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K/Ar and 40 Ar/ 39 Ar Thermochronology

K/Ar and 40 Ar/ 39 Ar Thermochronology. K/Ar and 40 Ar/ 39 Ar Thermochronology. Currently and traditionally the most widely used and most versatile thermochronologic technique for constraining moderate T cooling histories. Wide use also makes it most widely abused and misapplied technique??.

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K/Ar and 40 Ar/ 39 Ar Thermochronology

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

  2. K/Ar and 40Ar/39Ar Thermochronology Currently and traditionally the most widely used and most versatile thermochronologic technique for constraining moderate T cooling histories. Wide use also makes it most widely abused and misapplied technique??

  3. Why use K-Ar method? K is 8th most abundant element in the crust (~1 wt%) K is a major constituent of many common minerals in igneous and metamorphic rocks

  4. K-Ar method Developed in the late 1940’s Used widely from the 50’s through the 80’s (may still be used today for certain applications)

  5. K-Ar Thermochronology Remember this #!! K has 3 naturally occurring isotopes: 39K (93.258%) – stable 40K (0.0117%) – radioactive 41K (6.730%) – stable K ratios are constant in all terrestrial samples Ar has 3 naturally occurring isotopes: 40Ar (99.600%) – stable 38Ar (0.063%) – stable 36Ar (0.337%) – stable Atmospheric 40Ar/36Ar ratio: 295.5

  6. Decay of 40K 40K has a half life of 1250 Ma (l = 5.543x10-10a-1) 40K decays into two different isotopes: 40Ca by beta decay (b-): 89.52% (lb = 4.962x10-10a-1) 40Ar by electron capture (b+): 10.48% (le = 0.581x10-10a-1) 40K-40Ar decay chain only takes advantage of a small percentage of the # of decays: why not use 40K–40Ca?

  7. Decay of 40K 40K has a half life of 1250 Ma (l = 5.543x10-10a-1) 40K decays into two different isotopes: 40Ca by beta decay (b-): 89.52% (lb = 4.962x10-10a-1) 40Ar by electron capture (b+): 10.48% (le = 0.581x10-10a-1) Because we’re interested in the amount of Ar produced by K, we use the ratio of le to l in calculating the age from the Ar/K ratio:

  8. Nomenclature for Ar: Atmospheric Ar (ArA): Ar with isotopic composition found in present day atmosphere (40Ar/36Ar:295.5) Radiogenic Ar (40Ar*): Ar formed from in situ decay of 40K For K-Ar dating, 40Ar*=40ArT-(36Ar*295.5) Trapped Ar: Ar of atmospheric composition trapped or incorporated in a rock or mineral Excess Ar (40ArE): component of 40Ar, apart from atmospheric, incorporated into sample by process other than in situ decay of 40K

  9. K-Ar dating Because K is solid and Ar is a noble gas, have to measure 40K and 40Ar separately and by different methods: 40K: flame photometry, atomic absorption spectrometry, isotope dilution, X-ray florescence, gravimetric chemistry, nutron activation 40Ar: Noble gas mass spectrometer (with isotope dilution using spike of Ar of atmospheric composition 40Ar/36Ar: 295.5) Problems: Can only get “total gas” ages No indication of diffusive profiles No way to evaluate presence of excess 40Ar Precision is ~1%

  10. Critical development came in the mid 1960’s: Merrihue (1965) and Merrihue and Turner (1966) demonstrated that 39Ar was produced by neutron bombardment of 39K (most abundant K isotope) Use neutron produced 39Ar as a proxy for 39K as a proxy for 40K Can use same technique for parent and daughter isotopes A new approach became possible!

  11. 40Ar/39Ar Dating technique: • Sample is subjected to neutron bombardment (several days to weeks) in a nuclear reactor to transform small proportion of 39K atoms to 39Ar. Wt. % K Amount of sample used proportional to age and weight % K

  12. Duration of sample irradiation is ideally a function of the age of the sample

  13. 40Ar/39Ar Dating technique: (2) A standard of known K-Ar age is irradiated with unknown to monitor neutron flux Used to calculate the extent to which 39Ar was produced from 39K. (3) Wait! ~1 week (and sometime ~1 month) for activity of sample to decrease.

  14. (4) Neutron interactions with K and Ca also produce Ar isotopes. K and Ca salts are irradiated with unknown to correct for these "interfering" argon isotopes.

