The importance of the mantle lithosphere for preservation of

Document Sample
The importance of the mantle lithosphere for preservation of Powered By Docstoc
					    The importance of the mantle
lithosphere for preservation of early
          Archaean ‘arcs’

               Balz Kamber

        Department of Earth Sciences
147Sm-143Nd   ‘age’ of CC



       2.0-2.2 Ga
    CC vs. MORB Pb isotopes
           16.0

           15.8
     Pb



           15.6                                  Sediment
    204
     Pb/




           15.4                              MORB
    207




           15.2

           15.0
               16 16.5 17 17.5 18 18.5 19 19.5 20
                          206         204
                                Pb/         Pb
•  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%
                     0.12

                     0.10
 Flux (e g 100Ma )
-1




                     0.08

                     0.06
25




                     0.04

                     0.02

                     0.00
                            0     1     2         3   4
                                      Age (in Ga)
Nb/Th systematics of basalts

        18
Nb/Th




        14


        10


         6
             0   1000   2000   3000   4000

                        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
                     103
                                  W
                     102                     Pb
 N-MORB normalised




                                                  Be Li
                     101
                                        Ta
                                      Nb
                     100
                                                            Ti

                     10-1

                     10-2
                             Tl Ba Th Nb La Pb Sr Be Hf SmSn Gd Dy Ho Tm Lu Co Ni
                            Cs Rb W U Ta Ce Pr Nd Zr Li Eu Ti Tb Y Er Yb Sc Cr
Most plausible environment


                QuickTime™ an d a
       TIFF (Uncompressed) decompressor
          are need ed to see this picture.
Modern sediment vs. 3.71 Ga
     mafic metapelite
                     103
                                  W
                     102                        Pb
 N-MORB normalised




                     101                             Be Li


                     100
                                           Ta
                                      Nb
                                                             Ti
                     10-1

                     10-2
                             Tl Ba Th Nb La Pb Sr Be Hf SmSn Gd Dy Ho Tm Lu Co Ni
                            Cs Rb W U Ta Ce Pr Nd Zr Li Eu Ti Tb Y Er Yb Sc Cr
       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

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:22
posted:12/11/2011
language:
pages:13