2. Tectonics and climate of the Precambrian Geology 103

3. When Did the Solar System Form? 4.56 billion years ago How do we know? (evidence for formation) Meteorite photo by Carl Allen at http://ares.jsc.nasa.gov/Education/Activities/ExpMetMys/..%5C..%5CSlideSets/ExpMetMys/Slides1-9.htm Lunar samples - 4.5 to 4.6 Ga Meteorites - 4.56 Ga Earth – 3.9 (or 4.4 Ga) Lunar meteorite at http://meteorites.wustl.edu/lunar/stones/mac88105.htm

4. How Did We Get a Solar System? Huge cloud of cold, thinly dispersed interstellar gas and dust – threaded with magnetic fields that resist collapse – solar nebula theory of Swedenborg (1734), Kant (1755) and Laplace (1796). Hubble image at http://hubblesite.org/newscenter/archive/releases/nebula/emission/2006/41/image/a/ Image: LPI

5. Concentrations of dust and gas in the cloud; material starts to collect (gravity > magnetic forces) How Did We Get a Solar System? Hubble image at http://hubblesite.org/newscenter/archive/releases/nebula/emission/2005/35/image/a/ Image: LPI

6. How Did We Get a Solar System? Gravity concentrates most stuff near center Heat and pressure increase Collapses – central proto-sun rotates faster (probably got initial rotation from the cloud) Image: LPI http://www.lpi.usra.edu/education/timeline/gallery/slide_1.htmlhttp://www.lpi.usra.edu/education/timeline/gallery/slide_1.html

7. How Did We Get a Solar System? NASA artwork at http://en.wikipedia.org/wiki/Image:Ra4-protoplanetary-disk.jpghttp://en.wikipedia.org/wiki/Image:Ra4-protoplanetary-disk.jpg Rotating, flattening, contracting disk - solar nebula! Equatorial Plane Orbit Direction

8. After ~10 million years, material in center of nebula hot enough to fuse H “...here comes the sun…” How Did We Get a Solar System? NASA/JPL-Caltech Image at http://www.nasa.gov/vision/universe/starsgalaxies/spitzer-20060724.html

9. How Did We Get a Solar System? Hubble photo at http://hubblesite.org/newscenter/archive/releases/star/protoplanetary-disk/2005/10/image/a/layout/thumb/ Metallic elements (Mg, Si, Fe) condense into solids at high temps. Combined with O to make tiny grains Lower temp (H, He, CH4, H2O, N2, ice) - outer edges Planetary Compositions

10. How Did We Get a Solar System? Inner Planets: Hot – Silicate minerals, metals, no light elements, ice Begin to stick together with dust  clumps Image: LPI http://www.lpi.usra.edu/education/timeline/gallery/slide_3.htmlhttp://www.lpi.usra.edu/education/timeline/gallery/slide_3.html

11. How Did We Get a Solar System? Accretion - particles collide and stick together … or break apart … gravity not involved if small pieces Form planetesimals, up to a few km across Image: LPI http://www.lpi.usra.edu/education/timeline/gallery/slide_3.htmlhttp://www.lpi.usra.edu/education/timeline/gallery/slide_3.html

12. How Did We Get a Solar System? Gravitational accretion: planetesimals attract stuff Large protoplanets dominate, grow rapidly, clean up area ( takes ~10 to 25 My) Image: LPI http://www.lpi.usra.edu/education/timeline/gallery/slide_4.htmlhttp://www.lpi.usra.edu/education/timeline/gallery/slide_4.html

13. A Magma Ocean Lunar evidence – Textures, Uniform Composition, Age – Crystallization of well-mixed magma ocean produces uniform layered crust Terrestrial Magma Ocean – Existence of large amount initial heat – Outer part of Earth melted during accretion – Depth estimates 100 to >1000 Km – Ultramafic (high Fe & Mg) – Crystallization complete in 100 my

14. Composition of the Early Crust Composition largely speculative, no examples Oldest lunar crustal rocks may represent early earth’s crust – anorthosite, gabbro (both mafic) Komatiites: volcanic, ultramafic (high Fe & Mg concentrations) rocks Rapid break-up and recycling of crust – Due to vigorous convection – Impacts Existence of some form of plate tectonics that does not resemble the modern version

15. Outgassing Oceans are byproducts of heating and differentiation: as earth warmed and partially melted, water locked in the minerals as hydrogen and oxygen was released and carried to the surface by volcanic venting activity

