenly throughout the thickness of the rock mass. The reliability of the obtained age of the target increases with an increase in the number of dated samples.
When sampling from a layer with a thickness of several centimeters, it is necessary to trace its dip so that the pipe does not pass by. Make sure that the thin layer does not wedge inward from the outcrop wall.
In case of very dense deposits, where it is difficult to clog the pipe (for example, dry loess-like rocks), a monolith of ground is cut out, which is fixed with gypsum for transportation. Sometimes such hard rocks are also sampled at night in opaque sealed bags. Doypack-type aluminium compound foil packs with a triple zip-lock are ideal (Fig. 9). To ensure that the pack is hermetic and light-proof, it would be good to use 2 or 3 bags for each sample.
If a young age of the samples (centuries) is assumed, then in order to increase the accuracy of dating, it is recommended to take a sample of recent sediment of a similar genesis (for example, if young alluvium is dated, this may be subsurface sand from a recent point bar). It will be used as a basis for determining the level of zeroing of electronic traps for appropriate error corrections (Panin, 2014).

Fig. 8. Correctly selected sample for OSL dating


Fig. 9. Doypack-type aluminium compound foil pack with a triple zip-lock

Similarly, a sample from recent sediments can be taken if dating is based on feldspar. When exposed to light, electronic traps in feldspars are reset to zero much more slowly than in quartz, so the probability of getting an older age increases. The feldspar is usually used for dating in the absence or low content of quartz in the sample, or when quartz gives out-of-limit dates. The lower limit of quartz dating is 100-150 kyr, and while feldspar dating, it can reach 500 kyr.


References

1. A.V. Panin. Methods of Paleogeographic Studies: Quaternary Geochronology. Training manual. M.: Geography Department of Moscow State University, 2014, 116 p.