Transcript The Sedimentary Rock Record

Radioisotopic Methods for Dating Rocks
FACTORS AFFECTING ISOTOPIC DATING
Works best when a rock or mineral represents a
“closed” system.
Parent and daughter isotopes cannot move in or out of
a mineral or rock.
Igneous rocks best fit this criteria.
Radioisotopic Methods for Dating Rocks
FACTORS AFFECTING ISOTOPIC DATING
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 the
rocks themselves.
Radioisotopic Methods for Dating Rocks
FACTORS AFFECTING ISOTOPIC DATING
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 some
problems.
Very young rocks may not have had enough time to
accumulate enough daughter isotope to measure.
Need to choose a radioactive isotope with t½ that fits the
approximate age of the rock.
Radioisotopic Methods for Dating Rocks
FACTORS AFFECTING ISOTOPIC DATING
The minerals in the rock also determine which isotope
that is best for dating the rock.
Radioisotopic Methods for Dating Rocks
TYPES OF ISOTOPIC DATING TECHNIQUES
Uranium (U) - Thorium (Th) - Lead (Pb) Dating
238U
decays to 206Pb
235U decays to 207Pb
232Th decays to 208Pb
Rocks containing Uranium provide three possible
techniques.
Because all three occur together, it allows a method
to cross-check the dates.
Radioisotopic Methods for Dating Rocks
Concordia Diagram
Radioisotopic Methods for Dating Rocks
Discordant
Points not on curve
Radioisotopic Methods for Dating Rocks
TYPES OF ISOTOPIC DATING TECHNIQUES
Uranium (U) - Thorium (Th) - Lead (Pb) Dating
238U
decays to 206Pb
Radioisotopic Methods for Dating Rocks
Radioisotopic Methods for Dating Rocks
TYPES OF ISOTOPIC DATING TECHNIQUES
Uranium (U) - Thorium (Th) - Lead (Pb) Dating
238U
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.
Radioisotopic Methods for Dating Rocks
TYPES OF ISOTOPIC DATING TECHNIQUES
Uranium (U) - Thorium (Th) - Lead (Pb) Dating
235U
decays to 207Pb
Half-life (t1/2) is 713 million years.
Can be applied to igneous and metamorphic rocks.
Uses zircons, uraninite and uranium ores.
Radioisotopic Methods for Dating Rocks
TYPES OF ISOTOPIC DATING TECHNIQUES
Uranium (U) - Thorium (Th) - Lead (Pb) Dating
232Th
decays to 208Pb
Half-life (t1/2) is 14.1 billion years.
Can be applied to igneous and metamorphic rocks.
Uses zircons, uraninite and uranium ores.
Radioisotopic Methods for Dating Rocks
TYPES OF ISOTOPIC DATING TECHNIQUES
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.
Radioisotopic Methods for Dating Rocks
TYPES OF ISOTOPIC DATING TECHNIQUES
Potassium (K) - Argon (Ar) Dating
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.
Radioisotopic Methods for Dating Rocks
TYPES OF ISOTOPIC DATING TECHNIQUES
Potassium (K) - Argon (Ar) Dating
Problem with K-Ar dating is that the Argon produced
is a gas and with fracturing, weathering, or
metamorphism, the gas can be lost, resetting
the clock.
Radioisotopic Methods for Dating Rocks
TYPES OF ISOTOPIC DATING TECHNIQUES
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.
Radioisotopic Methods for Dating Rocks
TYPES OF ISOTOPIC DATING TECHNIQUES
Rubidium (Rb) - Strontium (Sr) Dating
Whole rock analysis
Isochron
Ratio of 87Rb/86Sr is graphed
Against the 87Sr/86Sr
The older the rocks, the
Greater the slope of the
isochron
Radioisotopic Methods for Dating Rocks
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, paper
and charcoal.
Radioisotopic Methods for Dating Rocks
14Carbon
14C
(C) Dating
is produced in the
atmosphere.
Cosmic rays hit other atoms in
atmosphere, giving off
neutrons.
Neutrons hit 14N and  decay
occurs producing 14C.
Radioisotopic Methods for Dating Rocks
14Carbon
(C) Dating
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 over
time.
Needs to be calibrated with other techniques.
OTHER NUMERICAL DATING TECHNIQUES
FISSION-TRACK DATING
FISSION is the division of radioactive nuclei (usually
238U) 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 250C 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)
OTHER NUMERICAL DATING TECHNIQUES
FISSION-TRACK DATING
13.5 m
1 m = 0.001 mm
OTHER NUMERICAL DATING TECHNIQUES
DENDROCHRONOLOGY
Trees in temperate regions produce light and dark
annual growth rings.
By counting the rings, the tree’s
age can be determined.
OTHER NUMERICAL DATING TECHNIQUES
DENDROCHRONOLOGY
Climate and other events are
also recorded.
By comparing the ring counts
and chronology from
living and fossil trees
a dendrochronology for
a region can be formed.
Goes back about 9000 years.