Download In-Situ Chlorine-36

In-Situ Chlorine-36
Nicole Dix
HWR 696T
Outline
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Introduction
Production Mechanisms
Sample Collection Methods
Laboratory Analysis
Applications
Introduction
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Chlorine has three isotopes. Two of which are
stable (chlorine-35 and 37) and the third is a
cosmogenic isotope (chlorine-36).
However, for this presentation we will only focus
on chlorine-36, which is produced in the solid
materials on the Earth’s surface.
http://www.sahra.arizona.edu/programs/isotopes/chlorine.html
Introduction Continued…
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Because chlorine-36 has a half-life of 310,000
years it is useful in age dating ground water and
solid materials on the Earth’s surface.
Like all cosmogenic nuclides the production of
chlorine-36 depends on the intensity of incident
cosmogenic rays, availability of target nuclei in
the exposed material, and the probability with
which a nuclear reaction produces the nuclide of
interest.
Zreda et al., 2000
Production Mechanisms
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Chlorine-36 is produced in solid materials on the
Earth’s surface primarily through cosmic-ray
induced reactions with chlorine-36, potassium39 and calcium-40.
The three mechanisms of formation are: 1)
spallation reactions, 2) muon reactions and 3)
thermal neutron absorption.
Zreda et al., 1991 & http://www.sahra.arizona.edu/programs/isotopes/chlorine.html
Production Mechanisms
Conti..
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In the top few meters of the Earth’s surface thermal
neutron activation of chlorine-35 and spallation of
potassium-39 and calcium-40 are the dominant means of
production for chlorine-36.
Below that depth, slow negative muon capture, by
calcium-40, becomes more important than the other
mechanisms.
In carbonates chlorine-36 is produced by Ca and in
silicates it is produced by K, Ca, and Cl.
Zreda et al., 1991 & Zreda et al., 2000
Production Mechanisms
Conti..
Zreda et al., 2000
Production Mechanisms
Conti..
Zreda et al., 1991
Sample Collection Methods
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First and foremost, determine the rock/mineral type you
want to sample, from what surface and how many
samples you need to collect.
Because chlorine-36 is produced from several target
elements, virtually all rock types are suitable for
sampling.
The number of samples is related to geological
characteristics of the surface dated, specifically, its
history of burial and erosion.
Zreda et al., 2000
Sample Collection
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Sampling sites should be assessed due to their
geomorphic stability and geometry.
Preferably sampling should take place on flat,
horizontal surfaces that are likely to have been
continuously exposed since the surfaces
formation such as, large tall morainal boulders.
For chlorine-36 samples should be far from
edges because of a possible leakage of thermal
neutrons form the sides.
Zreda et al., 2000
Sample Collection
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Samples need to be collected from the top few
centimeters of rock minimizing the variability of
production rates with depth.
The least weathered surfaces are ideal for
sampling.
Once collected samples should be stored in
plastic bags until preparation.
Zreda et al., 2000
Laboratory Analysis
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First, samples need to cleared of any organic
growth.
They then need to be ground to a size fraction
smaller than the mean phenocryst size of each
rock.
Rock type
Granites etc.
Characteristics
Size
Coarse-crystalline 0.5-1.0 mm
Basalts etc.
Fine-crystalline
Carbonates
0.25-0.5 mm
0.25-1.0 mm
Laboratory Analysis Conti…
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The samples are then leached for 24 hours.
-Silicates are leached in 5% nitric.
-Carbonates are leached in deionized water.
The leaching is done to remove any chlorine
resulting from handling in the field or secondary
carbonates, in the case of silicates, from the
microscopic pore or grain boundaries in the rock
sample.
Laboratory Analysis Conti…
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After leaching, the samples are then dissolved in
airtight capsules or “bombs”.
Silicate samples are dissolved using hydrofluoric
acid and are incased in the “bomb” for 6 hours,
at a temperature of 130 degrees Celsius.
The carbonate samples are dissolved using
concentrated nitric acid and are incase in the
“bomb” for 3 hours, at room temperature.
Laboratory Analysis Conti…
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Once the samples have been digested, AgNO3 is
added to the solution to precipitate out AgCl.
This solution sits overnight.
Next, the liquid is removed and NH4OH is added
to dissolve the solid.
BaNO3 is added to the solution to remove any
sulfur present.
The solution sits overnight.
Laboratory Analysis Conti…
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The next day HNO3 is added until a white precipitate
forms and again AgNO3 is added to precipitate AgCl out
of solution.
This solution stands overnight.
The above steps, using NH4OH, BaNO3, and HNO3, are
repeated two more times.
Finally, the nearly sulfur-free AgCl is rinsed in deionized
water five times, to eliminate any unwanted chemicals,
and dried in an oven at 60 degrees Celsius.
The resulting sample is then weighed and sent to Purdue
University. The amount of chlorine-36 in each sample
will be measured using Accelerator Mass Spectrometry
(AMS).