TCN -
for beginners
THIS SITE IS CURRENTLY AND WILL BE ALWAYS ;-) UNDER CONSTRUCTION
(particularly the
theory-part)!
The Real-Part is quite finished...
Theory
Cosmic radiation
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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!) |
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Examples of what can be/has been yet
dated: |
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Prehistoric rockfalls
(here at Köfels, Austria) |
Movement of glaciers
glacier
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roche moutonnée |
What has to be kept in mind when dating: |
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self-shielding
and shielding |
erosion |
build-up of cosmogenic nuclides in ideal case of
"no erosion" (data
calculated by R. Braucher) |
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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. |
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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. |
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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. |
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Being home again, one spends some days with crushing and sieving the
taken samples to appropriate grain sizes (e.g. 250-500 µm; 250-1000
µm, <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). |
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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. |
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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. |
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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!) |
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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
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my colleagues @ CEREGE, especially the B3s,
for their patience and help!
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my CRONUS-EU colleagues for all the "geology
for dummies"-lessons
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my AMS colleagues world-wide for
answering all the "AMS physics for dummies"-questions
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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
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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!
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