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In-Situ Chlorine-36

In-Situ Chlorine-36. Nicole Dix HWR 696T. Outline. Introduction Production Mechanisms Sample Collection Methods Laboratory Analysis Applications References. Introduction.

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In-Situ Chlorine-36

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  1. In-Situ Chlorine-36 Nicole Dix HWR 696T

  2. Outline • Introduction • Production Mechanisms • Sample Collection Methods • Laboratory Analysis • Applications • References

  3. 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

  4. 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

  5. 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

  6. 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

  7. Production Mechanisms Conti.. Zreda et al., 2000

  8. Production Mechanisms Conti.. Zreda et al., 1991

  9. Production Rates • Case Study (Phillips et al., 1996): Measured the chlorine-36 content in 33 rock samples of well-constrained exposure histories and ages. • Production parameters were made by minimizing the squared deviation (2) between the chlorine-36 and independent ages (found through carbon-14, argon, or thermoluminescence). Phillips et al., 1996

  10. Production Rates Continued… Phillips et al., 1996

  11. Production Rates Continued… • The following production rates were found: • Spallation and muon production from Ca 2940+200 atoms 36Cl (mole Ca)-1 yr-1. • Spallation from K 6020+400 atoms 36Cl (mole K)-1 yr.. • Neutron production in air 586+40 fast neutrons (g air)-1 yr-1. • The new production constants found in Phillips et al., 1996 for the spallation of Ca and thermal neutron activation are in agreement with previous constants. However, the new production constant for K is now about 50% larger than previously thought. • Previous constants were: 4160+310 atoms 36Cl yr-1 mol-139K, 3050+210 atoms 36Cl yr-1 mol-140Ca, and (3.07+0.24)*105 neutrons (kg of rock)-1 yr-1. Phillips et al., 1996

  12. Production Rates Continued... • However, in Phillips et al., 2001, the value for the neutron production in air increased from 586 to 626 fast neutrons (g air)-1 yr-1 and the value for the spallation of Ca decreased by 9% from 73.3 to 66.8 atoms 36Cl (g Ca)-1 yr-1, due to the inclusion of production by muon absorption of Ca (unlike the 1996 study). The production rate value for the spallation of K stayed the same. • These values seem to be the most accurate to date, but will most likely be changed in the future. Phillips et al., 2001

  13. Production Rates Continued.... • Problems in calculating production rates include: • The lack of a perfect calibration site. The calibration cannot be more accurate than the chronology on which it is based. At sites erosion occurs, burial, etc. • Problems with scaling factors for location and geomagnetic strength. *If scaling factors were known with an absolute certainty or the “perfect” site were to be found then production rates would no longer be controversial and ever changing.

  14. 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

  15. 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

  16. 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

  17. 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.

  18. 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.

  19. 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.

  20. 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.

  21. 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).

  22. Applications • Chlorine-36 can be used to date impact craters, paleoseismic events, glacial moraines, young paleoshorelines and young volcanic events. • Impact Craters are formed instantly and deep enough to ensure that there was no prior cosmogenic nuclide build up at the base of the crater. Therefore, this allows the date of the impact to be determined using chlorine-36.

  23. Applications Continued… • Paleoseismic Events can be dated by using samples from the face of a fault scarp. During an earthquake previously buried rock is brought up to the surface exposing it to cosmic rays. While dating a fault it is important to recognize the fact that faults can be active more than once and to collect samples accordingly. • The bedrock that the fault scarp is composed of is ideal for dating using chlorine-36 due to it resistance to weathering.

  24. Applications Continued… • Glaciers uplift bedrock ask they move down a valley. The material is then later deposited, some of which is deposited as glacial moraines. -The assumption in all of this is that the glacial erosion extends deep enough to bring material to the surface that was previously shielded from comic rays. • Once deposited the material becomes bombarded by cosmic rays and chlorine-36 begins to accumulate. • It is possible to correlate the deposition of moraines located in different areas during the same time interval. Zreda et al., 2000

  25. Applications Continued… • Two types of materials from ancient lake shorelines are suitable for surface exposure dating: (1) clasts transported by streams and redeposited at the shore; and (2) tufa deposits precipitated directly from lake water. • A problem with this experiment is the fact that some fluvial deposits may have already accumulated chlorine-36 and this must be assessed before sample analysis. • The tufa, at the time of deposition, will have the same concentration of chlorine-36 as the lake water. This amount can be subtracted out when calculating the shoreline age. Zreda et al., 2000

  26. Applications Continued… • Lava flows are ideal for surface exposure dating for three reasons: (1) they originate deep within the Earth, (2) they form almost instantaneously, and (3) they have surface structures that help assess their geomorphic stability. • Youngest lava flows= most recent volcanic activity • Mt. Erciyes, Turkey Zreda et al., 2000

  27. References Phillips, F., Zreda, M., Montgomery, F.R., 1996, A Reevaluation of Cosmogenic Chlorine-36 Production Rates in Terrestrial Rocks, Geophysical Research Letters 23 No. 9, 949-952. Phillips, F.M., Stone, W.D., Fabryka-Martin, J.T., 2001, An Improved Approach to Calculating Low-Energy Cosmic-Ray Neutron Fluxes Near the Fluxes Near the Land/Atmosphere Interface, Chemical Geology 175, 689-701. Zreda, M.G., Phillips, F.M., Elmore, D., Kubik, Sharma, P., and Dorn, R.I., 1991, Cosmogenic Chlorine-36 Production Rates in Terrestrial Rocks, Earth and Planetary Science Letters 105, 94-109. Zreda, M.G., and Phillips, F.M., 2000, Cosmogenic Nuclide Buildup in Surficial Materials, in J.S. Noller, J.M. Sowers and W.R. Lettis, eds., Quaternary Geochronology: Methods and Applications, AGU Reference Shelf 4, American Geophysical Union, 61-76.

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