Argon–Argon (or 40Ar/39Ar) Dating Brought to you by amazing, wonderful Mark and sidekick Sarah

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

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=93.2581% 40K=0.01167% 41K=6.7302% 40Ar=99.6% 38Ar=0.063% 36Ar=0.337% Noble gas mass spectrometer Dickin, 1997, after Dalrymple and Lanphere, 1969

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.

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).

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 350-700 degrees C.

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*

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

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.

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.

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

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.

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

In conclusion Thank you for your time, have a wonderful day.