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

linear relationship between

distance and red-shift

demonstrates uniform expansion,

implying a point-source origin

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

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

http://hubblesite.org

Supernova 1994DYoung Magellan stars

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.

Classification of stars

our sun

• 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.)

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)

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

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 nuclidesNaturally 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

num

ber

of p

roto

ns

p=n

line

Chart of the Nuclides

number of neutrons

nu

mb

er

of

pro

ton

s

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

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

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Density and Size of Planets

Distance from sun, 108 km

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Condensation sequence

Mercury

Venus

EarthMars

Jupiter

Saturn

Condensing the ices is what gave the giant planets the

mass to gravitationally capture H and He from nebula

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

Irons(cores of differentiated planetesimals)

Stony-irons(mechanical mixes of Fe and rock)

Basaltic Achondrites(Crusts and mantles of

differentiated planetesimals)

Ordinary Chondrites(Aggregates of chondrules, CAIs, metal, matrix)

Carbonaceous Chondrites(Primitve, organic rich, contain CAIs)

Carbonaceous Chondrites

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

•99% of solar system mass is in the sun, so solar composition is good approximation to bulk solar system composition

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