factors affecting isotopic dating

FACTORS AFFECTING ISOTOPIC DATING

Works best when a rock or mineral represents a “closed” system.

Parent and daughter isotopes cannot move in or out ofa mineral or rock.

Igneous rocks best fit this criteria.

Radioisotopic Methods for Dating Rocks


Metamorphic rocks are not always closed systems.During metamorphism, heat, pressure, and circulating

fluids affect mineral grains.Daughter isotopes are generally lost in the process.

Dating metamorphic rocks provides the age of the metamorphic event rather than the age of therocks themselves.



Accuracy of isotope dating also depends on the condition of the material dated

Fractured or weathered rock is not a good candidate.

Age of the rocks being considered also presents someproblems.

Very young rocks may not have had enough time toaccumulate enough daughter isotope to measure.

Need to choose a radioactive isotope with t½ that fits theapproximate age of the rock.



The minerals in the rock also determine which isotopethat is best for dating the rock.


TYPES OF ISOTOPIC DATING TECHNIQUES

Uranium (U) - Thorium (Th) - Lead (Pb) Dating

238U decays to 206Pb235U decays to 207Pb232Th decays to 208Pb

Rocks containing Uranium provide three possibletechniques.

Because all three occur together, it allows a methodto cross-check the dates.



Concordia Diagram


DiscordantPoints not on curve


Uranium (U) - Thorium (Th) - Lead (Pb) Dating238U decays to 206Pb




Half-life (t1/2) is 4.5 billion years.

Can be applied to igneous and metamorphic rocks.

Uses zircons, uraninite and uranium ores.




Half-life (t1/2) is 713 million years.





Uranium (U) - Thorium (Th) - Lead (Pb) Dating232Th decays to 208Pb

Half-life (t1/2) is 14.1 billion years.





Potassium (K) - Argon (Ar) Dating

Potassium (K) is an extremely common element.One isotope, 40K, is radioactive.Found in muscovite, biotite, orthoclase and glauconite.Used to date volcanic rocks.




Produced by electron or beta () capture.

Half-life (t1/2) is 1.3 billion years.Range is 100,000 to 4.6 billion years.Useful for relatively young and very old rocks.




Problem with K-Ar dating is that the Argon producedis a gas and with fracturing, weathering, ormetamorphism, the gas can be lost, resettingthe clock.



Rubidium (Rb) - Strontium (Sr) Dating

Rubidium (Rb) decays to Strontium (Sr).

Half-life (t1/2) is 47 billion years.

Found in muscovite, biotite, feldspars and hornblende.Used to date volcanic and metamorphic rocks.Because of large half-life, rocks between 10 million and

4.6 billion years can be dated.



Rubidium (Rb) - Strontium (Sr) Dating

Whole rock analysis


Isochron

Ratio of 87Rb/86Sr is graphedAgainst the 87Sr/86Sr

The older the rocks, the Greater the slope of the isochron


14Carbon (C) Dating

Produced by Beta () decay.

Half-life (t1/2) is 5,730 years.

Age range is 100 to 70,000 (really ~50,000) years.Used to date carbon-based remains like bones, plant

remains (wood, pollen, seeds), shells, cloth, paperand charcoal.


14Carbon (C) Dating14C is produced in the

atmosphere.Cosmic rays hit other atoms in

atmosphere, giving offneutrons.

Neutrons hit 14N and decayoccurs producing 14C.


14C in atmosphere combines with O2 to produce 14CO2.Plants and animals ingest or breathe in 14CO2 and it

becomes incorporated in the organism.Upon death, 14C decays back into 14N.

The rate of cosmic ray bombardment has varied overtime.

Needs to be calibrated with other techniques.

14Carbon (C) Dating

FISSION is the division of radioactive nuclei (usually238U) into two equally-sized fragments.

Process releases and particles.When splitting occurs, particles rip through the mineral

lattice (crystal structure) producing tracks or tears in the lattice.

Occurs continuously in minerals with radioactive substances.

OTHER NUMERICAL DATING TECHNIQUES

FISSION-TRACK DATING

The older the mineral, the more tracks are produced.Age range is 50,000 to billions of years.Can be applied to volcanic glass, zircons and apatites.

Limitations do exist.Temperatures above 250C cause tracks to heal.Can’t be used to date medium- to high-grade

metamorphic rocks.Fills the gap between 14C and K-Ar techniques.

(between 50,000 and 1,000,000 years)




13.5 m

1 m = 0.001 mm


Trees in temperate regions produce light and dark annual growth rings.

By counting the rings, the tree’s age can be determined.

DENDROCHRONOLOGY


Climate and other events arealso recorded.

By comparing the ring countsand chronology fromliving and fossil treesa dendrochronology fora region can be formed.

Goes back about 9000 years.

DENDROCHRONOLOGY


factors affecting isotopic dating

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