Transcript Lecture 1: Nucleosynthesis, solar composition, chondrites, volatility

12.710 Introduction to Marine Geology & Geophysics
Alison Shaw, Dan Lizarralde, Andrew Ashton & Bill Thompson
1) Formation and Structure of the Earth
2) Petrogenesis and Volcanism
3) Plate Tectonics, Geophysical methods and Geodynamics
4) Sedimentation in the oceans and Coastal processes
5) Paleo-oceanography and Climate
Grading:
Mid-term exam: 25%
Final exam: 25%
Labs/Problem Sets: 50%
Lecture 1: Formation of the Universe, the elements,
the solar system, and Earth
1) The Big Bang – what is the evidence for it?
2) Nucleosynthesis – how did the elements form?
3) What is the bulk composition of the solar system and
how did it form?
4) How did bulk solar system stuff condense into solids and
eventually planets?
5) What evidence is available from meteorites?
The Big Bang
based on:
1) the observation of Edwin Hubble (1889-1953)
that the galaxies were moving away from us
2) background cosmic microwave radiation can be
“heard” – discovered by Penzias and Wilson
linear relationship between
distance and red-shift
demonstrates uniform expansion,
implying a point-source origin
Big Bang: Main steps
1) Universe started ~15 Ga, the size of
an atom, at temperatures (or energy)
too hot for normal matter > 1027 K –
it start expanding extremely rapidly
2) Within 10-32 seconds, it cools enough to form a quark soup +
electrons and other particles
3) At about 1 second, the universe was a hot and dense mixture of
free electrons, protons, neutrons, neutrinos and photons.
4) At about 13.8 seconds, temperature has decreased to 3 x 109 K
and atomic nuclei began to form, but not beyond H and He.
The universe was a rapidly expanding fireball!
5) 700 000 years later electrons became attached to nuclei of H
and He – formation of true atoms. Matter became organized
into stars, galaxies and clusters
Periodic Table:
Developed by Russian chemist, Dmitri Mendeleev in 1869
to illustrate recurring ("periodic") trends in the properties of
the elements
Z = atomic number or
number of protons
A = mass number or
number of protons
+ neutrons
Nucleosynthesis: the process of creation of the elements
Our understanding of nucleosynthesis comes from a combination of
observations of the abundances of the elements (and their
isotopes) in meteorites and from observations on stars and related
objects:
– Until stars formed, there was nothing except H and He
– Gravitational instabilities developed which lead to formation
of galaxies and collapse of molecular clouds to form stars
– At sufficient temperature and density (~107 K), nuclear
fusion begins in star cores
Young Magellan stars
Supernova 1994D
http://hubblesite.org
Classification of stars
our sun
The Hertzsprung-Russell diagram of the relationship between luminosity and
surface temperature. Most stars, like the sun fall on the main sequence, but
can evolve to red giants and supernovas (if they are at least 5 x as massive
as our sun) or to white dwarfs, pulsars or even black holes.
•
Most stars produce energy by 1H burning – first generation stars
produce 4He by proton-proton-chain process: fusion
•
Second generation stars have already incorporated other elements
beyond 4He and 1H burning takes place by C-N-O cycle: four protons
fuse using carbon, nitrogen and oxygen isotopes as a catalyst to
produce one alpha particle (4He), two positrons and two electron
neutrinos. Fusion processes (He burning, C burning, O burning, Si
burning can form elements up to mass 40Ca)
•
Eventually all H will be fused to He (our sun has fused 10-20% of its H)
•
If the star is < ~8 solar masses, the star will undergo swelling to form a
red giant, followed by gravitational collapse to a white dwarf - when
thermal radiation reaches gamma-ray energies it drives rapid nuclear
rearrangement creating everything up to 56Fe
Nucleosynthesis (cont.)
