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K-Ar and 40 Ar- 39 Ar Dating8/28/12 What are the principles behind K-Ar dating? What problems can K-Ar dating address? What are the main limitations of the method? What are the recent advances? Lecture outline: 1)K-Ar dating applications 2)K-Ar dating principles & techniques 3)limitations, open-system behavior 4) 40 Ar- 39 Ar dating and step heating 5)lasers and micron-scale analyses Hornblende thin section

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K-Ar applications - K one of eight most abundant elements on Earth - dating good from 2ky-4500Ma - Earliest application in dating magnetic reversals in lavas - intercalibration of decay constants - used to date K/T boundary, deep-sea cores with magnetic record - thermochronometry: Ar loss occurs during metamorphosism in diffusive process Dickin, 1997, after Berger and York, 1981a Dickin, 1997, after Mankinen & Dalrymple, 1979

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40 K decay scheme Unknowns: 40 Ar* : radiogenic 40 Ar from 40 K decay (isotope dilution) 40 K : a small fraction of total K (measure K conc. and use abundance %) Why not use 40 K- 40 Ca as dating system?

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K-Ar measurement via isotope dilution 39 K=93.2581% 40 K=0.01167% 41 K=6.7302% 40 Ar=99.6% 38 Ar=0.063% 36 Ar=0.337% Natural Abundances Method: 1. Add a spike that is very enriched in one isotope to the sample 2. Measure ratios with mass spectrometer 3. Calculate contributions from contamination (natural ratios), spike (enriched ratios), and isotope of interest Example: Dickin, 1997, after Dalrymple and Lanphere, 1969 Noble gas mass spectrometer Goal: separate radiogenic 40 Ar from air 40 Ar

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Closed vs. Open System Behavior The K-Ar age is only accurate if the sample has remained a CLOSED SYSTEM: i.e. there has been no gain or loss of K or Ar through time. In reality, this is almost never the case, because Ar is a noble gas and is highly mobile. You will get an inaccurate K-Ar age if: 1. Your initial Ar was not zero (the mantle contains appreciable 40 Ar that may not have been completely degassed during rock formation). 2. You lose Ar because of low-temperature alteration. 3. Your sample is contaminated by atmospheric Ar (which is ~97% 40 Ar). We can address #3 by monitoring 36 Ar (~20,000 more abundant in air than in the mantle)

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Ifx=( 40 K/ 36 Ar) m Andy=( 40 Ar/ 36 Ar) m We have y=b+mx Where interceptb=( 40 Ar/ 36 Ar) I And slope m=( e / t )(e - t -1) measured calculated Isochron Method Dickin, 1997 after McDougall et al., 1969 xy

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40 Ar- 39 Ar Dating - based on K-Ar dating - bombard sample with fast neutrons, 39 K --> 39 Ar Converting 39 K into 39 Ar brings the following advantages: 1. You can obtain K ( 39 Ar) and 40 Ar data from the same sample 2. Ar isotopic ratios are the only measurements required (high precision) 3. You can measure Ar ratios as you slowly heat the sample where J calculated from bombarding and measuring samples of known age (T) So… Older samples have higher 40 Ar*/ 39 Ar values and Altered regions of samples have lower 40 Ar*/ 39 Ar values due to loss of 40 Ar*

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Step-wise heating and 40 Ar- 39 Ar Dating vs Plot ( 40 Ar*/ 39 Ar) vs heating steps Or Plot Apparent Age vs. fraction 39 Ar released

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Plot ( 40 Ar*/ 39 Ar) vs heating steps Or Plot Apparent Age vs. fraction 39 Ar released Step-wise heating and 40 Ar- 39 Ar Dating plateau gives most reliable crystallization age low-temperature steps reveal sample has lost 40 Ar* higher T steps give consistent 40 Ar*/ 39 Ar ratios

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Laser applications in Ar-Ar dating laser spot Allows for step-wise heating of different zones within grains Lee et al., 1991

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... or measure many grains and use isochron method

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