TSK 11 Göttingen 2006 Dunkl et al.
 (U-Th)/He thermochronol-
 ogy — methodology and a
 case study: dating of faulting
 in the Southern Alps Vortrag
 István Dunkl1 Martin Dani˘sík2 Vin-
 cenzo Picotti3 Wolfgang Frisch2
 Hilmar von Eynatten1 Alberto
 Castellarin3
 History of He thermochronology
 The radiogeic 4He isotope is continu-
 ously forming in the lithosphere mainly
 by the alpha-decay of U, Th and Sm.
 This decay process was discovered al-
 ready at the beginning of the 20th cen-
 tury, and the first U/He dating was
 made by Rutherford (1905). Although
 his result indicated that the dimen-
 sions of the Earth’s history ranges to
 millions of years, the U/He method
 was used only scarcely later on be-
 cause the minerals are usually not closed
 to the decay product. The helium is
 extremely mobile and diffuse through
 the crystal lattices. Thus, the appar-
 ent U/He ages were always younger
 than the radiometric ages determined
 by other isotope geochronometers like
 U/Pb, Rb/Sr or K/Ar. The current
 renaissance of the method has been
 started at the end of eighties, when
 H. Lippolt, P. Zeitler, K. Farley and
 their co-workers have described the pa-
 rameters of diffusion of some uranium-
 bearing minerals (e.g. Lippolt & Weigel
 1987, Zeitler et al. 1987). It turned
 out that the (U-Th)/He apparent ages
 do carry meaningful geological informa-
 tion. The closure temperature of the
 most widely used apatite-He system is
 around 70°C. Thus, by the usage of this
 1 University of Göttingen, Germany 2 Uni-
 versity of Tübingen, Germany 2 University
 of Bologna, Italy
 Figure 1: Schematic vertical trend of ap-
 atite FT and He ages in the uppermost part
 of the lithosphere in a tectonically stagnat-
 ing block (after Stockli et al. 2000).
 mineral/method pair it became possible
 to date low-temperature geological pro-
 cesses, which were undatable by other
 geochronometers.
 Typical applications
 The vertical trends of apparent fission
 track (FT) and He ages in a stagnat-
 ing lithosphere show convergence to the
 age of the latest exhumation phase at
 the surface and they have total reset
 (= zero age) at the depth where the
 temperature does not allow the accu-
 mulation of decay products and nuclear
 tracks (Fig. 1). In between these val-
 ues there is a gradually changing section
 where the apparent ages are the result of
 the balance of continuous accumulation
 of decay products and their disappear-
 ance. This transitional section is called
 as ‘partial annealing zone’ (PAZ) in
 the fission track thermochronology and
 ‘helium partial retention zone’ (PRZ),
 which is defined as a temperature range
 where between 5% and 95% of the He
 is retained in a crystal. The rapid ex-
 pansion of the application of (U-Th)/He
 1
Dunkl et al. TSK 11 Göttingen 2006
 method in different fields can be related
 to its low closure temperature.
 • This allows determining the final
 phase of exhumation of structural
 blocks in the upper crust (e.g.
 Stockli et al. 2000).
 • In ideal conditions (proper gran-
 itoid lithology, possibility of 3D
 sampling in a high mountains, in
 a subsurface mine or in bore-
 holes) the sensitivity of the He-
 thermochronology allows to date
 and determine the magnitude of
 vertical offset along faults (McInnes
 et al., 1999).
 • In cratonic areas, where the typi-
 cal lithologies are high-grade rocks
 and the sedimentary record is usu-
 ally poor the post-orogenic ther-
 mal events can be well recorded
 by apatite fission track and He-
 thermochronology.
 • Not only cooling and exhum-
 ing geodynamic scenarios, but the
 early phase of burial warming of
 sedimentary basins can also be well
 dated. This has prominent impor-
 tance in the hydrocarbon prospect-
 ing, because the temperature range
 of He reset in apatite corresponds
 to the beginning of ‘oil-window’.
 • The depth of the ca. 70°C isotherm
 depends on the geothermal gra-
 dient, the local surface morphol-
 ogy and the areal variation of
 rate of erosional removal. This
 opens the door for apatite He-
 thermochronology in geomorpho-
 logical and morphotectonic studies
 (House et al. 1997).
