Isotope Geology

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Isotope Geology

• Louisa Bradtmiller

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Overview

• Intro to isotopes and elements
• Methods of Decay
• Methods of Dating
• Stable Isotopes and their uses

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Nomenclature

• NUCLIDE one particular atom
• A NUCLIDE is made up of a NUCLEUS and surrounding
  electrons (-)
• NUCLEUSprotons () and neutrons (0)
• These three parts are important for charge
  balance (ions) and decay

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Methods of organization

• Periodic table
• Chart of the nuclides
• Z protons- this defines the element
• N neutrons
• A protons neutrons Atomic Number

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What is an isotope?

• Isotope- line of equal Z. It has the same
  protons (ie. they are the same element) but a
  diff. of neutrons.

14N
15N
12C
13C
14C
10B
11B
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How did all this stuff get here?

• 4 types of isotopes, based on how they formed
• Primordial (formed w/ the universe)
• Cosmogenic (made in the atmosphere)
• Anthropogenic (made in bombs, etc)
• Radiogenic (formed as a decay product)

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Primordial Isotopes

• Big bang formed 1H, 2H, 3H, and 4He Also, 3H
  4He 7Li --gt 7Be
• He burning in stars 3 4He 12C, etc Forms 16O,
  20Ne, 24Mg, 28Si
• Si burning in stars creates elements up to 56Fe

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Primordial Isotopes

• To get atoms of masses over 56, you need energy
  input
• This comes from supernovae, usually
• Once larger nuclides are formed, they can be
  incorporated into new stars, and added onto

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Radioactive decay

• Unstable nuclides spontaneously decay to form
  stable nuclides
• Radioactivity is the spontaneous transformation
  of an unstable nuclide, usually involving the
  emission of particles and energy
• Conservation of mass and energy apply

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Types of Radioactive decay

• Alpha decay (a particle 4He nucleus)
• Beta- decay (b- electron)
• Positron emission (b positron)
• Electron capture (e electron)
• Nuclear Fission
• Gamma emission (g high energy photon)

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a Decay

• Occurs for unstable nuclides with A ? 56 (except
  5He, 5Li and 6Be)
• P--gtD 4He Q g (Q energy)

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b- decay

• Parent and daughter are isobars
• P --gt D b- ? Q g (? antineutrino)
• neutron --gt proton electron ( antineutrino)

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b decay

• Analogous to b- decay
• proton --gt neutron positron ( neutrino)
• P --gt D b ? Q g (n neutrino)

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e capture

• Nucleus captures an orbiting electron
• Proton electron--gt neutron neutrino
• Same result as b

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Nuclear Fission

• Nucleus splits into 2 or more smaller nuclei,
  plus a??n, Q
• Usually not an even split
• Products often have excess n, decay b-
• Occurs spontaneously when Z ? 100
• Can be induced by neutron bombardment

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Rules of decay
N atoms, t time, ? decay constant
-dN/dt ? N
N0 N at t0
N N0e-lt
Half-life time it takes for half a sample to
decay (t when N ½N0)
t½ (ln 2) / l .693/l
D radiogenic daughter daughter atoms
produced by radioactive decay of a parent, P (D
D0 D)
D N0 N N0 N0e-lt N0 (1- e-lt)
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Rules of Decay
Divide by a stable, non-radiogenic isotope of the
daughter element to get ratios e.g. for 87Rb --gt
87Sr b- 87Sr/86Sr (87Sr/86Sr)0 87Rb/86Sr
(elt 1) We do this because ratios are much
easier to measure than actual counts of isotopes
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Dating Methods

• To use the radioactive decay equation to
  calculate the age of a sample, we measure the
  present day ratios, and we know ?.
• There are still 2 unknowns- R0 and t
• What to do?

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Dating Methods

• Assume zero initial daughter
• This only works if you know the daughter is
  excluded for some physical reason
• Use two different isotope systems
• Assume one to calculate the other
• This is called a model age
• Use an isochron

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Isochron

• Radioactive decay equation is like a line
  DD0P(elt 1) OR y mxb
• So, plot D vs P/D for samples to get a line
• The y intercept is the initial D ratio
• The slope is related to t
• Assumptions
• System was a isotopic equil. At t0
• It has been a close system
• Can do whole rock or mineral isochrons

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Isochron
Initial Ratios
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Dating methods

• Rubidium-Strontium
• Pro high conc., easy to measure.
• Con soluble, closed system often not valid
• Samarium-Neodymium
• Pro relatively immobile
• Con low conc., harder to measure

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Dating methods

• Uranium-Thorium-Lead
• Pro LOTS of uses in rocks and seawater
• Very long half life, so you can date old stuff
• Potassium-Argon
• Pro K is very common
• Con split decay to Ca and Ar, so often turn 39K
  into 39Ar and do 40Ar-39Ar

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Cosmogenic Nuclides

• Cosmogenic Nucleosynthesis Formation of isotopes
  in earths atmosphere by interaction w/ cosmic
  rays
• Dating using these gives date of last contact
  with atmosphere or extent of time in contact
  w/atm. (exposure age)

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Anthropogenic Nuclides

• 14C, 3H (tritium) are the main species of
  interest
• Peak production of both with bomb tests in the
  1960s
• Now they can be used as a tracer in the ocean or
  other reservoirs (GW)

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Stable Isotopes

• Lighter (stable) isotopes behave differently-
  they are easily fractionated during chemical and
  biological reactions, as well as phase changes
• FRACTIONATION separation between isotopes on the
  basis of mass (usually)
• Bonds between heavier isotopes are harder to break

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Stable Isotope Examples

• Rayleigh fractionation light isotopes evaporate
  more easily, and heavy isotopes rain out more
  quickly

d (Rsample Rstandard) / Rstandard x 103
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Stable Isotope Examples

• d18Ocarbonate in forams depends on d18Oseawater
  as well as T, S
• d18Oseawater depends on how much glacial ice
  there is
• Glacial ice is isotopically light b/c of Rayleigh
  fract.
• More ice means lower
• d18Oseawater

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Stable Isotope Examples

• Stable isotopes can also tell you about biology
• Organisms take up light isotopes preferentially
• So, when an organism has higher ?30Si, it
  means that it was feeding from a depleted
  nutrient pool

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Most asked isotope question(s)

• How old is the earth? How do we know?
• Answer(s) 4.56 Gy, and meteorites
• Chondrites are meteorites with small glass
  inclusions (chondrules)
• They are thought to represent the uniform
  composition of the early universe
• C1 carbonaceous chondrites have the same
  composition as the sun
• Therefore, their age is used for the age of the
  earth, sun and solar system