(U-Th)/He chronometry (not just for breakfast anymore)

1. “The helium observed in the radioactive minerals is almost certainly due to its production from the radium and other radioactive substances contained therein. If the rate of production of helium from known weights of the different radioelements were experimentally known, it should thus be possible to determine the interval required for the production of the amount of helium observed in radioactive minerals, or, in other words, to determine the age of the mineral.” Ernest Rutherford, March 1905, Silliman Lectures, Yale University

2. (U-Th)/He chronometry(not just for breakfast anymore) • Tectonics and geomorphology • erosional and tectonic exhumation, landscape evolution pubs with “(U-Th)/He” in title or abstract 2. Detrital He dating provenance, source terrane evolution, erosion gradients Wolf et al., 1996a,b 3. Volcanology, stratigraphy Zeitler et al., 1987 4. Dating chemical weathering 5. Meteorite thermal histories 6. Economic mineralization, basin thermal histories

3. Outline 1. Calibrations: He diffusion, complexities 2. Analytical techniques 3. Closure temperatures, closure depths, thermal models 4. Examples 5. Prospects

4. U, Th decay chains l238 = 1.55125 x 10-10 yr-1 … t1/2 = 4.468 x 109 yr l235 = 9.8485 x 10-10 yr-1 … t1/2 = 0.7038 x 109 yr l232 = 4.9475 x 10-11 yr-1 … t1/2 = 14.010 x 109 yr 4He = 8·238U(elt-1) + 7·235U(elt-1) + 6·232U(elt-1) + 147Sm(elt-1)

5. Thermochronology of a rock from southeast Alaska

7. <38 mm, Tc = 175 °C 43-74 mm, Tc = 196 °C 74-140 mm, Tc = 200 °C 250 mm, Tc = 203 °C He diffusion Apatite: Ea = 33 kcal/mol; D0 = 32 cm2/s Titanite: Ea = 45 kcal/mol; D0 = 60 cm2/s He in apatite(Farley, 2000) Zircon: Ea = 40 kcal/mol; D0 = 0.46 cm2/s He in zircon (Reiners et al., 2004) He in titanite (Reiners & Farley, 1999)

8. He diffusion: Importance of grain and/or domain size typical apatites a = 0.1 micron a = 1 micron a = 10 micron a = 100 micron a = 1000 micron

9. Apatite complexities • decreasing diffusivity at low f is observed sometimes • weak anisotropy • Rollover at ~265 °C Farley, 2000 Shuster et al., 2003

10. zircon complexities • decreasing diffusivity observed in all specimens • increasing diffusivity observed only for one whole-grain sample • No rollover observed • Slight continuous diffusivity decrease

11. 4He diffusion in other minerals Farley and Stockli, 2002 xenotime monazite goethite epidote Nicolescu and Reiners, unpub data Shuster et al., 2005

12. 1. Apatite: Ca5(PO4)3(OH,F,Cl) 2. Zircon: ZrSiO4 Tc ~ 70°C; U,Th ~ 100-101 ppm Tc ~ 180°C; U,Th ~ 102-103 ppm Other phases: rutile, fluorite, garnet, epidote, magnetite, monazite 3. Titanite: CaTiSiO5 Tc ~ 200°C; U,Th ~ 101-102 ppm

13. measurements Apatite: l, w1, w2, broken? Zircon: l, w1, w2, h1, h2

14. Alpha ejection For a sphere: FT = 1 – (3/4)(S/R) + (1/16)(S3/R3) For R >> S: FT = 1-(S/4) ∙ b Farley et al., 1996 Hourigan et al., 2005

15. Complications to alpha ejection • Broken, abraded, or polished crystals • a. With assumptions, corrections for this are typically <5-10% • Interaction of diffusion with alpha ejection • a. Negligible (?) unless crystals have resided in partial retention zone for long periods • 147Sm contribution • a. 147Sm stopping distance is only ~6-7 mm • U-Th zonation • a. Systematic core-rim zonation can produce up to ~35% inaccuracies

16. Gas extraction/processing/measurement • 4He blanks ~0.05-0.10 fmol • 0.1 or 0.4 pmol of 3He spike added • 3He corrected for 3H and HD by monitoring m/z = 1 • 4He determined by comparing (4He/3He)m of sample with (4He/3He)m of standard with manometrically-calibrated 4He delivery • In a single day, (4He/3He)m standard reproducibility is ~0.1-0.5%

