Radiometric Dating:

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Radiometric Dating

• Clocks in Rocks!

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Definitions

• Radiometric dating calculating the absolute age
  of a material by measuring the abundance of a
  radioactive isotope and/or its radiogenic product
  relative to a stable isotope.
• Radioactive isotope (a.k.a. parent) an unstable
  isotope that undergoes spontaneous nuclear decay.
  
• Radiogenic isotope (a.k.a. daughter) an isotope
  produced by the nuclear decay of a radioactive
  isotope.
• Cosmic rays streams of high-energy particles
  (mostly electrons and hydrogen nuclei traveling
  at nearly the speed of light) that constantly
  bombard objects in space.

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Modes of Radioactive Decay

• a-decay A 4He nucleus (2 protons 2 neutrons)
  is ejected.
• decreases the atomic number by 2
• decreases the atomic mass by 4
• b-decay A neutron splits into a proton and an
  electron.
• increases the atomic number by 1
• conserves atomic mass
• electron capture An electron and a proton
  combine to form a neutron.
• decreases the atomic number by 1
• conserves atomic mass
• chain decay A series of radioactive decays.

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a-decay
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b-decay
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Electron Capture
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Chain Decay
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Exponential Decay
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Equations

• N0 is the initial number of parent atoms
• N is the number of parent atoms at time t

t1/2 is the half-life of the radioactive atom
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Carbon

• Isotopes of carbon
• 12C stable (non-radioactive) and abundant
• 13C stable but less abundant
• 14C radioactive
• Production of 14C
• 14N, which resides in Earths upper atmosphere,
  absorbs neutrons from cosmic rays.
• Usually when a neutron is absorbed, a proton is
  ejected and 14C is formed.
• 14N n p ? 14C
• The amount of 14C in the atmosphere is small but
  fairly constant.

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This is how living organisms maintain the same
14C/12C ratio as the atmosphere photosynthesis,
food consumption, respiration.When the organism
dies, the exchange stops!

• Living organisms constantly exchange carbon with
  the atmosphere
• Chemically, 14C and 12C behave similarly.
• Both 12C and 14C combine with oxygen to make CO2.
• CO2 is absorbed by plants for photosynthesis.
• Animals consume the CO2 in plants either directly
  or indirectly.

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Radiocarbon Dating

• Over time, the 14C trapped in a dead organism
  decays back into 14N.
• Scientists can measure the amount of 14C and the
  total amount of carbon (12C 13C 14C) in a
  sample.

Since the initial amount of 14C, the final amount
of 14C, and the rate of decay are all known, the
age can be calculated!
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The U of A Physics Departments Accelerator Mass
Spectrometry Laboratory is one of the worlds
best facilities for radiocarbon dating. Many
important artifacts have been dated there.

• The Dead Sea Scrolls (200 B.C.E. to 68 C.E.)
• The Shroud of Turin (The Middle Ages circa 1300
  C.E.)
• The Vinland Map (1434 C.E.)

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Rubidium-Strontium Dating(More Equations)

• 87Rb (the parent) decays to 87Sr (the daughter)
  by the process of b-decay

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D0 is the initial number of daughter atoms D is
the number of daughter atoms at time t D is the
number of daughter atoms produced by decay
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and
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Combine equations 2 and 3
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Combine equations 1 and 4
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Applied to Rb-Sr dating
Ratio with stable isotope 86Sr
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This is just the equation of a line y the
value on the y-axis x the value on the x-axis
m the slope of the line b the y-axis
intercept
y b (x m)
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How old are the...

• oldest rocks on Earth?
• The Acasta gneisses in Canadas Northwest
  Territories are 4.03 Ga.
• oldest mineral grains on Earth?
• Zircon grains from western Australia are 4.3 to
  4.4 Ga.
• oldest lunar rocks?
• Rock samples from the moon are as old as 4.4 to
  4.5 Ga.
• oldest meteorites?
• The meteorite Allende is 4.559 Ga.

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What else can we learn from age dating meteorites?

• The date that they fell to Earth
• The date that the meteorite parent body broke
  apart
• The age of the last cooling event
• The age of formation

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ALH84001
Meteorite from Mars
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Determining when a meteorite fell to Earth
While cosmic rays bombard objects in space (and
cause reactions such as 14N ? 14C), objects on
Earths surface are shielded from cosmic rays by
the atmosphere. ALH84001 landed on Earth
13,000 years ago (14C-dating)
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The date that the meteorite parent body broke
apart

• ALH84001 broke off of its parent body (Mars) 16
  to 17 Ma.

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Age of the last cooling event

• ALH84001 last cooled 4.0 Ga

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Age of formation

• Tells when the material that makes up the
  meteorite solidified.
• Most meteorites are 4.55 Ga.
• Allende, the oldest, is 4.559 Ga.
• This is taken to be the age of the solar system.