Formation and Structure of the Earth Petrogenesis and Volcanism

1. 12.710 Introduction to Marine Geology & GeophysicsAlison Shaw, Dan Lizarralde, Andrew Ashton & Bill Thompson Formation and Structure of the Earth Petrogenesis and Volcanism Plate Tectonics, Geophysical methods and Geodynamics Sedimentation in the oceans and Coastal processes Paleo-oceanography and Climate Grading: Mid-term exam: 25% Final exam: 25% Labs/Problem Sets: 50%

2. Lecture 1: Formation of the Universe, the elements, the solar system, and Earth • The Big Bang – what is the evidence for it? • Nucleosynthesis – how did the elements form? • What is the bulk composition of the solar system and how did it form? • How did bulk solar system stuff condense into solids and eventually planets? • What evidence is available from meteorites?

3. The Big Bang • based on: • the observation of Edwin Hubble (1889-1953) that the galaxies were moving away from us • 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

4. Big Bang: Main steps 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 Within 10-32 seconds, it cools enough to form a quark soup + electrons and other particles At about 1 second, the universe was a hot and dense mixture of free electrons, protons, neutrons, neutrinos and photons. 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! 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

5. 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

6. 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 Supernova 1994D Young Magellan stars http://hubblesite.org

7. 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.

8. 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

9. 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 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) Crab nebula – 1054 supernova

10. Elements stability

11. Solar abundance of the elements • Only 4% of universe is made up of elemental matter – the rest is dark energy (73%) and dark matter (23%) • General decrease in abundance with atomic number (H most abundant, U least abundant) • Big negative anomaly at Be, B, Li - moderate positive anomaly around Fe, sawtooth pattern from odd-even effect

12. Chart of the nuclides 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. p=n line number of protons number of neutrons

13. Chart of the Nuclides Shows the nuclear, or radioactive, behaviour of nuclides number of protons Isobar: nuclides of equal mass number Isotope: nuclides of the same chemical element having different atomic masses number of neutrons

14. Formation of the solar system The Nebular hypothesis: • a diffuse roughly spherical, slowly rotating nebula begins to contract • As it contract and rotates more rapidly, it flattens and matter gets concentrated at the center - protosun • The disk of gas and dust start to form grains, which collide and form planetesimals • The terrestrial planets build up by multiple collisions and accretion due to gravitational attraction – gas giants form d

15. Density and Size of Planets Distance from sun, 108 km 17

16. 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 18

17. Meteorite Classification 19

18. 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

19. 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%.

20. 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