16. What might the first continents have been like? Continental crust resists recycling due to buoyancy But continental (not mafic) materials can be produced by partial melting of oceanic crust in subduction zones Tonalites are more felsic igneous rocks - abundant plagioclase, quartz, high in Ca,Na, Al Then, small islands can accrete into bigger continents Oldest remnants 3.8 to 4.0 by < 500km diameter Remnants are made of tonalites and granodiorites

17. Early Continental Crust Amitsoq Gneiss Isua, Greenland 4.0-3.8 by

18. Thus, the Precambrian divisions are defined broadly by atmospheric changes Hadean: Lots of carbon dioxide, water vapor and methane Archean: Water vapor forms oceans, oxygen starts to be made by photosynthetic organisms Proterozoic: Significant oxygen in atmosphere, massive drop in carbon dioxide

19. Graphically…

20. Period of major accretion (~ 10-30 my) { Period of heavy bombardment Present-day plate tectonics “begins” Some boundaries coincide with other events

21. Period of major accretion (~ 10-30 my) { Period of heavy bombardment Present-day plate tectonics “begins” period of rapid crustal growth Archaen-Proterozoic transition To modern plate tectonics 1. Early plates became bigger and thicker 2. Continued recycling of oceanic crust formed large amounts of buoyant continental crust Continued partial melting/distillation Separation of Si and other elements from Mg and Fe Conversion of mafic material to felsic material through rock cycle 3. Decrease in heat production slowed mantle convection Drove system to larger convection cells Allowed larger plates to travel farther on the Earth’s surface and cool more Led to subduction rather than collision of plates Modern plate tectonics

22. The Witwatersrand (South Africa) goldfields may have been generated by atmospheric changes

23. Evidence against the theory Not all gold deposits are the same age Clearly, some other mechanism deposits gold in this fashion – anoxic inland seas?

24. More evidence for atmospheric change in Archean Banded iron formations (BIFs) are interlayered alternating chert (jasper) and iron oxide Mostly found in Archean, some in Proterozoic, almost none in the Phanerozoic

25. Mechanism for generating BIFs

26. Still more evidence for atmosphere changes Redbeds - sandstones and shales w/ iron oxides require enough oxygen to oxidized. – Absent from geologic record until 2.4 by and only abundant after 1.5 by Sulfates (gypsum and anhydrite) require free oxygen; not present in geologic record until 2 by Uraninite (uranium mineral) & pyrite unstable under oxidizing conditions; present in rocks 2.3 to 2.8 by, none younger

27. In Oklo, Gabon, a natural nuclear reactor was generated in a depositional basin Rising oxygen levels allowed uranite to dissolve and reprecipitate at the bottom of an anoxic basin, allowing the criticality of uranium

28. Meanwhile, plate tectonics settles down Archean rocks worldwide are of only two types: granite/gneiss complexes (a high-grade metamorphic rock) and intervening greenstones (metamorphosed basalt and some sedimentary rock) Superior province in North America is among the biggest in the world

29. Greenstone Belts of the Superior Province

30. What does a greenstone belt remind you of? (Hint: ophiolite)

31. Plate Tectonic Model for the Development of Greenstone Belts and Growth of Continental Crust

32. But still different than today’s plate tectonics Komatiites are ultramafic igneous rocks that are common in the Precambrian but unknown today Hotter mantle? Wetter mantle? Diamonds!

33. First continents form and stick around

34. Archean Life Organic compounds and macrofossils or microstructures Carbon isotopes (C-13 to C-12) in kerogen similar to modern organisms; 3.8 by – Isua, Greenland Rodshaped and filamentous structures, spheroidal bodies common in Archean cherts from 3.6 by – Warrawoona Group, Pilbara Region Australia – Fig Tree Group, Barberton, South Africa Stromatolites: laminated domed-shaped mounds deposited by cyanobacteria 3.6 by, Pilbara region of Australia

35. Fig. 9.8f

36. So, by the Proterozoic… Division between Archean and Proterozoic is based on oxidizing conditions found in surface waters (1.8 by) Tectonics is more similar to today’s; evidence for rifting and subduction and terrane accretion

37. Mechanisms of continental growth Magma addition in arcs Terrane accretion Continental collision Welding of marginal sediments Quick definition: “shield” is the exposed crystalline, typically Precambrian, part of any continent; the “craton” is the shield plus any areas of crystalline Precambrian rock overlain by sedimentary deposits (the “platform”).

