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
238 92 U
atomic number (Z):
amount of protons
Part 1 - The method

6. yellow nuclides with a decay
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
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 %
0.721 % %
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
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 / la ln[(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 / la ln[(la / lf)(rs/ri) Q G I s f + 1]
Part 1 - The method

28. … simplified, for the last 150 m. y
… 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 s f / (rmlf)
rm = density of tracks on dosimeter
zeta age equation
t = 1 / la ln[(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: age length
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: °C
time: minutes to years ( 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

51. Data presentation: vertical profiles
apatite FT ages along bore hole (Wagner et al. 1989)
PAZ
Part 1 - The method

52. Data presentation: horizontal profiles
Part 1 - The method

53. Data presentation: Radial plot - I
Part 1 - The method

54. Data presentation: Radial plot - II
Part 1 - The method

55. Data presentation: Boomerang plot - I
Part 1 - The method

56. Data presentation: Boomerang plot - II
Part 1 - The method

57. Data presentation: Boomerang plot - examples
(from Gleadow
et al. 2002)
Part 1 - The method

58. Thank you for your attention !
Part 1 - The method