Course Goals

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Course Goals

• At the end of this course
• You will be able to identify a number of
  different potential reactions in any environment
  involving elements in solutions, solids, or
  gases, as affected by abiotic and biotic
  processes
• You will be able to utilize thermodynamic,
  kinetic, and transport calculations to determine
  if individual reactions are feasible/important
  under any given condition
• You will be able to appreciate both the dynamics
  and complexity of geochemistry yet utilize what
  you know to ascertain processes important in the
  stability, movement, and reactivity of elements
  in the earth and other planets

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FIELDS OF GEOLOGY RELYING ON GEOCHEMISTRY

• Mineralogy
• Igneous Petrology
• Metamorphic Petrology
• Environmental Science

• Sedimentology
• Geochronology
• Ore deposit studies
• Planetary Science

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ORIGIN OF THE UNIVERSE AND THE ABUNDANCE OF
ELEMENTS
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BIG BANG THEORY

• Most astronomers and astrophysicists now accept
  that the Universe was created in the so-called
  Big Bang.
• Not an explosion. More accurate to think of a
  growing bubble or balloon analogy.
• All the mass and energy that the Universe
  contains today was present at the moment of its
  inception.

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AFTER THE BIG BANG

• At t 10-32 seconds, pressure and temperature
  were so high that matter existed as a mix of
  quarks (fundamental components of matter).
• At t 13.8 seconds, the Universe cooled to about
  3 x 109 K, and the quarks combined to form
  neutrons, protons, etc., and then H and He
  nuclei. This continued for 30 minutes, but only
  H and He produced. Could not create Li, Be or B.

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WHAT IS THE EVIDENCE SUPPORTING THE BIG BANG
THEORY?

• The redshift of the stars
• Blackbody remnant radiation

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REDSHIFT OF STARS - THE DOPPLER EFFECT

• The wavelength of waves emanating from a moving
  source appears to be longer or shorter, depending
  on whether the source is moving towards or away
  from the observer.

?? - wavelength from moving source ? -
wave-length from stationary source v - velocity
of source c - speed of light.
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• If v gt 0, i.e., the source is moving away from
  the observer, ?? gt ?, so the light appears
  redshifted.
• If v lt 0, i.e., the source is moving towards the
  observer, ?? lt ?, so the light appears
  blueshifted.

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ELECTROMAGNETIC SPECTRUM
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HUBBLES DISCOVERY

• Hubble noticed that spectral lines from stars
  undergo a redshift owing to the Doppler effect.
• From the apparent intensity of the star, the
  distance can be estimated.
• From a knowledge of the composition of stars,
  i.e., H and He, we know the expected wavelength
  of emission.
• Hubble found that the amount of redshift
  increased with distance, i.e., stars further away
  are moving away from us (and each other) at
  greater speeds.

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HUBBLES EQUATION

• H 15 km/sec/106 light years

The accepted age of the Universe is ? (14.51.0)
x 109 years
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BLACKBODY REMNANT RADIATION

• Penzias Wilson (1964) discovered a background
  microwave radiation corresponding to a blackbody
  temperature of 3 K.
• This radiation is thought to be a remnant of the
  radiation that filled the Universe for 700,000
  years when T gt 3000 K.

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STELLAR EVOLUTION

• Evolution of stars is described by
• luminosity ? mass
• surface temperature ? volume
• Stars form from the contraction of interstellar
  gas. As contraction proceeds, temperature
  increases and IR and visible radiation is
  emitted.
• When T 20 x 106 K, H-fusion is possible. Most
  stars derive energy from H-fusion and fall on the
  main sequence of a Hertzsprung-Russell diagram.

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HERTZSPRUNG-RUSSEL DIAGRAM
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NUCLEOSYNTHESIS

• Nucleosynthesis - refers to the creation of the
  nuclei of the chemical elements.
• Only H, D and He were created in the initial big
  bang.
• Other elements are generated in stars during
  their life or during supernovas that end the
  stars life.

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HOW DO WE KNOW THE ABUNDANCES OF ELEMENTS IN THE
SOLAR SYSTEM?

• Spectroscopic studies of sun and other stars.
• Analysis of meteorites, terrestrial rocks, and
  lunar rocks.
• Indirect inferences based on physical properties

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ELEMENTAL ABUNDANCES IN SOLAR SYSTEM
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IMPORTANT OBSERVATIONS ABOUT ELEMENTAL ABUNDANCES
IN THE SOLAR SYSTEM

• H and He are by far the most abundant elements,
  with H/He 12.5.
• Abundances of the first 50 elements decrease
  exponentially.
• Abundances of elements with Z gt 50 are very low
  and do not vary greatly with Z.
• Abundances of Li, Be, and B are anomalously low.

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• Abundances of Fe and Pb are anomalously high.
• Tc and Pm do not occur naturally in the solar
  system.
• Elements with Z gt 83 (Bi) have no stable
  isotopes they only occur naturally because they
  are decay products of long-lived radioactive
  isotopes of U and Th.
• Even atomic number elements are more abundant
  than odd (Oddo-Harkins rule).

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Nuclide Distribution
From Faure (1986)
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H-FUSIONPROTON-PROTON CHAIN

• T gt 107 K low probability because 2 3He nuclei
  must react (note in stars, all atoms are
  stripped of electrons).

This is the only source of nuclear energy for 1st
generation stars.
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H-FUSIONCNO CYCLE

• After the supernova of 1st generation stars,
  processes involving elements with higher Z were
  possible.
• CNO cycle is higher probability process than
  proton-proton chain.

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He-FUSIONTRIPLE-ALPHA PROCESS

• At T 108 K, He is the fuel for the triple-?
  process.

• This process bridges the gap in the stability of
  Li, Be and B.

• For 8Be, t½ 10-16 seconds. Thus, 8Be must
  absorb an ?-particle very quickly to get 12C.

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ALPHA-CHAIN PROCESS

• With further increases in temperature,
  ?-particles fuse with 12C to form higher atomic
  number atoms in an ?-chain process.

etc.
The process stops at 56Fe. Thus, Fe is the last
element produced in normal stars.
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NEUTRON-CAPTURE REACTIONS

• During the final stages of red giant evolution,
  neutron-capture reactions produce atoms with Z gt
  26 (Fe). The following represent the slow process
  or s-process.

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RAPID OR r-PROCESS

• s-processes bypass stable 70Zn. To get this
  nuclide we need to speed up the neutrons. This
  requires a higher neutron flux, which requires
  higher temperatures and pressures.
• Occurs in the last few minutes of a stars life.

s-process
stable
unstable
r-process
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p-PROCESS