In-Situ Chlorine-36 Nicole Dix HWR 696T

Outline Introduction Production Mechanisms Sample Collection Methods Laboratory Analysis Applications

Introduction 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… 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 Chlorine-36 is produced in solid materials on the Earth’s surface primarily through cosmic-ray induced reactions with chlorine-36, potassium-39 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.. 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 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 Continued... 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 Continued... 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 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 Characteristics Size Granites etc. Coarse-crystalline 0.5-1.0 mm Basalts etc. Fine-crystalline 0.25-0.5 mm Carbonates 0.25-1.0 mm

Laboratory Analysis Conti… 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… 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… 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… 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).