Natural radioactivity

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• Natural radioactivity
• Discovered by Henri Becquerel in 1896
• Lead to a way to determine actual ages of rocks
• Unfortunately, we have to deal with a bit of
  chemistry/physics here

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• Atom
• Smallest particle of matter that can exist as a
  chemical element
• Made up of a nucleus, containing protons
  neutrons, orbited by a cloud of electrons
• Atomic number
• The number of protons in the nucleus
• Mass number
• Total number of protons neutrons in the nucleus

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• Isotopes
• Atoms of the same element with differing mass
  numbers (same atomic number, but different
  numbers of neutrons)
• Some isotopes are stable nothing happens to
  them thru time
• Other isotopes are unstable, and will
  eventually break apart (decay)
• For dating, these are the ones were interested in

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• Radioactive decay starts with a parent element
• Decay occurs by release of particles from nucleus
• Alpha decay a helium nucleus (2 protons 2
  neutrons)
• Beta decay an electron (neutron splits into a
  proton an electron)
• Neutrons may also be emitted
• Gamma radiation (a result of an electron
  combining with a proton - electron capture)
• New atom formed by decay is the daughter element
• There may be several daughter elements formed
  before a final, stable element is formed

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Decay of U-238 to Pb-206 (14 steps)
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• How radioactivity allows age dating
• Rate of decay is uniform
• Unaffected by changes in temperature, pressure,
  or chemical environment
• Certain amount of a radioactive element is
  incorporated into a growing mineral crystal
• Once there, that amount decays at a steady rate
• Over time, the amount of parent isotope
  decreases, and the amount of the final daughter
  isotope increases
• Assumes a closed system within that mineral
  grain, neither isotope can be removed, nor is any
  added
• The mineral grain needs to be very
  weathering-resistant, and/or refractory (very
  hard to melt)

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• What does that all mean?
• Basically, we need to measure how much parent
  isotope and how much daughter isotope is present
  (the ratio of the two)
• These two numbers, plus the parent isotopes rate
  of decay (or, half life)
• allow us to calculate an age before the present

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• Half-life
• The parameter by which an age is determined
• Related to the rate of decay
• The time required for half of a parent isotope to
  decay to its daughter isotope
• Each isotope has its own specific half-life
• Admittedly, our basic assumption is that it does
  not (and has not) change(d) with time

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Radiometric dating graph showing the decline in
the number of atoms of radioactive parent isotope
(dark blue line) and the increase in the number
of atoms of stable daughter isotope (red line).
Numbers on the vertical axis refer to numbers of
atoms. Numbers on the horizontal axis refer to
numbers of half lives. (Years are given for the
decay of carbon-14 over a period of 10 half
lives). The numbers on the graph in red and blue
refer to the number of atoms present at each
half-life.
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• What can we date radiometrically?
• Igneous rocks are best
• Tells us when the magma/lava crystallized
• Potassium-40
• Potassium feldspar
• Muscovite
• Amphibole
• Glauconite (actually, sedimentary very rare)
• Uranium Thorium
• Zircon
• Uraninite
• Monazite
• Apatite
• Sphene (titanite)

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• Isotopes are not useful for dating sedimentary
  rocks
• Radioactive grains in them are derived from older
  igneous rocks
• Ages would indicate the age of the igneous rocks
• Could have several sources
• Exception is the sedimentary mineral glauconite,
  which contains potassium

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• How to determine ages of sedimentary rocks
  fossils
• A combination of both relative absolute dating

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Layer of volcanic ash (bentonite) between two
layers of limestone (Ordovician age)
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• There must be a lot of elements used for dating
• Actually, no
• Six main ones (there are other minor ones, but
  they are special cases, and are used for other
  purposes)

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• Carbon-14 - a bit of a different beast
• Most isotopes are used for dating materials that
  are millions to billions of years old
• C-14 has a short half-life (5730 yrs) maximum
  age is about 50,000 yrs
• For most materials, measures the ratio of C-14 to
  C-12 (common carbon)
• For the past 50 years, we measure the actual
  activity of a sample (tell ya why in a bit)

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Carbon-14 cycle
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• C-14 dating not useful from 1950s to present
• Yet, why would we bother?
• It will be important in the far future, for them
  monkeys that take our place
• Reason
• Mans infinite wisdom (or desperation)..

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Trinity Site, New Mexico, 16 July 1945
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• Nukes have added C-14 to the atmosphere
• Kinda messed up the natural way, yet still useful
• Allows us to track groundwater movement, for one
• Large amounts of C-14 entered groundwater systems
• Can also track infiltration (vertical movement of
  water through the subsurface)

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• So, with all this knowledge, how old is this ball
  of rock?
• We can only date things that solidified and
  incorporated radioactive isotopes
• Zircon grains in a sandstone from western
  Australia 4.2 billion years (by) old
• Granites from sw Greenland, metamorphic rocks
  from Minnesota, rocks in Northwest Territories
  (Canada) are 4 by old
• Moon rocks 3.3 4.6 by old
• Meteorites likely formed at same time as Earth
  dated at 4.6 by old
• Thus, the accepted age of the Earth at present
  4.6 by (or 4.5depends on the source)

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• Why are most Earth rocks younger than space
  rocks?
• Earth is geologically active
• Oldest rocks may be deeply buried
• Higher chance that oldest rocks have been heated,
  metamorphosed, or melted, resulting in a
  resetting of the clock