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Fission tracks and their application in Geology

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  1. Fission tracks and their application in Geology Lecture on fission track dating in three parts I The method - what is fission tracks about II Application of the fission track method III Fission tracks and very low-grade metamorphism

  2. What is the aim of the three courses ? learn about the physical background of track formation learn how to read and interpret fission track data and evaluate FT data from the literature learn about the potential of this method - and its flaws eventually apply the method in your research

  3. 3 key questions What process creates fission tracks? How do we measure fission tracks? How are fission track data presented? Part 1 - The method

  4. The periodic table of elements Part 1 - The method

  5. Description of a nuclide atomic mass (N): amount of protons + neutrons 23892 U atomic number (Z): amount of protons Part 1 - The method

  6. The nuclide chart black stable nuclides red nuclides with b+ decay blue nuclides with b- decay green nuclides with spontaneous fission yellow nuclides with a decay last stable element Part 1 - The method

  7. Radioactive decay Part 1 - The method

  8. a-decay of Uranium Part 1 - The method

  9. spontaneous fission of 238U Part 1 - The method

  10. The “camel“ curve Part 1 - The method

  11. Fission on the nuclide chart Part 1 - The method

  12. Induced decay of 235-Uranium Part 1 - The method

  13. Natural ratios and half life times T1/2 a decay 4.47·109 y 7.04·108 y 1.40·1010 y T1/2 spont. fission 8.19·1015 y 1.0·1019 y 1.0·1021 y Nuclide 238U 235U 232Th Element ratio 99.275 % 0.721 % 100.000 % Only 238U is important for fission track formation ! Part 1 - The method

  14. track formation in crystal lattice track etching to make them visible Part 1 - The method

  15. Can we see fission tracks without etching ? TEM image from Yada et al. (1987) Part 1 - The method

  16. fission track annealing track length as a second important parameter Part 1 - The method

  17. Age versus U content Part 1 - The method

  18. minerals for FT dating: apatite apatites from the Durango thermal field apatite Ca5(PO4)3(F, CL, OH) U Part 1 - The method

  19. minerals for FT dating: zircon zircon ZrSiO4 U Part 1 - The method

  20. Sample preparation procedure Part 1 - The method

  21. Counting the tracks apatite crystal mica print Part 1 - The method

  22. Not all grains can be counted... tracks on plane II a, b We only count tracks on grain surfaces II c axis, i.e…. - with blade shaped tracks - with sharp polish scratches - with parallel etch pits - with corresponding geometry tracks on plane II c Part 1 - The method

  23. Why do we have to irradiate our samples? Before irradiation, we only observe tracks generated by the spontaneous decay of 238U. These tracks are counted per area unit. The number of tracks are a function of age and U content. For age calculation, we need to know the grain's individual U content. The sample is irradiated with a dosimeter of known U content. During irradiation, the 235U in the grain undergoes a fission decay and produces induced tracks. Part 1 - The method

  24. What do we count Induced tracks on dosimeter mica (rd): each irradiated sample package has at least two dosimeters (top, bottom)  estimation of irradiation gradient Spontaneous tracks on mineral grain (rs) Induced tracks on mica print of the mineral grain (ri) Part 1 - The method

  25. Induced tracks on white mica Part 1 - The method

  26. The FT age equation - 1 dNP/dt = -lNP NP = number of parent atoms NP = (NP )0e-lt l = decay constant ND = Np (e-lt -1) ND = number of daughter atoms ld= la + lf where ld ≈ la Ns = lf / la238N(e-lat -1) Ns = number of spont. tracks t= 1/ la238ln[(la/ lf)(Ns/238N) + 1] Part 1 - The method

  27. The FT age equation - 2 Ni = 235N s f s = neutron cross section f = neutron fluence Ni = 238N I s f I = 235U / 238U general age equation t= 1/ laln[(la/ lf)(Ns/Ni) I s f + 1] G= geometry factor = 0.5 (2p/4p geometry) Q = revelation factor = 1 practical age equation t= 1/ laln[(la/ lf)(rs/ri) Q G I s f + 1] Part 1 - The method

  28. … simplified,for the last 150 m.y., track production has been constant Part 1 - The method

  29. The zeta (z) age approach Problem: Many of the factors in the age equation are loosely constrained or not accurately known. It was proposed to use age standards and irridiate the standards together with dosimeter glasses of known U content  calibration factor z (zeta) z= Q I sf / (rmlf) rm = density of tracks on dosimeter zeta age equation t= 1/ laln[(la)(rs/ri) rmG z + 1] Part 1 - The method

  30. What is the error of a fission track age ? error(age) = error(rs) + error(ri) + error (rd) + SD(z) Poissonian statistics: error (n) = n-0.5 Generally, errors for fission track ages are large compared to other radiologic dating methods, in the range of 5-10%. Part 1 - The method

  31. The partial annealing zone Part 1 - The method

  32. Crossing the partial annealing zone Part 1 - The method

  33. Track length histogram Part 1 - The method

  34. How can we observe tracks in full length ? Part 1 - The method

  35. Confined fission tracks under the microscope TINCLE = track in cleavage TINT = track in track Part 1 - The method

  36. Long and short tracks, gaps (from Barbarand et al. 2003) Part 1 - The method

  37. The shape of the time-temperature path Part 1 - The method

  38. data production:agelength Part 1 - The method

  39. final data for each locality Part 1 - The method

  40. What rocks are suitable for FT dating? Well suitable: granites, pegmatites, syenites, granodiorites, orthogneisses from these lithologies. Less suitable: sandstones, greywackes, paragneisses, andesites, basalts Not suitable: very pure qtz or cal sandstones, limestones, silt- and claystones, ultramafics Part 1 - The method

  41. Rules of thumb for sample collection: Well suitable: granites, pegmatites, syenites, granodiorites, orthogneisses from these lithologies. Less suitable: sandstones, greywackes, paragneisses, andesites, basalts Not suitable: very pure qtz or cal sandstones, limestones, silt- and claystones, ultramafics Part 1 - The method

  42. Cl content of apatite Ca5(PO4)3(F, CL, OH) Part 1 - The method

  43. Zircon FT annealing and radiation damage young or U poor zircon  strong retentivity for FT  high closure temperature old or U rich zircon  lower retentivity for FT  low closure temperature Part 1 - The method

  44. Closure temperature and PAZ Part 1 - The method

  45. Annealing studies annealing experiments under laboratory conditions: field studies and comparison with other indicators for time and temperature • temperature: 100-1000 °C • time: minutes to years (102 - 109 s) • bore holes • contact aureoles around intrusive bodies • areas with regional metamorphic gradients Part 1 - The method

  46. Laboratory annealing studies Part 1 - The method

  47. Combining laboratory and field information Part 1 - The method

  48. What does a fission track age mean? It dates a fast to very fast cooling event (with complete crossing of the partial annealing zone) It dates a moment during cooling across the partial annealing zone It forms a mixed age between an old and a young age component It provides an age information about the detrital origin the single grains in a sample Part 1 - The method

  49. When does the FT method fail to provide an age information if the time-temperature history is very complex (e.g. contains short-term hydrothermal events) if the sample has been heat treated after cooling (e.g. gem quality zircons in Sri Lanka) if a sample contains many different age populations or age origins if apatite grains in a sample are very variable in Cl content if zircon grains are very variable in U content and therefore accumulated radiation damage Part 1 - The method

  50. Data presentation - I: maps Part 1 - The method