FACTORS AFFECTING ISOTOPIC DATING

1
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.
2
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.
3
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.
4
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.
5
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.
6
Radioisotopic Methods for Dating Rocks
Concordia Diagram
7
Radioisotopic Methods for Dating Rocks
Discordant Points not on curve
8
Radioisotopic Methods for Dating Rocks
TYPES OF ISOTOPIC DATING TECHNIQUES
Uranium (U) - Thorium (Th) - Lead (Pb) Dating
238U decays to 206Pb
9
Radioisotopic Methods for Dating Rocks
10
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.
11
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.
12
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.
13
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.
14
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.
15
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.
16
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.
17
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
18
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.
19
Radioisotopic Methods for Dating Rocks
14Carbon (C) Dating
14C is produced in the atmosphere. Cosmic rays
hit other atoms in atmosphere, giving
off neutrons. Neutrons hit 14N and ?
decay occurs producing 14C.
20
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.
21
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.
22
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. Cant 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)
23
OTHER NUMERICAL DATING TECHNIQUES
FISSION-TRACK DATING
1 ?m 0.001 mm
24
OTHER NUMERICAL DATING TECHNIQUES
Trees in temperate regions produce light and dark
annual growth rings. By counting the rings, the
trees age can be determined.
25
OTHER NUMERICAL DATING TECHNIQUES
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.