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

2. 147Sm-143Nd ‘age’ of CC Ga

3.  207Pb/206Pb imposes a limit on CC recycling
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 50%/65% % CC
% DM
% PM U 50
15/23
35/27 Pb 45
22/33
33/22

5. Maximum recycling of mature CC 25-50%

6. Nb/Th systematics of basalts18 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