The Archean and Proterozoic

1. The Archean and Proterozoic Part 2

2. A Quick Review • The Big Bang • Red Shift • Accretion of Solar System • Earth • Moon

3. A Quick Overview • Atmosphere and Oceans • Continental and Oceanic Crusts • Protocontinents of the Archean • Assembly of major continental pieces in the Proterozoic • Rifting and Suturing events • Rodinia - Supercontinent

4. The Hadean • A time of major changes and Earth formation. No rock record. • Origin of the Earth and solar system • Differentiation of the Earth to form crust, mantle and core • Cold accretion model, heating from impacts and radioactivity lead to molten Earth and gravitational differentiation

5. The Hadean • Origin of the atmosphere • Condensation of water vapor • Origin of continental crust • Oldest dated Earth rocks are 3.96 by old (Canada)

7. Evolution of the Atmosphere • Atmosphere - Envelope of gases that surrounds the Earth. Used by life as a reservoir of chemical compounds used in living systems. Atmosphere has no outer boundary, just fades into space. Dense part of atmosphere (97% of mass) lies within 30 km of the Earth (so about same thickness as continental crust).

8. Evolution of the Atmosphere • Chemical Composition Today - Nitrogen (N2)- 78%, Oxygen (O2)- 21%, Carbon Dioxide (CO2) - 0.03 %, plus other miscellaneous gases (H2O for one).

9. First Atmosphere • Composition - Probably H2, He • Fate? • Either: • Blown away during ignition of the sun • Not enough gravitational pull to hold it in when the Earth was still small and accreting pieces

10. Second Atmosphere • Produced by volcanic outgassing. • Gases produced were probably similar to those created by modern volcanoes (H2O, CO2, SO2, CO, S2, Cl2, N2, H2) and NH3 (ammonia) and CH4 (methane) • No free O2 at this time (not found in volcanic gases).

11. The relative amounts of gases in the primordial atmosphere were different from the abundances vented to the exterior of the Earth during differentiation.

12. Addition of O2 to the Atmosphere • Today, the atmosphere is ~21% free oxygen. How did oxygen reach these levels in the atmosphere? • Photochemical dissociation - breakup of water molecules by ultraviolet • Produced O2 levels approx. 1-2% current levels • At these levels O3 (Ozone) can form to shield Earth surface from UV • Photosynthesis - • CO2 + H2O + sunlight = CH2O + O2 • produced by cyanobacteria, and eventually higher plants - supplied the rest of O2 to atmosphere.

13. Fate of O2 in the Atmosphere • Chemical Weathering - through oxidation of surface materials (early consumer) • Largely Fe during the Archean • Animal Respiration (much later)

14. History of O2 in the Atmosphere • Throughout the Archean there was little to no free oxygen in the atmosphere (<1% of presence levels). What little was produced by cyanobacteria, was probably consumed by the weathering process. Once rocks at the surface were sufficiently oxidized, more oxygen could remain free in the atmosphere.

15. Evidence from the Rock Record • Iron • Other Minerals

16. Sedimentary Iron • Iron (Fe) is extremely reactive with oxygen. If we look at the oxidation state of Fe in the rock record, we can infer a great deal about atmospheric evolution.

17. Sedimentary Iron • Banded Iron Formation (BIF) - Deep water deposits in which layers of iron-rich minerals alternate with iron-poor layers, primarily chert. Iron minerals include iron oxide, iron carbonate, iron silicate, iron sulfide. BIF's are a major source of iron ore, because they contain magnetite (Fe3O4) which has a higher Fe/O ratio than hematite. These are common in rocks 2.0 - 2.8 B.y. old, but do not form today.

18. Banded Iron Formations

19. Banded Iron Formations

20. Banded Iron Formations

21. Other minerals • Archean - Find occurrence of minerals that only form in non-oxidizing environments in Archean sediments: Pyrite (Fools gold; FeS2), Uraninite (UO2) . • These minerals are easily dissolved out of rocks under present atmospheric conditions.

22. Iron Again • Red beds (continental siliciclastic deposits) are never found in rocks older than 2.3 B. y., but are common during Phanerozoic time. Red beds are red because of the highly oxidized mineral hematite (Fe2O3), that probably forms secondarily by oxidation of other Fe minerals that have accumulated in the sediment.