  15. (5) Relative abundances of Ar isotopes are measured in a gas source mass spectrometer for both standard (to calculate J) and unknown: 40Ar, 39Ar, 37Ar, 36Ar, 38Ar (6) Ratios are corrected for atmospheric argon (which may have been incorporated in sample and in the vacuum system) and Ar isotopes produced during irradiation. 40ArT = 40ArA + 40ArK + 40Ar* 39ArT = 39ArK + 39ArCa 38ArT = 38ArCl + 38ArA 37ArT = 37ArCa 36ArT = 36ArA + 36ArCa -Composition of atmospheric argon known -Analysis of CaF2 provides: (36Ar/37Ar)Ca and (39Ar/37Ar)Ca -Analysis of K2SO4 provides: (40Ar/39Ar)K -Analysis of 38Ar provides info regarding amount of Cl, which has been shown to correlate with excess 40Ar

  16. Isotopic ratios corrected using the equation: (6) 40Ar*/39ArK from the standard is used to calculate J and J is used to calculate age from unknown:

  17. Advantages of the 40Ar/39Ar method: • K and Ar measured simultaneously by measurement of only Ar isotopic ratios. Isotope ratios can be measured more precisely than concentrations of K and Ar (as in the K-Ar method) Aren’t analyzing different aliquots! - more precise age! • **However accuracy of 40Ar/39Ar age is limited by uncertainty in age of standard, determined by K-Ar technique, i.e. ~1%. • Not extremely accurate, but extremely precise. • Step-heating method: • (2) Allows presence of excess Ar to be recognized in some cases • (3) Allows Ar loss to be recognized in some cases • (4) May reveal concentration distribution of Ar in sample!! • i.e. thermal history information!

  18. Step Heating experiments: Performed by measuring Ar isotopic ratios of the gas released during individual temperature steps during incremental heating using a furnace or laser (generally heating time of 10-20 minutes per step). A series of apparent ages is determined Fluid inclusions (often sites of excess argon) degas at lower temperature. For minerals which do not breakdown during heating, step-heating leads to a diffusion experiment with information regarding the internal distribution of 40Ar* relative to 39Ar (assumed to be homogeneous because K has much lower diffusion rates than Ar).

  19. Step-heating method: (1) Allows presence of excess Ar to be recognized in some cases (2) Allows Ar loss to be recognized in some cases (3) May reveal concentration distribution of Ar in sample!! i.e. thermal history information!

  20. Distribution of 40Ar* and 39Ar and resulting age spectra

  21. Materials suitable for K-Ar and 40Ar/39Ar dating and relevant characteristics: Material%K2OEst. TcAffects on TcPoss. ProblemsHeating Behavior K-feldsparup to 14 175-375°C exsolution lamellae initial excess Ar Stable to 1350°C U-shaped spectra Plagioclase ≤1 <350°C? excess Ar very stable U-shaped spectra Useful for dating mafic rocks Leuciteup to 18 ? alters easily breaks down ~550°C Useful for dating very young volcanic rocks (<1 Ma) Biotite 7-8 275-350°C composition excess Ar Unstable! irregular spectra Useful for metamorphic cooling ages and dating volcanic rocks Muscovite up to 10 300-400°C pressure excess Ar at high P debated Useful for metamorphic cooling ages; *less affected by excess Ar than biotite Amphibole 0.1-1.0 450-550°C composition mica intergrowths breaks down exsolution excess Ar hump-shaped spectra Useful for timing of relatively high-T cooling Whole rock variable variable alteration clays – atmospheric argon Useful for dating fine-grained, unaltered volcanic rocks

  22. 40Ar/39Ar data presentation and interpretation Simple Behavior: closed system THE PLATEAU: comprised of steps that do not differ in age at the 2s level. Plateau age is often cited as weighted mean age of steps comprising the plateau.

  23. 40Ar/39Ar data presentation and interpretation Isochron plot for closed system Age spectra assume non-radiogenic Ar is atmospheric in composition. Can check assumption using isochron plots. Inverse isochron plot: Y-intercept: trapped Ar component X-intercept: proportional to age Standard isochron plot: Y-intercept: trapped Ar component Slope: proportional to age

  24. 40Ar/39Ar data presentation and interpretation Advantage of Inverse Isochron plot Uses 40Ar to correlate between atmospheric and radiogenic components. 40Ar is much more abundant and can be measured with much more precision Inverse isochron plot: Y-intercept: trapped Ar component X-intercept: proportional to age

  25. 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.

  26. NEXT: 40Ar/39Ar thermochronology and applications 40Ar/39Ar K-spar thermochronology, MDD modeling, and applications

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