38. North American Craton- shield, and platform

39. The Assembling of North America Collision and suturing of provinces to make a continent Assembly of Archean plates took only 10 my 50% Late Archean (2.5-3.0 by) 30% Early Proterozoic (1.6-2.0 by) <10% Mid to late Proterozoic (0.9-1.2 by) <10% Phanerozoic (<544 my)

40. Mechanism for Continental Growth (a) Magma addition in arcs (b) Seaward migration of ocean plate (c) Terrane accretion through suturing (d) Continental collisions (e) Welding of marginal sediments

41. Continental Growth Rates Rapid early growth – recycling not feasible Linear growth Episodic growth – 2.7 by, 2.0 by, 1.0 by correspond to major orogenic episodes in North America time vol

42. Since the Archean Intensity of plate tectonics has varied over time Wilson cycles – 500 my cycles – Evidence of a supercontinent at 600-900 my (Rodinia) – Pangea formed ~ 300 my Periods of rapid sea floor spreading (and vice versa) – Sea level rises because large amounts of shallow basalt form and don ’ t cool (and subside) much – High CO 2 release – released at spreading centers when new crust forms and subducting crust has sediment on it including calcite which releases CO 2 when it melts

43. What evidence exists for Rodinia? Grenville orogeny rocks (sometimes called “mobile belts”), originally defined to explain Canadian shield rocks, were found to exist on many other continents All this mountain-building implies some large-scale tectonic event, like the creation of a supercontinent (name was suggested in the 1990s) Rodinia is constructed at 1.1 by, rifts apart by 0.85 by

44. The Grenville orogeny rocks Primarily marine sandstones and carbonates (limestones) No bioturbation Since then, these rocks have been metamorphosed, but the original rock is easily inferred

45. Conventional reconstruction Line up all the Grenville orogenic belts and create the supercontinent Note that Antarctica and the US (Laurentia) are quite separated

46. The SWEAT hypothesis Rodinia joined the southwest (SW) US (West Texas, specifically) with eastern Antarctica (EAT) Shown through lead isotope measurements of similar age rocks that were part of a rift in both areas Key point: there was not just one zone of orogeny as in the conventional theory

47. BIF upper peninsula MI BIF Wadi Kareim, Egypt The Animikie Group located on the western shores of Lake Superior contains BIFs and other sedimentary rocks BIFs record presence of free oxygen some deposits over 1000 m thick and over 100 km in extent Gunflint Chert contains a series of interesting assemblage of cyanobacteria Proterozoic life

48. Proterozoic Life By 2 by unicellular organisms widespread 1.9 by Gunflint fauna – thread bacteria & cyanobacteria Abundant stromatolites – reached peak diversity 750 my By 1.8 by evolution of eukaryotic cells – Acritarchs, unicellular, spherical microfossils, planktic, photosynthetic, common in rocks <1.5by A: Eoasterion B: Eophaera C: Animikiea D: Kakabekia Organisms from the Gunflint Chert

49. Precambrian climate Positions of continents, especially existence of polar continents, determines when ice ages occur

50. Gowganda Fm 240 my period of glaciation evidence in UT, NV, w. Canada AK, Greenland, S. Am Scandinavia, Africa Major Glaciations Witwatersrand Fm 2.8 by

51. Positive feedback If glaciers can build extensively to within 30° of the equator, the extensive ice will reflect a large portion of the Sun’s energy back into space, cooling the surface and allowing more glaciers to grow “Icehouse Earth” or “Snowball Earth” hypothesis (W. Brian Harland, Cambridge, 1964)

52. How to get out of the Icehouse Joe Kirschvink (Caltech, 1992) argued that volcanic activity and carbon dioxide production would not cease even during an Icehouse event, and nothing would “scrub” the carbon dioxide out of the atmosphere, enhancing the greenhouse effect

53. More detail about CO 2 scrubbing

54. Advent of the Metazoans Metazoans – multicellular, differentiated cells with tissues and organs – established by 1 by Ediacaran Fauna – Flinders Range, South Australia – Wilpena Pound Quartzite – 31 species; soft bodied annelids, cnidarians, arthropods, echinoderms – Late Proterozoic Ediacaran Fauna Spriggina floundersi segmented worm Kimberella- mollusc-like

55. Life alters as Rodinia breaks up Ediacaran fauna appears – first evidence of multicellular life No hard parts, all preserved as molds Unclear if they are all truly related to modern phyla, or represent extinct phyla Ediacaran period is a recognized division of the Proterozoic eon (630 – 542 my)