Isolation of Quartz Grains for Optically Stimulated Luminescence (OSL) Dating of Quaternary Sediments for Paleoenvironmental Research

This protocol is, was developed for isolating quartz grains for OSL dating. These procedures were developed over the past 20 years. This is not a static protocol, but we welcome additions, suggestions, and improvements.

This contribution includes detail protocols for use of imagery and Raman technology to isolate quartz fractions for luminescence dating. These protocols are designed for different applications. The researchers should take appropriate cores to separate fraction of sediments and to be able to isolate quartz for OSL applications.

To begin, open, describe, and interpret sediment cores. Evaluate the variation and sedimentologic features, such as particle size changes, sedimentary and diagenetic structures, bedding, if visible, Munsell colors, the basis for unit boundaries, and identify the sequences of strata. Transfer the core sections to the luminescence lab to sample for OSL dating and safe light conditions.

To collect the OSL sample, use a spatula to score a two-centimeter diameter circle from the centerpoint of the core face to define the sampling area. Scrape off the upper one centimeter of light-exposed sediment with a utility knife, and put the sediment in a labeled ceramic evaporation dish. Use this dried sediment sample for dose rate calculations.

Extract 10 to 30 grams of the light-shielded sediment with a spatula from the circular, scored, central area of the core, and place it in a labeled 250-milliliter polyethylene beaker for luminescence dating. After pulverizing and homogenizing the dried sample, remove the organic matter by slowly adding 30 milliliters of 25%hydrogen peroxide to 30 to 60 grams of the sediment in a 250-milliliter polyethylene beaker. Then stir well with a glass rod to facilitate the reaction.

To remove calcium carbonate and magnesium carbonate from the sediment, slowly add less than one milliliter of 15%hydrochloric acid to the sediment, and assess the effervescences. Then add up to 30 milliliters of hydrochloric acid for every five grams of sediment, and stir well with a glass rod to facilitate the completion of the reaction. Add more hydrochloric acid if necessary until the effervescence production stops, and keep the mixture inside the fume hood for at least 12 hours.

For removing the magnetic grains in a water-based solution, place the sediment in a 250-milliliter glass beaker containing approximately 100 milliliters of 0.3%sodium-pyrophosphate solution, and stir thoroughly until the sediment is well-disaggregated. Stir the mixture on a magnetic stir equipped with a hot plate at 8, 000 RPM for five minutes at ambient laboratory temperature. After removing the magnetic rods, rub the rods with a cloth or another magnet to separate the attracted magnetic grains.

Then return the magnate to the mixture, and repeat until no magnetic minerals are recovered. To separate the desired sand fraction, for example, 150 to 250 microns, add approximately 100 milliliters of 0.3%sodium-pyrophosphate solution to a 250-milliliter beaker containing the non-magnetic sediment, and stir thoroughly with a glass rod to facilitate particle dispersion. Place the assembled circular sieving guide tightly with framed mesh, followed by setting a one-liter beaker with a 250-micrometer mesh guide.

Separate the sediment into two sizes:greater and lesser than 150 microns. Store the samples less than 150 microns, and continue separating the particles larger than 150 microns to obtain the target range of 150 to 250 microns. Slowly pour the dispersed sediment mixture over the 250-micrometer mesh while manually swirling the mixture.

The sediment of particles less than 250 microns corresponds to the target size of 150 to 250 microns. Archive the sediment remaining on the mesh that is larger than 250 microns, and dry overnight for possible future analysis. Once the sediment has been separated at the desired size, add 70 to 80 milliliters of heavy liquid to the dried sediment fraction.

Following mixing, pour the mixture into a labeled 100-milliliter graduated cylinder, and cover the top of the cylinder with a wax sealant to avoid evaporation. Place the cylinder inside a fume hood to remain undisturbed and shielded from light, and allow the sample to separate in two markedly different zones for at least one hour. The higher floating, lighter minerals are often enriched in K-feldspar and sodium-rich plagioclases;and the lower, heavier grains are rich in quartz and other heavier minerals.

Next, leave the two distinct, separated sediments to dry. Use the sediment lighter than 2.6 grams per cubic centimeter for future assays and the heavier sediment for further separation with heavy liquid at 2.7 grams per cubic centimeter. Repeat the separation process as demonstrated earlier.

Store the heavier sediment and proceed with the acid digestion for the lighter fraction. Next, wearing an appropriate PPE kit, place a 250-milliliter, heavy-duty polypropylene beaker containing the sample inside the fume hood. After lowering the sash, add 20 milliliters of hydrofluoric acid to the beaker by pump increments for every two grams of quartz, and cover the beaker with wax paper sealant.

After 80 minutes of hydrofluoric acid digestion, wash the samples with the ionized water and immerse the undigested mineral grains in concentrated hydrochloric acid. Wearing the PPE kit, place the beaker containing the sample in the fume hood, followed by the addition of hydrochloric acid, and seal the beaker as demonstrated earlier. Use a dissecting needle to place 200 to 400 mineral grains on a glass slide, and inspect under a 10-times or 20-times binocular or PETRA-scopic microscope to identify grain minerals.

Quantify the percentage of quartz grains by point-counting, and record the mineralogy of 100 individual grains. And if a subsample exhibits greater than 1%non-quartz minerals, or is an unwanted material with high photon output, or remains unidentified, queue the sample for Raman spectroscopy. For Raman spectroscopy, place the sample in the spectrophotometer.

Use a blue beam with a width of five micrometers and 100 grain-point counts to assess the percent purity of quartz. Identify the unknown grain minerals, and analyze them to find quartz. To assess the quartz purity spectra by infrared stimulation, shake the grains onto a circular, aluminum disc to prepare five ultra-small aliquots of quartz.

Load the disc on a sample carousel for stimulation by infrared LED. Compare the obtained spectra with the spectra obtained by blue-light excitation which is preferential for quartz. The white sand and Mongolia core sections were processed in the current study.

The sample from white sand contains sulfates, mainly gypsum, halides, and very little quartz. The process sample showed a separate fraction that contains mostly quartz. However, the presence of some vestiges of gypsum was detected by the Raman spectroscopy.

The infrared blue ratio was 9%corroborating that further processing of the sample is required. The Mongolian sample is very rich in felsic feldspars, predominantly K-feldspar. After the cleaning procedures, abundant quartz was isolated, rendering a satisfactory infrared blue ratio of 3.7%The fast ratio in three samples representing different degrees of quartz fraction purity was compared.

The fast component in a pristine aeolian sample from Red River was 72. A sample with incomplete quartz and plagioclases represented that the L2 and L3 components were a significant percent of the L1 component. In contrast, a shine-down curve for feldspathic quartz had a dominant medium component L2.Choosing the right sample is paramount to get the best dating results.

It's important that the sample has tight stratigraphic context, it remains unlight-exposed prior to preparation laboratory, and there's enough particle size of quartz for effective dating.