K ar and 40 ar 39 ar thermochronology
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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

K/Ar and 40Ar/39Ar Thermochronology


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


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.


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


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


40Ar/39Ar data presentation and interpretation

Examples of analyses with ambiguous 40ArE component

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  • Age spectra is variable with clear excess 40Ar contamination

  • Inverse isochron does not show clear components of contamination

  • “real” age is probably not recoverable


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?


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!


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


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


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


40Ar/39Ar data presentation and interpretation

Can we tell difference between slowly cooled diffusion profile and reheating profile?


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


MDD modeling of K-Feldspar

What about K-feldspar?

What is the Arrhenius plot telling us?


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


MDD modeling of K-Feldspar

Schematic age spectra and arrhenius plots for different numbers of domains and relative abundance of each domain


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)


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


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


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


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


Example of application of MDD modeling: modeling resultsKongur Shan extensional system


log (r/r modeling results0) plot used to constrain correlation between age spectra and size of each diffusion parameter



Example of application of 40Ar/39Ar dating: and uplift Quxu plutonsouthern Tibet


Example of application of MDD modeling: and uplift Gangdese Thrust, southern Tibet


Yin et al., 1999 and uplift



Wong and Gans, 2003 and uplift

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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