THE GOALS OF GEOCHEMISTRY

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INTRODUCTION
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THE GOALS OF GEOCHEMISTRY

• Determine the distribution of elements in the
  Earth and Solar System.
• Determine the causes for the observed chemical
  compositions of Earth and Space materials.
• Study chemical reactions of geologic relevance.
• Assemble this information to learn how
  geochemical processes worked in the past and will
  operate in the future.

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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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TOPIC 1ORIGIN 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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AT 700,000 YEARS

• The temperature cooled to 3 x 103 K.
• Electrons finally could become attached to nuclei
  to form atoms.
• Eventually matter became organized into stars,
  galaxies, etc.
• The Universe continued to expand.

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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
Blue
Red
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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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THE HUBBLE CONSTANT AND THE EXPANDING UNIVERSE

• Wendy Freedman
• American Scientist
• v. 91 (2003) p. 36-43

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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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STEFANS LAW

• For a blackbody emitter
• I ? T4
• ?max 0.29/T(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 are
  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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• Eventually a star starts to burn He and may move
  off the main sequence to become a red giant.
• When T 108 K, He-fusion occurs by the
  triple-alpha process, converting 3He nuclei to
  a 12C nucleus.
• The length of time a star stays on the main
  sequence depends on
• its mass
• the H/He ratio of the initial gas cloud
• Late in a stars life, fuel for a particular
  reaction may be exhausted, so the star collapses
  and contracts, and its core temperature rises.
  Then a new set of nuclear reactions take over.
• If a star has sufficient mass, a large explosion
  (supernova) may eventually occur.

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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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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.
• Even atomic number elements are more abundant
  than odd (Oddo-Harkins rule).

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• Abundances of Li, Be, and B are anomalously low.
• 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.

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ELEMENTAL ABUNDANCES IN SOLAR SYSTEM
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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.

stable
unstable
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p-PROCESS