TCN - for beginners

THIS SITE IS CURRENTLY UNDER CONSTRUCTION
(particularly the theory-part)!

The Real-Part is quite finished...
 

Theory

Cosmic radiation
 


Sorry, under construction.
See so far Kosmische Strahlung (in German!)

Cosmogenic nuclides

Radioactive cosmogenic nuclides with half-lives longer than 1 year (the ones marked in red can be more or less routineously measured by accelerator mass spectrometry).  See so far Kosmogene Nuklide (in German!)

 

Examples of what can be/has been yet dated:

 

Prehistoric rockfalls
(here at K÷fels, Austria)

Movement of glaciers

glacier

 

 

 

roche moutonnÚe

What has to be kept in mind when dating:

 

self-shielding and shielding

erosion

build-up of cosmogenic nuclides in ideal case of
 "no erosion" (data calculated by R. Braucher)

   

Real world

But how does the application of terrestrial cosmogenic nuclides work in reality?

It all starts - as most geology projects - with field work. You visit some interesting places on Earth with a small group of people (not as much as during the first CRONUS-EU workshop field-trip!).

Some helpful (?) details about sampling can be downloaded here.

One takes by more or less sophisticated technique a sample from e.g. a lava flow to get an age of the volcano eruption or from a rock fall to know when all the rocks came down.

Very often people take only surface samples, i.e. the upper 3-5 cm. This is for sure okay for lava samples and boulders which came down after a rockfall.

But sometimes people prefer to take more samples of one location from different depths e.g. from a "fresh" bedrock to be able to correct precisely for a possible pre-irradiation (inheritance) and the erosion rate. Of course taking samples at different depths means more work, and very often specialised tools like drills to take with you.

Being home again, one spends some days with crushing and sieving the taken samples to appropriate grain sizes (e.g. 250-500 Ám; 250-1000 Ám, <1 mm). For some applications one prefers to start with mineral separates (or at least enriched mineral phases) instead of bulk samples. For this purpose, one can use a Frantz magnetic separator, heavy liquids for density separations or chemicals like dilute acids which dissolve the non-interesting materials. 

For stable isotope measurements, i.e. noble gas measurements, there are different techniques to get rid of non-cosmogenic noble gas sources. To learn more about these techniques and the application of TCN using stable nuclides you should visit other websites or read the papers.

Using long-lived radioactive cosmogenic nuclides will keep you busy for some weeks in a chemistry laboratory - even if your samples are  of "pure" quartz or calcite - to produce a chemical compound which can be measured by accelerator mass spectrometry (AMS).

Because there is a possible contamination of the sample with radionuclides originating from the atmosphere, especially 10Be and 36Cl, the sample material has to be cleaned to get rid of this partition of the nuclides before total dissolution of the samples.

For quartz samples which will be analyzed for 10Be, one leaches about 30 % of the total mass to be sure to remove all atmospheric 10Be. For atmospheric 36Cl a simple water-leaching is assumed to be sufficient, but usually 10 % of calcite is partially dissolved by dilute nitric acid after water leaching.  These steps are performed while grains are kept in motion, e.g. by a shaker-table, an ultrasonic bath or (the latest invention of our American colleagues!) a hot-dog roller.

For the dissolution of the samples we use simple disposable polyethylen bottles. The size depends on total samples mass (250-1000 ml). We add stable isotope carrier (9Be, 35/37Cl, sometimes 27Al) and try to totally dissolve the sample. For quartz dissolution fluoric acid is used and the bottles are put on a shaker-table over-night. For calcite we add as slowly as possible (also cooling the liquid with ice) dilute nitric acid to prevent the loss of stable Cl-carrier before equilibration is reached. Sometimes the sample doesn't dissolve totally and the remaining residues has to be separated and the weight taken into account.

Now, "real chemistry" starts: The goal is always to get as much as possible from the element of interest (e.g. Cl or Be) and as least as possible from the element which contains a disturbing isobar like 10B (for 10Be) or 36S (for 36Cl).  For 36Cl we reach this goal by repeated precipitation and/or redissolution of BaSO4 and AgCl. The latter one being the final chemical compound which is used for AMS measurements.

Before one can start the chemical separation of Be and Al from quartz samples, we need to remove the HF and dissolve the sample in HCl (not necessary for calcite samples!). If you want to determine the stable Al concentration in your sample (which is necessary to calculate the numbers of 26Al from your 26Al/27Al ratio), you should take an aliquot now. Then, a first hydroxide precipitation by adding aqueous ammonia solution takes place leading very often to a brownish precipitate (the colour comes mainly from iron impurities). Each time we do a a hydroxide preparation the purity of Be and/Al will be enriched. The hydroxide is dissolved in HCl and a following anionic exchange will mainly diminish B, Mn and Fe. The hydroxides precipitated now are white and will be redissolved in dilute HCl for a cationic exchange to separate Be and Al from eachother. If the sample contains too much Al, one needs to repeat the cationic exchange.  A final hydroxide precipitation, drying and ignition at 800-900░C g give BeO and Al2O3 as final chemical compounds, which are used for AMS measurements.

After all this work in the field and the chemistry lab, you have the final AMS sample "within your hands", usually not more than 0.5-6 mg. The material will be directly pressed into metal sample holders (e.g. Cu) or mixed with metal powders (e.g. Ag, Nb) for gaining better heat and electronic charge transfer and then pressed.  Now, AMS can start! So you just need one of these fancy AMS machines to measure isotope ratios as low as 10-14. (Do not forget that AMS will need an isotopic standard to quantify the result. So your result cannot be better as the standard the AMS facility uses!) 

 If you think now, "great, I can do it, too" congratulations! But please read the original papers first to understand what/why you are doing this. Contact TCN experts and please acknowledge also the former chemistry work appropriately (e.g. Conard et al., Stone et al., Vogt and Herpers, Merchel and Herpers, Kohl and Nishiizumi, Brown et al. etc.). Good luck for you!

Acknowledgements

  1. my colleagues @ CEREGE, especially the B3s, for their patience and help!

  2. my CRONUS-EU colleagues for all the "geology for dummies"-lessons

  3. my AMS colleagues world-wide for answering all the "AMS physics for dummies"-questions

  4. all the TCN experts (e.g. J. O. Stone, F. M. Phillips, J. Gosse) for describing what they have done in their papers and sharing their knowledge with all of us

  5. all the people who have worked on cosmogenic nuclides in extraterrestrial material and artificially irradiated targets like S. Vogt, M. Honda, K. Nishiizumi. Without their work the development of chemical separation steps and AMS measurements would have been a nightmare for in-situ applications!

  Last modified 25.06.2013     With comments or questions on this homepage please mail to webmaster 'at' meteoroids.de