Introduction to Geochemistry

Introduction to Geochemistry
! Geochemistry
• Focuses on the chemical compositions of planets and other rocky bodies
• Minerals, geology, properties and chemical behavior of natural materials
• Cyclic processes that physically mobilize mass or transform chemical species
• Life - origins of life, composition of life relative to its formation environment
• Biogeochemistry - interaction of life and geology on a planetary scale
! This course
• Examines the origin of chemical elements, processes forming Earth and other
planets, thermodynamics, quantitative depiction of geochemical reactions, and
environmental processes.
• Requires: working knowledge of chemistry, familiarity with chemical equations
and working mathematical problems, interest in geology and planetary science.

Geochemistry Lecture Topic 1:
Cosmic Origins

Origin of the Universe
! “Big Bang”
• Present understanding of the Universe’s s origin
• Initial rapid inflation of spacetime through three physical dimensions of
space and one of time, within the larger bulk (11-dimensional space)
! Timeline
• 10 -35 to 10 -6 seconds
• Inflation, rapid expansion
• Fundamental forces separate
• 1 second
• Quarks, photons, neutrinos and more exotic particles congeal
• 3 seconds
• Protons, neutrons combine to form ionized H (~75%), He (~25%), trace Li (~1 ppb)
• 300,000 years
• Background radiation temperature drops to where electrons can become captured to
form neutral atoms
• Universe becomes transparent
• 300 million years
• Stars and galaxies form
• 8.7 billion years (5 billion years ago)
• Our solar system forms

How do we know?
! Red shift
• Spectral lines emitted by distant galaxies (actually supernovae in
galaxies) are shifted to the red by the Doppler Effect
• 1929: Edwin Hubble measured redshifts of light from 18 galaxies
in the Virgo cluster… calculated recessional velocities
• Found that recessional velocity increases with distance
• Doppler equation
!/!’ = 1 + "/c
Where !’ = wavelength of spectral line emitted by moving source
! = wavelength of same line emitted by stationary source
c = speed of light
" = recessional velocity

Hubble constant
v = Hd
H = Hubble constant
d = distance
• Provides a limit to the age of the universe
• Time required to separate two objects is…
t = d/v = 1/H
• Properly calibrated, estimate comes out to be

Cosmic microwave radiation
! Discovered by accident in 1965 by Penzias and Wilson
• Won Nobel prize in 1978 for discovery
• Universe is permeated by microwave radiation at a blackbody
temperature of ~2.7 K
• Remnant from when universe became transparent…
wavelengths are stretched out by cosmic expansion since then
• Max Planck equation for blackbody radiation
! max = 0.29/T
! Radiation varies slightly in temperature throughout the
universe
• Due to quantum fluctuations in the vacuum energy, occurring in
the first fraction of a second
• Inflation fixed these fluctuations in place on a cosmic scale

COBE data
! COsmic
Background
Explorer
• Microwave observatory launched in 1989
• Measured the cosmic background throughout the sky
• Using these data, quantum calculations could more accurately
date the origin of the universe
• 13.7 billion years (±0.1(
billion years)
NASA

Wilkinson Microwave Anisotropy Probe
(WMAP)
! “Baby picture” of the Universe
• Records a high-resolution image of light from 379,000 years after
the Big Bang
NASA
NASA

Hubble Deep Field
! Long exposure image of a section of sky
containing no stars (at least 2º 2 of arc
away from any star of magnitude 2 or
greater)
! Field width is very small, equivalent to a
dime held 75’ away (analogy from NASA
press briefing)
! Image contains at least 1,500 galaxies

NASA's Spitzer Space Telescope
• Observations of light from the first stars in the Universe

Stellar Evolution
! Stars form as a result of gravitational collapse of gas
clouds in space
• During collapse and contraction, core temperature and pressure
increases
• Eventually, a critical density is reached where atoms in the core
of the cloud reach pressures and temperatures that allow fusion
• Stellar fusion converts (primarily) Hydrogen into Helium, emitting
energy in the process
• The expansive force released by fusion balances the contractive
force of gravity, stabilizing the star as a discrete object in space

! Eagle Nebula - NGC 6611 - 6,500 light years away
! Stellar nursery - star-forming region
! Cloud is several hundred light-years across, and numerous protostars are visible
within (in IR light)
! Our solar system would have begun in a similar nebula

Hertzsprung-Russell diagram
! Solar luminosity v. log surface temperature
• Shows a definite relationship among observable stars
• Linear relation (main sequence) if H fusion is the dominant
process
• Red dwarfs => Blue giants
• Sun (~6,000 K)

Stellar Life Cycle
! Birth
• Contraction begins fusion at a core temperature of ~20 million K
• Early form of star is a “T-Tauri” star, with a strong solar wind
! Midlife
• Fusion of H in stellar core, slow expansion of stellar diameter
! Senescence
• Exhaustion of core H, fusion of H continues in expanding shell in mid-depth of
star… significant stellar expansion
• Star moves off of “main sequence” and becomes a Red Giant
• At the red giant stage, core temperature increases while outer mantle of star
cools by expansion
• Eventually, star begins to fuse He to form C in core
! Death
• Supernova - star detonates, may leave a pulsar or neutron star at its core, or a
black hole (>3 solar masses)
• White dwarf - lower mass stars dim to form a white dwarf star, which cools and
fades as surface T decreases, fuel runs out

