DETERMINING NUMERICAL OR ABSOLUTE AGE

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DETERMINING NUMERICAL OR ABSOLUTE AGE
FACTORS AFFECTING ISOTOPIC DATING
Most useful in igneous rocks. As minerals
crystallize, radioactive isotopes
become incorporated in the minerals. No daughter
isotopes at that time. Crystallization sets the
isotopic clock. Doesnt work in sedimentary
rocks. How come?
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DETERMINING NUMERICAL OR ABSOLUTE AGE
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.
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DETERMINING NUMERICAL OR ABSOLUTE AGE
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.
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DETERMINING NUMERICAL OR ABSOLUTE AGE
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.
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DETERMINING NUMERICAL OR ABSOLUTE AGE
FACTORS AFFECTING ISOTOPIC DATING
The minerals in the rock also determine which
isotope that is best for dating the rock.
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DETERMINING NUMERICAL OR ABSOLUTE AGE
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.
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DETERMINING NUMERICAL OR ABSOLUTE AGE
TYPES OF ISOTOPIC DATING TECHNIQUES
Uranium (U) - Thorium (Th) - Lead (Pb) Dating
238U decays to 206Pb
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DETERMINING NUMERICAL OR ABSOLUTE AGE
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.
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DETERMINING NUMERICAL OR ABSOLUTE AGE
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.
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DETERMINING NUMERICAL OR ABSOLUTE AGE
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.
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DETERMINING NUMERICAL OR ABSOLUTE AGE
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.
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DETERMINING NUMERICAL OR ABSOLUTE AGE
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.
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DETERMINING NUMERICAL OR ABSOLUTE AGE
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.
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DETERMINING NUMERICAL OR ABSOLUTE AGE
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.
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DETERMINING NUMERICAL OR ABSOLUTE AGE
TYPES OF ISOTOPIC DATING TECHNIQUES
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.
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DETERMINING NUMERICAL OR ABSOLUTE AGE
TYPES OF ISOTOPIC DATING TECHNIQUES
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.
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DETERMINING NUMERICAL OR ABSOLUTE AGE
TYPES OF ISOTOPIC DATING TECHNIQUES
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.
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DETERMINING NUMERICAL OR ABSOLUTE AGE
OTHER NUMERICAL DATING TECHNIQUES
FISSION-TRACK DATING
FISSION is the division of radioactive nuclei
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.
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DETERMINING NUMERICAL OR ABSOLUTE AGE
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.
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DETERMINING NUMERICAL OR ABSOLUTE AGE
OTHER NUMERICAL DATING TECHNIQUES
FISSION-TRACK DATING
1 ?m 0.001 mm
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DETERMINING NUMERICAL OR ABSOLUTE AGE
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.
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DETERMINING NUMERICAL OR ABSOLUTE AGE
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.
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DETERMINING NUMERICAL OR ABSOLUTE AGE
OTHER NUMERICAL DATING TECHNIQUES
VARVE CHRONOLOGY
Lakes can produce annual layers. Usually occur in
glacial lakes or those that freeze over in
winter. Coarser sediments are deposited in
summer. Winter-summer layers are called
COUPLETS. Couplets in lakes are known as
VARVES. Count the couplets back from the sediment
surface to determine numerical age.
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DETERMINING NUMERICAL OR ABSOLUTE AGE
OTHER NUMERICAL DATING TECHNIQUES
VARVE CHRONOLOGY
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DETERMINING NUMERICAL OR ABSOLUTE AGE
OTHER NUMERICAL DATING TECHNIQUES
LICHENOMETRY
Lichens are plant-like organisms that grow on
rocks. Grow at a measurable rate. By measuring
size on items of known date, the size
is plotted against size on unknown aged
objects. Good for the last 9000 years.
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DETERMINING NUMERICAL OR ABSOLUTE AGE
OTHER NUMERICAL DATING TECHNIQUES
LICHENOMETRY