17. U, Th, Sm analysis by ICP-MS • Each sample spiked with 0.4-0.8 ng 233U, 0.6-1.2 ng 229Th, 0.6 ng 147Sm • Apatite dissolved in Pt-foil in ~50% HNO3 at ~80 °C • Zircon (and Nb) dissolved in Parr bomb with 2-stage HF-HNO3 and HCl method • ~1 ml of 5% HNO3 (+0.6% HF for zircons) analyzed on HR-ICP-MS • 2000 measurements of 238U/233U, 232Th/229Th, 152Sm/147Sm, 238U/235U made on 6 samples in middle 5% of peaks • Typical RSDs on measured ratios: 0.1-0.5% • Blanks for apatite: 1-2 pg for U and Th; blanks for zircon: 2-3 pg for U, 3-5 pg for Th

18. U and Th concentrations in apatite

19. U and Th concentrations in zircon

20. Durango apatite mean = 32.1 Ma 1 stdev = 1.0 Ma (3.1%) n = 156 Jonckheere et al., 1993 McDowell et al., 2004 House et al., 2000 Schmitz and Bowring, 2001 Fish Canyon Tuff zircon mean = 28.3 Ma 1 stdev = 1.2 Ma (4.2%) n = 98

21. How low can He dating go? 4He blank is typically 0.05-0.1 fmol... Zircon: 11 ± 1 ka for Powder River Basin clinker Apatite: ~300 ka Garnet: 1885 ± 188 a (Aciego et al., 2004) How old? Meteorites: Acapulco: 4.54 ± 0.02 Ga (Min et al., 2003) St. Severin: 4.31 ± 0.19 Ga (Min and Reiners, unpub.) Terrestrial: Apatite: 0.86 ± 0.11 Ga (Flowers et al., unpub.) Zircon: 1.76 ± 0.06 Ga (Flowers et al., unpub.) (Note: these zircons have 10-60 ppm U…) Radiation dosage and He diffusivity in apatite and zircon?

22. Thermochronologic signals and noises • Timing, rate of movement towards surface (exhumation) • Paleotopography • Structures • Fluid flow • Magmatism • Surface effects: • Wildfire • Volcanics • Diurnal heating • Shear heating

23. Using (U-Th)/He in vertical transects to look at uplift / exhumation Isostatic uplift of the Coast Range, BC Farley et al., 2001

24. Exhumation of normal fault blocks From Reiners et al., 2000

25. Slow exhumation of the Dabie Shan, China, using (U-Th)/He in lateral transect Reiners et al., 2003

26. Using (U-Th)/He to look at paleotopography Relationship between Helium ages, depth, and temperature

27. Sierra Nevada paleo-topography

28. Interpreting (U-Th)/He ages: Spatial variations across a landscape

29. Interpreting (U-Th)/He ages: Closure temperature (Tc)

30. Interpreting (U-Th)/He ages: Closure depth (Zc)

31. Interpreting (U-Th)/He ages: Steady erosion rate (dz/dt)

32. (U-Th)/He dating example: Washington Cascades

33. FE = Erosional flux FA = Accretionary flux • Surface erosion: Channels and hillslopes • Bedrock incision: • Bedrock landsliding: Drew Stolar, 2004

34. Non-uniform precipitation Rock uplift rate [km/Myr] • Landscape response is weak. • Uplift and deformation response is localized and strong. Drew Stolar, 2004

35. Detrital apatite (U-Th)/He dating: Whole-catchment erosion histories

36. Zircon He-Pb double dating Missouri and Mississippi Rivers

37. Oak Creek vent X, Big Pine Volcanic Field, CA Dating young basalts by xenolithic zircons Little Bear Mountain volcano, BC wtd. mean = 179 ka 1 wtd stdev = 18 ka (8.2%) MSWD = 15 wtd. mean = 157 ka 1 wtd stdev = 4 ka (2.5%) MSWD = 1.7 Oak Creek vent X, BPVF

38. Wildfire dynamics by (U-Th)/He dating Mitchell & Reiners (2003) bedrock wildfire effects

39. FT-He kinetic crossover and wildfire resetting

40. Prospects: • Spatially-resolved intracrystalline ages

41. Method 1: 4He/3He thermochronometry (Shuster) Method 2: laser ablation (Hodges, Boyce)

42. Ongoing calibrations and technique development for other phases Other Prospects Dating chemical weathering Stockli, 2005 Combining thermochronometry and cosmochronometry in single detrital xtals Shuster et al., 2005

44. No (or few) new zircons formed in Himalayan orogeny, but exhumation rates > 1-5 mm/yr ~ Zircon He-Pb double-dating Orogenic histories of source terranes

45. Conclusions • Low temperature chronometers are FT and He; • Cover a wide range of applications, from traditional thermochronology applied to tectonics to novel apps to provenance studies, resolving paleotopography, etc. • More detailed tectonic apps will be presented in my case studies at the end of class.