Walter D. Mooney, Ph.D. US Geological Survey Menlo Park, California USA mooney@usgs

1. Lecture #1: Evolution of the Early Earth IPRCC and SinoProbe Short Course: Lithospheric Evolution through Time April 8-12, 2011 Walter D. Mooney, Ph.D. US Geological Survey Menlo Park, California USA mooney@usgs.gov

3. Acknowledements The International Precambrian Research Center of China (IPRCC) and SINOPROBE: Deep Exploration in China. .

9. What is the lithosphere? The rigid outer shell of the Earth that moves as a tectonic plate is called the lithosphere. However, there are several ways to define the lithosphere.

11. Lithospheric and Mantle Structure Source: Forte and Mitrovica (2001)

13. Key Events of Precambrian Time • Accretion of the Earth from planetesimals • Differentiation (core and mantle, little crust) • Continuous bombardment by comets and meteorites • Volcanic activity was ubiquitous • Early atmosphere formed (with no oxygen) • Oceans began to accumulate (once Earth cooled sufficiently)

14. Very Early Earth • Earth was a rapidly rotating, hot, barren, waterless planet • bombarded by comets and meteorites • with no continents and no oceans • intense cosmic radiation and • widespread volcanism Venus lavascape

15. Formation of the Moon by Impact (4.5 Ga)

16. Hadean Time In Western (Greek) literature, “Hades” was a mythology place of endless fire and molten lava.

18. Hadean Tectonics In Hadean time (4.6-3.6 Ga), there was no plate tectonics. Most likely there was mantle plume tectonics. (4.6 Ga = “Giga annum” = 4,600,000,000 years before present)

19. Continental Crust First Then Water out

20. Key Events of Precambrian Time • Acasta Gneiss, Yellowknife Lake, NWT Canada: dated at 3.96 bya • Zircons possibly a bit older in Australia

21. The Early Earth How and where can we study really ancient rocks?

22. Earth’s Oldest Rocks Banded iron formation Sedimentary rock composed of thin chert (quartz) bands interlayered with iron oxide minerals. One banded iron formation, Itsaq gneiss complex-Isua greenstone belt of Southwest Greenland deposited 3.7 to 3.8 billion years ago.

27. Electrostatic analyzer Primary ion source Magnet Collector Sample chamber Ion source OperationalUnit

28. Techniques Established: • Zircon U-Th-Pb age dating • Monazite U-Th-Pb age dating • Apatite U-Pb age dating • Opal U-Pb age dating • Zircon in-situ REE analyses Under development: • U age dating of young zircon • U-Th-Pb age dating of fluorite

29. ⑩ ③Canada Beijing ⑥Italy ⑦ ① ⑤ Australia ②Brazil ⑨ ④ ⑧ SROS Workstations established so far 1.Yichang, China（2005.8.10） 2.University of Sao Paulo, Brazil（2005.9.16） 3.Ontario Geological Survey, Canada（2007.1.26） 4.Curtin University of Technology, Australia（2007.9.8） 5.Nanjing University, China（2007.9.19） 6.University of Milan-Bicocca, Italy（2007.10.8）

30. Key Events of Precambrian Time • Acasta Gneiss, Yellowknife Lake, NWT Canada: dated at 3.96 bya • Zircons possibly a bit older in Australia

31. Earth’s Early Atmosphere • Today: Nitrogen 78%, Oxygen 21% • Earth’s early atmosphere: • H2, CO2, CH4 and less CO2 , H2O, H2S, N2 • Faint Sun (-25%), but sediments exist • CO2 is 0.038% today

32. The Early Atmosphere

33. Hydrosphere • Earth’s oceans = outgassing + comets • Sea Level = amount of H2O (vs. ice pack) + volume of ocean basin, related to length of ridges • Sea Level +/-200 m • 20m/1000 yr

34. Key Events of Precambrian Time

35. When did Plate Tectonics begin? possible tectonic settings in at the end of the Archean, 2.7 billion years ago Changes in the Mantle-Crust System Source: Kent Condie, at: http://www.ees.nmt.edu/Geol/precam.html

36. Early Continents (Cratons) • Pilbara Shield, Australia • Canadian Shield • South African Shield Mafic Greenstone Belts Felsic Islands 40 km

38. Continental Crustal Volume Throughout Time Source: Kent Condie, at: http://www.ees.nmt.edu/Geol/precam.html

39. Petrologic Constraints on Lithospheric Roots beneath Continents

40. High Temperature Melting: Archean Crust Basalt flood Partial melting of basalt crust

43. Model for Archean Lithospheric Evolution

44. High Temp. Melting: Archean Lithosphere Basalt flood Partial melting of basalt crust

45. Model for Proterozoic Lithospheric Evolution

46. Continental Lithosphere First Then Water out

48. Overview: Crustal Growth Models early extraction of all the crust from the mantle, long-term growth or episodic periods of crustal growth? Did most of the crust form by the end of the Late Archean? Source: Bowring, Samuel and Housh, The Earth’s Early Evolution, Science, 269, 1995

50. Geophysical Methods for Lithospheric Studies