23. Red Beds

24. Biological Evidence • Chemical building blocks of life could not have formed in the presence of atmospheric oxygen. Chemical reactions that yield amino acids are inhibited by presence of very small amounts of oxygen. • Oxygen prevents growth of the most primitive living bacteria such as photosynthetic bacteria, methane-producing bacteria and bacteria that derive energy from fermentation. • Conclusion - Since today's most primitive life forms are anaerobic, the first forms of cellular life probably had similar metabolisms.

25. Origin of the Oceans • Surface Temperature cools • Begin the Hydrologic Cycle • rain; runoff leads to lakes, rivers, oceans originally freshwater (rain); may have been acidic from sulfurous gases • slow accumulation of salts due to weathering

26. The constant circulation of water at and near the Earth’s surface.

27. Time Divisions for the Precambrian.

28. Origin of the Oceans and Continents • most of the early crust was mafic (Komatiites) • Komatiites are ultramafic mantle-derived volcanic rocks. They have low SiO2, low K2O, low Al2O3, and high to extremely high MgO.

29. Komatiite

30. Komatiite

31. Origin of the Continents • Continental crust was probably present prior to 4 billion years ago. • Detrital Zircon (common in granites) 4.4 - 4.1 bya • Oldest dated Earth rocks are 3.96 by old (Canada)

32. North American craton, shield, and platform.

33. Clicker TimeWhat is a key difference between Basalt and Granite? A. The amount of Fe and Mg bearing minerals B. The amount of SiO2 varies systematically C. Temperature of melting or crystallization varies

34. Origin of the Oceans and Continents • Continental crust developed secondarily • several models proposed involving partial melting and weathering

35. Origin of Continents • First crust • Basalt (oceanic) • Felsics differentiated • Formed nuclei of continental crusts • Iceland • Modern analogue

36. Origin of Continents • Small Archean fragments • High heat flow limited continental thickness • Zircon crystals • 4.1–4.2 B years old • Weathered from felsic rocks • Canadian Shield • 3.8–4.0 B years old

37. Can erosion help to form felsic continental rocks? A.Yes B. Definitely not because erosion produces sedimentary rocks and our continental core is granitic which is an igneous rock

38. Origin of Continents • Continental accretion • Deep water sediments accreted to continent • Marine sediments form wedge between continental masses

39. Major areas of exposed Precambrian rocks (shown in yellow).

40. ARCHEAN TERRANES • Archean Terranes represent the oldest continental and oceanic crust on earth. Virtually all of this original crust has since been deformed and metamorphosed.

41. Origin of Continents • Greenstone belts • Weakly metamorphosed • Abundant chlorite • Green color • Nested in high-grade felsic metamorphic rocks

42. Origin of Continents • Greenstone belts contain igneous rocks • Volcanics contain pillow basalts • Underwater extrusion • Formation of sediments in deep water • Graywackes, mudstones, iron formations, volcanic sediments

43. Origin of Continents • Banded iron formations • 3 B years old • Isua, southern Greenland

44. Greenstone belts of the Superior province.(After Goodwin, A. M. 1968. Proc. Geol. Assoc. of Canada 19: 1-14.)

45. ARCHEAN TERRANES • Gneissic Complexes • Archean gneissic complexes represent the highly metamorphosed and contorted relicts of original volcanic and sedimentary rocks that have undergone high-grade metamorphism and deformation. Archean gneisses are the highly deformed remnants of the early protocontinents.

46. ARCHEAN TERRANES • Greenstone Belts are elongated features composed of mildly metamorphosed volcanic (mostly komatiite and basalt) rocks and associated sediments. Greenstone belts were generally deposited in oceanic settings although the occurrence of associated andesitic and rhyolitic rocks suggests nearby volcanic arcs.

47. Formation of greenstone belts • Greenstone belts may have formed in back-arc extensional basins within the interiors of proto-continents. • Greenstone belts may also represent old oceanic crust between proto-continents near subduction zones. When the proto-continents collided, they collapsed the oceans filled with basalt and flysch to form greenstone belts.

48. Formation of greenstone belts

49. Generalized cross-section through two greenstone belts.

50. Plate tectonics model for the development of greenstone belts and growth of continental crust. (A) Plates are in motion, driven by convection cells in the upper mantle. (B) Compression has occurred to create the greenstone belts with their synclinal form and to aggregate small continental patches into a larger continental mass. Later, granites are intruded in and around greenstone belts.(Simplified from B. F. Windley, 1984. The Evolving Continents, 2/E, New York: J. Wiley & Sons.)