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K/Ar and 40 Ar/ 39 Ar Thermochronology

K/Ar and 40 Ar/ 39 Ar Thermochronology. 40 Ar/ 39 Ar data presentation and interpretation. Inverse isochron plot: Y-intercept: trapped Ar component X-intercept: proportional to age Allows you to asses possible contamination by excess 40 Ar

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K/Ar and 40 Ar/ 39 Ar Thermochronology

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  1. K/Ar and 40Ar/39Ar Thermochronology

  2. 40Ar/39Ar data presentation and interpretation Inverse isochron plot: Y-intercept: trapped Ar component X-intercept: proportional to age Allows you to asses possible contamination by excess 40Ar Sometimes allows you to extract “real” age from contaminated samples Increase in X-axis is decrease in age Intercept is dependent on J factor: Slightly different for each sample Significantly different for each irradiation

  3. 40Ar/39Ar data presentation and interpretation Non-Atmospheric Trapped Ar Age spectrum consists of model ages, normally calculated assuming trapped Ar is atmospheric in composition.

  4. Age spectra based on assumed trapped 40Ar/36Ar = 295.5 Age spectra based on trapped 40Ar/36Ar ratios determined from inverse isochron 40Ar/39Ar data presentation and interpretation Another clever use of inverse isochron diagram to assess age and nature of trapped Ar component Inverse Isochron shows two trapped Ar components with different non-atmospheric 40Ar/36Ar ratios

  5. 40Ar/39Ar data presentation and interpretation Examples of analyses of very young sample Inverse Isochron Age: 1.83+/-.02 • Total Gas, Weighted Mean, and Inverse Isochron more or less agree. • Inverse Isochron is dominated by atmospheric component (from blank in extraction line) • “Best” age is probably weighted mean

  6. 40Ar/39Ar data presentation and interpretation Examples of analyses with ambiguous 40ArE component ? ? • Age spectra is variable with clear excess 40Ar contamination • Inverse isochron does not show clear components of contamination • “real” age is probably not recoverable

  7. 40Ar/39Ar data presentation and interpretation Examples of analyses of older sample Trapped 40Ar/36Ar = 151+/-75 Trapped 40Ar/36Ar ~222 • Age spectra have ~20Ma variation in age • Signal is dominated by radiogenic component and data cluster near X-axis: only one point to use to correlate to trapped component • Trapped Ar component has 40/36<295.5 i.e. no “excess” 40, but “excess” 36… huh?

  8. 40Ar/39Ar data presentation and interpretation Total Gas Age = 65.0 ± 0.2 Ma (1s)Weighted Mean Age = 65.4 ± 2.3 Ma (1s); MSWD = 4227Weighted Mean Plateau Age = 66.8 ± 0.6 Ma (1s); = 78What is geologically meaningful age and uncertainty?Interpreted age: 65 ± 15 Ma; 67 ± 2 Ma There is ambiguity in interpretation of geochronologic data!

  9. 40Ar/39Ar data presentation and interpretation • Age spectra show well behaved increase in age over the first 20-30% of gas released • Age spectra could be interpreted as diffusion profile of slowly cooled or reheated sample • Interpretation assumes that mineral is stable during step heating process

  10. 40Ar/39Ar data presentation and interpretation Single Domain Diffusion Model Assumptions: (1) Uniformly distributed K (39Ar) (2) Single diffusion domain with size = crystal (3) Zero 40Ar* at boundary Can theoretically be used to model cooling histories of samples experiencing slow cooling or reheating For latter case: Youngest age = age of thermal event

  11. 40Ar/39Ar data presentation and interpretation Model is of limited utility for most minerals Hornblende samples collected at variable distances from a granite intruded at 114 Ma

  12. 40Ar/39Ar data presentation and interpretation Can we tell difference between slowly cooled diffusion profile and reheating profile?

  13. MDD modeling of K-Feldspar Sample with single diffusion domain (size and diffusive parameters of all components being degassed are same) Theoretical age spectra Diffusion experiment yields linear slope

  14. MDD modeling of K-Feldspar What about K-feldspar? What is the Arrhenius plot telling us?

  15. MDD modeling of K-Feldspar Initial linear slope represents degassing of several “domains” within K-feldspar with different diffusive properties As low retentive domains are degassed slope of arrhenius plot shifts to different values

  16. MDD modeling of K-Feldspar Schematic age spectra and arrhenius plots for different numbers of domains and relative abundance of each domain

  17. Uncertainties in number, size, volume fraction of domains- Forward models show that calculated thermal histories are insensitive to relatively large variations in these parameters.- The important thing is to define a domain distribution that simply describes the diffusion behavior- the "true" domain distribution may be different- but is not requiredDomain boundaries are maintained at zero Ar concentration- Reasonable considering fast diffusion pathways at boundaries40K (and hence 39Ar) uniformly distributedrequirements: (1) Initial temperature of sample must have been high enough so that no radiogenic Ar was present in sample before cooling(2) An involved heating schedule so that details of diffusion behavior and age spectra are resolved– takes ~2 days in lab/sample(3) Age spectra + log(r/ro) plots must correlate (~75% match ok; ~25% good)

  18. Evaluation of assumptions Laboratory vs. Natural Diffusion- Age spectrum is function of radiogenic Ar diffusion in nature over millions of years timescale- Arrhenius plots + corresponding log(r/ro) plots are produced in laboratory- reflect diffusion of reactor-induced Ar over timescales of hours to days (1) plots are commonly strongly correlated(2) empirical: MDD model yields similar results for different samples from same area + consistent with independent thermal history info

  19. MDD modeling of K-Feldspar Critical argument for validity of model: During step-heating, if sample is cooled then reheated, arrhenius plot follows different slope than before This shows that there is a lower retentive domain that has been exhausted

  20. log(r/ro) plot Can constrain number, relative size, and relative volume fraction of domains from analysis of Arrhenius and log(r/ro) plots.With this info, can forward model age spectra to find a unique, best-fit cooling history

  21. Shape of cooling history is very well constrained by modeling resultsAbsolute T in cooling history is not well constrained by modeling

  22. Example of application of MDD modeling:Kongur Shan extensional system

  23. log (r/r0) plot used to constrain correlation between age spectra and size of each diffusion parameter

  24. Inflection interpreted to date initiation of rapid cooling and uplift

  25. Example of application of 40Ar/39Ar dating:Quxu plutonsouthern Tibet

  26. Example of application of MDD modeling:Gangdese Thrust, southern Tibet

  27. Yin et al., 1999

  28. Harrison et al., 2000

  29. Wong and Gans, 2003 Sierra Mazatan core complex, Sonora, Mexico:15-35 km of fault slip, 30-60 degree initial fault dip, assuming 20-30C/km geothermal gradient

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