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Precambrian History

Precambrian History. The Archean Chapter 8. Geologic Column. Precambrian Archean and Proterozoic eons Eoarchean era, …. Eoarchean (4.6-3.6 Ga). Earth was hot Bombarded by meteorites Lots of volcanic activity Hard to make and keep crust 1st rocks were ultramafic. The Eoarchean Earth?.

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Precambrian History

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  1. Precambrian History The Archean Chapter 8

  2. Geologic Column Precambrian Archean and Proterozoic eons Eoarchean era, ….

  3. Eoarchean (4.6-3.6 Ga) • Earth was hot • Bombarded by meteorites • Lots of volcanic activity • Hard to make and keep crust • 1st rocks were ultramafic

  4. The Eoarchean Earth?

  5. Eoarchean (4.6-3.6 Ga) • Melt ultramafic rock, make basalt • Melt basalt a few times to make granite/rhyolite

  6. Jack Hills In search of the world’s oldest mineral

  7. Metamorphic sandstone and Conglomerate: 3 Ga

  8. World’s oldest mineral • This zircon is 4.404 billion years old ±4 million years • From a granite

  9. SHRIMP II Cameca 4f Ion MicroprobeUniversity of Edinburgh 4 3 5 0 ± 3 M a 4 3 6 3 ± 4 M a 4 2 6 7 ± 5 M a 1 8 O = 5 . 0 ‰ d 4 3 3 9 ± 3 M a 0 . 6 p p m L a 5 . 5 p p m L a 4 3 2 6 ± 4 M a 4 3 5 3 ± 8 M a S u r f a c e 1 0 . 3 p p m 4 3 2 1 ± 3 M a 4 3 5 5 ± 2 M a 4 2 8 3 ± 4 M a 4 3 6 4 ± 3 M a 4 2 8 4 ± 3 M a 9 . 2 p p m L a 1 3 . 6 p p m L a 1 8 O = 7 . 4 ‰ d 4 2 8 8 ± 4 M a 4 3 1 9 ± 3 M a 4 4 0 4 ± 4 M a Peck et al., 2001 S u r f a c e 2

  10. A Cool Early Earth ?

  11. Eoarchean • World’s oldest rocks, Acasta Gneiss • Slave Craton of Canada • ~4.0 Ga

  12. Archean • Making and keeping crustal fragments

  13. Arcs collide due to subduction to form larger land mass

  14. Land masses continue to grow

  15. Precambrian shields and cratons • Shield: area of exposed Precambrian rocks

  16. Precambrian shields and cratons • Shield: area of exposed Precambrian rocks • Craton: buried and exposed Precambrian rocks

  17. Superior Craton of Canada • Small fragments before 3.0 billion • Explosion of crustal growth 3.0-2.5 billion

  18. Model for assembly of cratons • Continentalaccretion: plates collide with volcanic arcs and other plates. • Continents grow along margins

  19. Archean rocks • Granite-greenstone belts • Granite often gneiss

  20. Archean greenstone

  21. Archean greenstone pillows

  22. Archean greenstone pillows

  23. Archean greenstone pillows

  24. Archean rock record • Evidence of volcanoes • greenstones • Evidence of water • Pillow basalts

  25. Archean conglomerate: more evidence for water

  26. Archean sediment layers: more evidence for water

  27. Canadian Shield Gneiss

  28. Volcanic arcs in oceans

  29. Precambrian accretion • Lots of crust 3.0-2.5 Ga

  30. Implications of accretion model • Plate tectonics active in Archean • Differences • Earth hotter • Hotter Earth: • Easy to make ultramafic rocks • Plates move faster • More volcanic activity

  31. Radiogenic heat production through time • By end of Archean • Earth cooler • Easier to keep crust • Easier to make and keep granite • No more ultramafic rocks

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