R cks as Cl cks: Principles of Geochronology Nesse Ch' 5, p'9394

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R cks as Cl cksPrinciples of
GeochronologyNesse Ch. 5, p.93-94
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Principles of GeochronologyNesse Ch. 5, p.93-94
What is geochronology? determination of time
intervals on a geologic scale, through either
absolute or relative dating methods (Oxford
Concise Dictionary of Earth Sciences)
absolute dating use of radioactivity to date
rocks and minerals (date in Ma millions
of years before present) relative dating use of
stratigraphy and cross-cutting relationships
(date in terms of geological timescale or
simply older than or younger than)
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Principles of GeochronologyNesse Ch. 5, p.93-94
1. Atoms and Isotopes2. Decay Processes3.
Geochronology4. Thermochronology
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Atoms and Isotopes
Atoms consist of protons and neutrons that make
up a nucleus, surrounded by an electron cloud.
The number of protons, the atomic number, Z,
defines an element. The atomic number specifies
the position in the periodic table. The number
of protons and neutrons combined defines the
atomic mass, which is called the mass number, A.
A-Z
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Atoms and Isotopes
The atomic number (Z) is fixed for each
element. In contrast, the number of neutrons in a
given element, and therefore mass number, can
vary. Some isotopes are unstable and undergo
spontaneous radioactive decay to form new
isotopes and/or elements. The original
isotope (87Rb) is called the parent and the new
decay product (87Sr) is called the daughter.
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Atoms and Isotopes
Decay constant
The number of isotopes that decay per unit time
is proportional to the number of parent
isotopes. This proportionality factor is a
constant for each radioactive isotope and is
called the decay constant, ?.
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Atoms and Isotopes
A more familiar concept is the half-life,
t1/2, the time required for half of a given
number of parent isotopes to decay.
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t1/2 is proportional to 1/l
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Atoms and Isotopes
There are many naturally-occurring radioactive
isotopes. The most important for geological
purposes are
238U ? 206Pb l 1.55E-10 235U ?207Pb l
9.85E-10 232Th ? 208Pb l 4.95E-11 40K ? 40Ar
l 5.81E-11
These isotopes occur in the crystal lattices of
minerals as essential components (K), or as
trace elements (U, Th) substituting for major
elements
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Atoms and Isotopes
These isotopes occur in the crystal lattices of
minerals as essential components (K), or as
trace elements (U, Th) substituting for major
elements.
substitutional impurity (a type of point defect)
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Principles of GeochronologyNesse Ch. 5, p.93-94
1. Atoms and Isotopes2. Decay Processes3.
Geochronology4. Thermochronology
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Decay Processes

• Alpha Decay
• Beta Decay
• Positron Decay
• Electron Capture
• Fission

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Alpha Decay
An a particle is ejected from the nucleus of the
parent atom, along with the emission of g rays
and release of heat.
a particle 2 protons 2 neutrons.
Decay of parent produces a daughter with atomic
number Z 2 less than the parent. The mass number
is reduced by 4.
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Beta Decay
This can be considered the breakdown of a neutron
into a proton and an electron. The electron, or
bparticle, is ejected from the nucleus along
with an antineutrino with very small mass.
The atomic number increases by 1 and the mass
number remains the same.
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Positron Decay
A proton breaks down into a neutron and a
positron, b, plus a neutrino. The positron
ultimately combines with a normal electron and is
annihilated, releasing g radiation.
The atomic number decreases by 1 and the mass
number remains the same.
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Electron Capture
This involves capture of an electron by the
nucleus, converting a proton into a neutron. The
atomic number decreases by 1 and the mass number
remains the same (as for positron decay).
About 11 of 19K40 decays to 18Ar40 by electron
capture. The remaining 89 undergoes beta
decay to form 20Ca40.
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Fission
spontaneous or induced decay of heavy radioactive
isotope (e.g., 235U, 232Th) into 2 lighter
nuclei particles energy
92U235 ? 56Ba140 36Kr92 3n energy
the two daughter isotopes generally differ
substantially in A and Z fission damages the
crystal lattice as the daughter atoms
recoil these fission tracks can be used to date
minerals (a type of thermochronology)
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Pleochroic Halos
Decay of radioactive elements in mineral
inclusions (e.g. zircon, apatite, or monazite in
biotite) can damage the host mineral, producing
pleochroic haloes
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