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Cosmogenic 3 He dating of accessory phases

Cosmogenic 3 He dating of accessory phases. William Amidon - Caltech Kenneth Farley - Caltech Dylan Rood- LLNL. Outline. Introduction Dealing with 3 He production via 6 Li Examples from calibration studies Summary and considerations. Why develop 3 He method?.

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Cosmogenic 3 He dating of accessory phases

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  1. Cosmogenic 3He dating of accessory phases William Amidon - Caltech Kenneth Farley - Caltech Dylan Rood- LLNL

  2. Outline • Introduction • Dealing with 3He production via 6Li • Examples from calibration studies • Summary and considerations

  3. Why develop 3He method? • Easy, fast, and inexpensive • High [production rate/ detection limit] • Stable isotope: - No saturation - Applications to Paleo-Erosion

  4. All 3H decays to 3He (T1/2 = 12.5 yrs) Sources of 3He • Thermal neutron capture on 6Li: 6Li(n,a)3H 3He • (Main challenge! Can be as large as spallation component!!) In-situ “spallation” production (3H and 3He) (i.e. high energy neutrons colliding with target nuclei)

  5. “CTN” Component Radiogenic Component

  6. Li-produced 3He components can be large...What do we need to know? • Thermal neutron flux (model: Phillips et al., 2001) • Grain size and shape (grain counting) • U-Th/He closure age (measure) • Internal Li content (ICPMS) • Average “Host” Li content (complicated!!!)

  7. Solution: Calculate apparent Li from shielded sample Ejected term Implanted term S = stopping range (~30 mm) R= equivalent grain radius measure measure calculate calculate measure

  8. Use Liapp to calculate the CTN fraction.... Invariant with erosion rate or exposure age!!! Solve for spallation component...

  9. Shielded Samples Zircon Sierra Nevada U-Th/He age = ~50 Ma Liapp~10 ppm Apatite Sierra Nevada U-Th/He age = ~35 Ma Liapp~4 ppm shielded pyroxene!! Zircon Shoshone Rhyolite U-Th/He age = ~6 Ma Liapp~ 3 ppm 2s standard error

  10. Surface samples- Coso volcanic field Ar/Ar age = 0.6 My Exp age = 50-100 Ky Measured data show spread due to different apparent Li values measured Spread collapses when corrected for Li-produced 3He CTN component 2-50% !! spall-produced

  11. Liapp = 1 ppm 1 My UTh/He=10 My UTh/He = 100 My Liapp = 10 ppm 1 My UTh/He = 10 My UTh/He = 100 My

  12. Li measurements

  13. Zircon: 117 ± 12 at/g*yr Apatite: 139 ± 19 at/g*yr

  14. Conclusions • 3He dating of zircon and apatite should be successful in many geologic contexts !! • Highest accuracy if.... 1) Large grain size (> 80 mm width) 2) Thousands of grains 3) Shielded sample of same lithology 4) Moderate U-Th/He age relative to exposure age 5) Relatively low Li (< 100 ppm bulk rock)

  15. Thanks to: • National Science Foundation • Doug Burbank, Beth Pratt-Sitaula, Frank Monastero, Kevin Uno, Don Burnett

  16. Future Work • More calibration studies... send us your <300 mm fraction!! • Understand 3H/3He spallation production ratio • Diffusive loss of 3H from common minerals

  17. neutron sources that drive 6Li(n,a)3H • Radiogenic: -Invariant with depth - Begins at He closure age • Muogenic: ~2x radiogenic rate -Tiny below ~ 20 m • Cosmogenic: ~2-20x radiogenic rate -Upper 2-3 meters

  18. Host Li / Internal Li = 1 Host Li / Internal Li = 100 Host Li / Internal Li = 10

  19. Approach #2: Calculate “apparent Li” • Acquire a shielded sample (same lithology) • Lightly crush sample (need intact grains!!) • Sieve into narrow size fractions • Measure 3He and U/Th-He closure age

  20. What is cosmogenic dating? Offset terraces Glacial Moraines basin-scale erosion rates Gosse and Phillips, 2001 Atmospheric neutrons collide with nuclei in rock, producing rare isotopes

  21. 10 20 30 50 70 100 150 500

  22. Comparison of two Liapp techniques 1:1 Line

  23. Approach #1: • Identify“host” minerals in polished sections • Measure Li in all of them... (ouch!) “Publishable” “Complicated” “Downright Terrifying”

  24. Site 1: Shoshone Falls Rhyolite Twin Falls, ID • U-Th/He Age: ~6.5 My • Exposure age: 14,500 yr • Low Li content

  25. Site 2: Fremont Lake Granodiorite Sonora Pass, CA • U-Th/He Age: ~40-55 My • Exposure age: ~18,000 yr • Moderate Li content Thesis work of Dylan Rood

  26. Ar/Ar Age: ~0.6 My • Apparent Exp. ages: 50 -100 Ky • Very High Li Site 3: Devil’s Kitchen Rhyolite, Coso Volcanic Field, CA

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