 Methodology
 The (U-Th)/He age determination is
 performed on single or more crystals
 in multiple aliquots. Only completely
 clear, mineral and fluid inclusion free
 crystals are datable. Crystals with
 fissures and damaged external surface
 have to be avoided. The documenta-
 tion of the dimensions and shape of the
 crystals is crucial. By the radioactive
 decay the alpha-particles has significant
 kinetic energy and ‘jump’ ca. 20–30 µm
 through the lattice of apatite crystals.
 Consequently, the rim of the crystals is
 depleted as the part of helium atoms is
 ejected from the crystals. This causes
 a predictable loss, but this can be cor-
 rected by consideration of the dimen-
 sions and shape (surface/volume ratio)
 of the dated crystals. After the selec-
 tion, the crystals are degassed in a full-
 metal vacuum oven and the released he-
 lium is purified from the reactive gases
 by getters. The He content is measured
 by mass spectrometer using isotope di-
 lution. The mother elements (U, Th and
 Sm) are measured after the dissolution
 of the degassed crystals by ICP-MS.
 Our case study in the Dolomites
 The Dolomites of the eastern Southern
 Alps were formed from the Permo-
 Triassic sedimentary cover of the
 slightly deformed South Alpine in the
 Tertiary. The different parts of the
 Dolomites have suffered two times
 Alpine deformation, and the shortening
 process is actually still active along the
 southernmost thrusts. The immediate
 dating of the deformation is possible
 only in the southern zones, where
 sediments are involved in the thrusts,
 while the northern areas no or rather
 scarce geological evidences exists on
 the timing of structural evolution.
 2
TSK 11 Göttingen 2006 Dunkl et al.
 Figure 2: Simplified geological map of
 Dolomites with the apatite ages, where
 FT chronometer suffered minor reset, but
 the more sensitive He ages indicate Late
 Miocene exhumation.
 The isotope geochronological dating is
 also difficult because the sedimentary
 successions are dominantly composed
 of carbonate rocks. We could use only
 the Triassic volcanic dikes and tuff
 horizons for thermochronology. The
 apatite fission track ages are ranging
 between 210 and 6Ma. The oldest
 ages indicate the presence of slightly
 reset areas and date the exhumation
 of some structural blocks to be Late
 Miocene. The apatite (U-Th)/He ages
 are younger than the FT ages in every
 sample. In the western Dolomites,
 where the Neogene thermal reset is not
 detected by the apatite fission track
 thermochronometer, the He ages show
 Late Miocene reset (Fig. 2) due to the
 lower closure temperature of the later
 method. From the significant contrast
 between the FT and He ages we can
 conclude that the dated stratigraphic
 horizons were deeper than the total
 reset depth of He method but shallower
 than the reset depth of the FT method
 Figure 3: Schematic burial history of the
 samples from western Dolomites. The thick
 black line represents the position of the
 dated samples. They were in a hotter en-
 vironment than the helium retention tem-
 perature, but below the complete FT reset
 temperature.
 between the Late Triassic and Late
 Miocene. We suppose that beyond the
 general Miocene uplift of the Dolomites
 mainly the displacements along the
 Stava Line and Schio-Vicenza Fault
 or Bassano Thrust are responsible for
 the exhumation of Passo Feudo and
 Recoaro area, respectively. Figure 3
 shows a supposed burial path for the
 samples presented in Figure 2.
 References
 Lippolt HJ & Weigel E (1987) 4He diffusion
 in 40Ar-retentive minerals. Geochim. Cos-
 mochim. Acta, 52, 1449–1458
 McInnes BIA, Farley KA, Sillitoe RH & Kohn
 B (1999) Application of apatite (U-Th)/He
 thermochronometry to the determination of
 the sense and amount of vertical fault dis-
 placement at the Chuquicamata porphyry
 copper deposit, Chile. Economic Geology,
 94, 937–948
 Stockli DF, Farley KA & Dumitru, TA (2000)
 Calibration of the apatite (U-Th)/He ther-
 mochronometer on an exhumed fault block,
 White Mountains, California. Geology 28,
 983–986
 Zeitler PK, Herczeg AL, McDougall I & Honda
 M (1987) U-Th-He dating of apatite: a po-
 tential thermochronometer. Geochim. Cos-
 mochim. Acta, 51, 2865–2868
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