1. Argon–Argon (or 40Ar/39Ar) Dating
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2. More accurate than K/Ar dating
Based on K/Ar with addition of bombarding sample with fast neutrons, 39K --> 39Ar
Requires only one rock fragment or mineral grain
Uses single measurement of Ar isotopes

3. K-Ar measurement via isotope dilution
Natural Abundances
Goal: separate radiogenic 40Ar from air 40Ar
Method:
Sample is crushed and single crystals or rock fragments are selected
Add a spike that is very enriched in one isotope to the sample
Measure ratios with mass spectrometer
Sample is heated to release trapped gases
Calculate contributions from contamination (natural ratios),
spike (enriched ratios), and isotope of interest
39K= %
40K= %
41K=6.7302%
40Ar=99.6%
38Ar=0.063%
36Ar=0.337%
Noble gas mass spectrometer
Dickin, 1997, after Dalrymple and Lanphere, 1969

4. Age Equation
λ is the radioactive decay constant of 40K (approximately 5.5 x 10−10 year−1, corresponding to a half-life of approximately 1.25 billion years),
J is the parameter associated with the irradiation process
R is the 40Ar*/39Ar ratio.

5. Problems
Limited application by rock type: No rocks from detrital grains such as most sandstones, mudrocks, and conglomerates
Ar is a noble gas and is highly mobile
You will get an inaccurate K-Ar age if:
Your initial Ar was not zero (the mantle contains appreciable 40Ar that may not have been completely degassed during rock formation).
You lose Ar because of low-temperature alteration.
Your sample is contaminated by atmospheric Ar (which is ~97% 40Ar).

6. Applications
Dating crystallization of metamorphic and igneous minerals
Dating movement of fault systems
Can confirm accuracy of K-Ar dating
Thermochronology; Decay Series “clock” is reset at temperatures that vary between different minerals from about degrees C.

7. Older samples have higher 40Ar*/39Ar values and
So…
Older samples have higher 40Ar*/39Ar values
and
Altered regions of samples have lower 40Ar*/39Ar values
due to loss of 40Ar*

8. U-Th/Zircon
Its one of the oldest dating technique and most refined of the radiometric dating schemes.
Used on rocks from 1 to 4.5 mya
Precision of 0.1 to 1 percent range
Relies on decay chains

9. Zircon (ZrSiO4)
Zircon mineral incorporates uranium and thorium atoms into its crystalline structure, but rejects lead.  allowing for assumption that its lead content is radiogenic.
Method can also be used on monazite, titanite, baddeleyite.

10. age of the zircon rock can be calculated by the equations for exponential decay like so:
where in this situation:
is the number of Uranium atoms measured now.
is the number of Uranium atoms originally - equal to the sum of Lead and Uranium atoms measured now.
is the decay rate of Uranium.
t is the age of the zircon rock, which one wants to determine.

11. Problems
Limited application by rock type: No rocks from detrital grains such as most sandstones, mudrocks, and conglomerates

12. Applications Dating provides information about provenance of sediment
Has established the age of Earth’s oldest rocks
It was revealed that the simultaneous U-Pb and U-Th zircon dating technique using LA-ICP-MS* is easy to apply and is useful for Quaternary tephras- it can give age information on the tephra itself and xenocrystic zircons in a quick and cost-effective manner.

13. Techneque
U-Pb and U-Th zircon dating uses laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS)

14. In conclusion
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