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The importance of the mantle lithosphere for preservation of early Archaean ‘arcs’

The importance of the mantle lithosphere for preservation of early Archaean ‘arcs’. Balz Kamber Department of Earth Sciences. 147 Sm- 143 Nd ‘age’ of CC. 2.0-2.2 Ga. CC vs. MORB Pb isotopes.  207 Pb/ 206 Pb imposes a limit on CC recycling. U and Pb mass balance?.

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The importance of the mantle lithosphere for preservation of early Archaean ‘arcs’

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  1. The importance of the mantle lithosphere for preservation of early Archaean ‘arcs’ Balz Kamber Department of Earth Sciences

  2. 147Sm-143Nd ‘age’ of CC 2.0-2.2 Ga

  3. CC vs. MORB Pb isotopes  207Pb/206Pb imposes a limit on CC recycling

  4. U and Pb mass balance? For mass of CC = 2 .1 x 10e25g

  5. Maximum recycling of mature CC 25-50%

  6. Nb/Th systematics of basalts 18 14 Nb/Th 10 6 Age (Ma)

  7. Conclusion 1 Continental crust with high survival potential is and was difficult to make Possibility of high infant mortality cannot be excluded Armstrong model of constant continental mass violates combined Nd&Pb isotope systematics

  8. Reason for nature of mantle depletion: modern UCC

  9. Most plausible environment

  10. Modern sediment vs. 3.71 Ga mafic metapelite

  11. Conclusion 2 Subduction operated at 3.7 Ga but was ineffective as evidenced by slow increase in preserved continental mass Continental geological record heavily biases towards subduction

  12. Conclusion 3 Throughout Archaean both continental and oceanic lithosphere represented minor proportion of plates The major plates (oceanic plateaus?) did not form a deep lithospheric mantle keel and were ultimately recycled

  13. Conclusion 4 Alternative mechanisms for creation of continental crust did exist. Geochemistry requires that crustal dense reside was recycled into the mantle

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