If the star is >8 solar masses, then it
collapses catastrophically, then explodes
into a supernova. Its eventual fate is either
a neutron star or a black hole is the mass
of the star is big enough
Crab nebula – 1054 supernova
The rest of the elements are produced by 2 pathways:
s-process (slow): addition of neutrons to nuclei one at a time (only
stable elements can be made)
r-process (rapid): addition of neutron at a rapid rate so as to bridge
areas of nuclear instability (only in supernovas and accounts for
about half of elements beyond 56Fe)
Elements stability
Solar abundance of the elements
1)
Only 4% of universe is made up of elemental matter – the rest is dark
energy (73%) and dark matter (23%)
2)
General decrease in abundance with atomic number (H most
abundant, U least abundant)
3)
Big negative anomaly at Be, B, Li - moderate positive anomaly around
Fe, sawtooth pattern from odd-even effect
Chart of the nuclides
number of protons
Naturally occurring nuclides define a path in the chart of the nuclides,
corresponding to the greatest stability of proton/neutron ratio. For nuclides
of low atomic mass, the greatest stability is achieved when the number of
neutrons and protons are approximately equal (N = Z) but as atomic mass
increases, the stable neutron/proton ratio increases until N/Z = 1.5.
number of neutrons
Chart of the Nuclides
number of protons
Shows the nuclear, or radioactive, behaviour of nuclides
Isobar: nuclides of equal mass number
Isotope: nuclides of the same chemical
element having different atomic
masses
number of neutrons
Formation of the solar system
The Nebular hypothesis:
a) a diffuse roughly spherical, slowly
rotating nebula begins to contract
b) As it contract and rotates more rapidly, it
flattens and matter gets concentrated at
the center - protosun
c) The disk of gas and dust start to form
grains, which collide and form
planetesimals
d) The terrestrial planets build
up by multiple collisions and
accretion due to gravitational
attraction – gas giants form
d
Density and Size of Planets
Distance from sun, 108 km
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Condensation sequence
Mercury
Venus
Earth
Mars
Condensing the ices is what
gave the giant planets the
mass to gravitationally capture
H and He from nebula
Jupiter
Saturn
Bulk oxidation state of a planet
is set by how much O is
condensed as FeO and how
much H is retained as H2O
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Meteorite Classification
Carbonaceous Chondrites
(Primit ve, organic rich, cont ain CAI s)
Ordinary Chondrites
(Aggregates of chondrules, CAI s, metal, matrix)
Irons
Stony-irons
(cores of dif f erentiat ed planet esimals) (mechanical mixes of Fe and rock)
Basaltic Achondrites
(Crusts and mant les of
dif f erent iated planetesimals)
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Carbonaceous Chondrites
•99% of solar system mass is in the sun, so solar composition is good
approximation to bulk solar system composition
Except for the most volatile
elements (i.e., more volatile than
nitrogen), CI carbonaceous
chondrites are excellent models of
bulk solar system composition
and hence may be close to bulk
earth composition
While the sun is basically H+He,
the Earth is dominated by O, Si,
Mg, Fe. Much Fe is in core,
leaving rocky earth dominated by
O, Si, Mg
Composition of Bulk Silicate Earth (=crust + mantle)
• Earth differentiated into crust, mantle,
outer core and inner core relatively quickly
(within 30 million years of formation)
• BSE should be similar to carbonaceous
chondrites in terms of refractory lithophile
elements (Al, Ca, Ti, Sc, V, REE, U, Th, ...)
• Sm-Nd & Lu-Hf isotope systems tell us
BSE’s Sm/Nd and Lu/Hf should not
deviation from chondritic values by more
than 5%.
Geochemical characterization of elements
• By nucleosynthetic origin and nuclear properties
• primordial, H burning, red giant processes, neutron capture
• stable, long-lived radioactive, short-lived (extinct?) radioactive
• By volatility in gas-solid equilibria, i.e. by condensation temperature
• refractory, moderately volatile, highly volatile
• By affinity during gross chemical differentiation of the earth
• Siderophile: like to be with iron (core),
• Lithophile: like to be with silicates (crust + mantle),
• Atmophile: likes to be in the atmosphere
• Chalcophile: likes to be with sulfur
• By compatibility (solid/melt concentration ratio) in igneous processes
• compatible, incompatible, very incompatible; generally functions of
charge and ionic radius…related to position in periodic table in
systematic ways
References for Lecture 1: Faure, Chapters 1, 2 and 3