Supernova remnant M1
(NGC 1952), the Crab Nebula
Explosion observed 1054 A.D.
(6,300 l.y. distant)

Nearby Starforming Region:
Orion Nebula (1,450 l.y.)
• Lower inset: includes Trapezium, a group
of four massive stars.
• In the larger Orion cloud complex:
~2,300 young stars w/
planetary discs
~200 “stellar embryos”

Nucleosynthesis
! Process forming elements heavier than He (or trace Li)
! Determined from nuclear fusion calculations, solar abundances of
elements, meteorite and comet compositions
! Solar (cosmic) abundances of the elements
• Obtained from spectrometric observation of the Sun’s s corona (emission
lines from ions in the solar photosphere)
• Not really “cosmic”,, because precise concentrations vary among stars
• H and He are most abundant elements (primordial H, some He)

Cosmic Abundances of the Elements
U
B

Nucleosynthesis
! Solar (cosmic) abundances of the elements
• H and He are most abundant elements (primordial H, some He)
• First 50 elements show exponentially decreasing abundances
• After first 50 elements, abundances are very low
• Even-number nuclides are more abundant than odd-numbered nuclides
• Oddo-Harkins rule, a result of nuclear “magic numbers”
• Li, Be and B are very low in abundance
• nuclei are destroyed in stars or are weakly stable
• Fe is of anomalously high abundance
• 56 Fe is the most stable of all possible nuclear configurations
• Tc and Pm do not occur naturally in our solar system
• Z > 83 (Bi) have no stable isotopes
• Once thought Bi 209 was heaviest stable, but in 2002 was shown to have a
half-life of 1.9 x 10 19 years…

Nucleosynthesis in the Sun & smaller stars
! Proton-Proton chain reaction
• Proton-Proton chain requires high Tº, T , low interaction probability,
regarded as principal reaction in stars of Solar mass or smaller
Step 1: 1
H + 1 H 2
H + # + + " (releases 0.422 MeV)
# + = positron, " = neutrino
Step 2: e - + # + $ (releases 1.02 MeV)
$ = gamma ray
Step 3: 2
H + 1 H 3
He + $ (releases 5.49 MeV)
Step 4: 3
He + 3 He 4
He + 1 H + 1 H (releases 12.86 MeV)
Overall: 4H form one He + neutrino + gamma ray + 19.794 MeV

Nucleosynthesis in massive stars
! The CNO cycle is the dominant mechanism of nucleosynthesis in
stars heavier than the Sun (1.5 solar mass and up)
Step 1: 12 C + 1 H
13 N + $
Step 2: 13 N
13 C + # + + "
Step 3: 13 C + 1 H
14 N + $
Step 4: 14 N + 1 H
15 O + $
Step 5: 15 O # + + " + N
Step 6:
15 N + 1 H
12 C + 4 He
Overall:
4 protons form one He, C acts as a catalyst…

He Fusion in Red Giants
! Triple alpha process
• A two-step reaction, both steps must occur in rapid succession,
because the 8 Be nuclide decays with a half-life of 10 -16 sec.
• Without this process, entire universe would be only H and He…
Step 1: 4
He + 4 He
Step 2: 8
Be + 4 He
He
12
8
Be
12
C + $
Overall: Three helium nuclei (triple(
%)) produce one carbon

Fusion of Heavier Elements
! Alpha capture reactions
• Mainly important in stars more massive than our Sun
• Concentric shells inside the star are dominated by different fusion
reactions, yielding products that are fused in the adjacent inward shell of
gas
• Produces elements up to the Fe-group (first row transition elements)
• Examples:
12 C + 4 He
16 O + 4 He
52 Fe + 4 He
He
16 O
He
20 Ne
He
56 Ni
• Some products undergo fission due to nucleon bombardment, neutron
capture… leads to a state of “nuclear statistical equilibrium”
• Does not lead to elements heavier than Ni…. . electrostatic repulsion
between positively-charged nuclei and alpha particles dampens fusion

Fusion of Elements Heavier than Ni
! Neutron capture, principally
! s-process: neutron flux in 2nd generation stars or red giants is low
enough that nuclides can undergo # decay between capture events
• Can yield nuclides up to Bi
Example step 1: 62 Ni + 1 n
63 Ni + $
step 2: 63 Ni 63 Cu + # - + "
! r-process: high neutron flux can allow sequential capture of many
neutrons before decay can occur
• Occurs in final minutes of a red giant, during supernova detonation
! p-process: low-probability reactions where multiple protons are
captured (by bombardment) simultaneously
• Characteristic of supernova detonations
• Yields particular isotopes such as 74 Se, 92 Mo

Explaining Cosmic Abundances
! H and He are most abundant
• Because they were formed in the Big Bang
! Exponential decrease in abundance of first 50 elements
• Reflects decreased productivity of He capture processes
! Lower abundances of heavier elements
• Caused by sluggishness of neutron capture reactions in normal stars
! Greater stability of nuclides with even-numbered numbers of protons and/or
neutrons
• Nucleons with paired spins have stronger nuclear binding affinities
! Low abundances of Li, Be, B
• Because production processes tend to bypass these elements, also they are
destroyed by nucleon bombardment inside the star