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

147Sm-143Nd ‘age’ of CC 2.0-2.2 Ga

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

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

Maximum recycling of mature CC 25-50%

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

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

Reason for nature of mantle depletion: modern UCC

Most plausible environment

Modern sediment vs. 3.71 Ga mafic metapelite

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

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

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