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Precambrian Geology

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					      Precambrian Geology
Lecture MSc course Earth Sciences

  Lithosphere tectonics, Petrology and Isotope
                                 Geochemistry
Early Earth
Two lines of evidence

   From accretion of the solar system
    forward

   Using the terrestrial rock record and work
    back in time
Solar system accretion

                Primitive matter in the
                solar system is mainly
                H and He, organized
                in galaxies, in very
                large systems of stars
Solar system accretion

                Within galaxies dispersed
                matter, mainly H, compacts
                due to gravitational forces.
                From this compaction process
                ultimately stars are formed
Solar system accretion

                Hydrogen stars are the
                main element factories of the
                Universe.
                Nuclear fusion of hydrogen and
                step by step heavier elements
                form elements up to Fe
Solar system accretion

                Hydrogen stars are the
                main element factories of the
                Universe.
                Nuclear fusion of hydrogen and
                step by step heavier elements
                form elements up to Fe
Solar system accretion

                In the case of very large stars
                the process ends cata-
                strophically as in a nova event.
                Much matter is dispersed from
                the star into the universe




               Cat’s eye nebula formed ca 1000 years ago
Solar system accretion

                Intense energy released during
                The nova event, mainly in the
                Form of fast moving neutrons
                Cause the formation of
                Elements heavier then Fe
                (neutron capture and decay
                Reactions, r, s, p processes)
Solar system accretion

                As a result of these
                Processes we are left with
                A galaxy filled with matter
Solar system accretion




Gravitational forces cause renewed contraction of the
cloud of matter and a second cycle of star formation
is initiated
Solar system accretion




Accretionary disk
Solar system accretion


                    Planet system




Accretionary disk
Accretion: the rock record




  Chondrites:
  Allende
Accretion: the isotope record




  Chondrite meteorites
  Are likely to represent
  The least modified sample
  Of the solar accretion nebula
Accretion: the rock record




  Achondrites
Accretion: the rock record




  Iron meteorites:
  Canyon Diablo
Accretion: the rock record




             Shergottite
             EETA79001, Elephant Moraine, Antarctica, Dec, 1979
Accretion: the isotope record

The age of the
Solar system, Earth,
Moon, Mars:
Isotopes with long half lifes


                                Rates of processes during
                                Planet formation:
                                Isotopes with short half lifes
Accretion: the isotope record

                The age of the
                Solar system, Earth,
                Moon, Mars


                Isotopes with large half-life's:
                235
                    U/207Pb, 238 U/206Pb
                87
                   Rb/ 87Sr, 40K/ 40Ar
                Lu-Hf, Re-Os
Solar system                 Meteorite Name
                             (billions
                             of years)
                             Allende
                                              Material Dated Method Age


                                              whole rock    Ar-Ar   4.52 +/- 0.02
                                              whole rock    Ar-Ar   4.53 +/- 0.02

accretion                                     whole rock
                                              whole rock
                                              whole rock
                                                            Ar-Ar
                                                            Ar-Ar
                                                            Ar-Ar
                                                                    4.48 +/- 0.02
                                                                    4.55 +/- 0.03
                                                                    4.55 +/- 0.03
                                              whole rock    Ar-Ar   4.57 +/- 0.03
                                              whole rock    Ar-Ar   4.50 +/- 0.02
                                              whole rock    Ar-Ar   4.56 +/- 0.05

The oldest meteorite ages:
                             Guarena          whole rock    Ar-Ar   4.44 +/- 0.06
                                              13 isochron   Rb-Sr   4.46 +/- 0.08
                             Shaw             whole rock    Ar-Ar   4.43 +/- 0.06
                                              whole rock    Ar-Ar   4.40 +/- 0.06
                                              whole rock    Ar-Ar   4.29 +/- 0.06
                             Olivenza         18 isochron   Rb-Sr   4.53 +/- 0.16
                                              whole rock    Ar-Ar   4.49 +/- 0.06
                             St. Severin      4 isochron    Sm-Nd   4.55 +/- 0.33
                                              10 isochron   Rb-Sr   4.51 +/- 0.15
                                              whole rock    Ar-Ar   4.43 +/- 0.04
                                              whole rock    Ar-Ar   4.38 +/- 0.04
                                              whole rock    Ar-Ar   4.42 +/- 0.04
                             Indarch          9 isochron    Rb-Sr   4.46 +/- 0.08
                                              12 isochron   Rb-Sr   4.39 +/- 0.04
                             Juvinas          5 isochron    Sm-Nd   4.56 +/- 0.08
                                              5 isochron    Rb-Sr   4.50 +/- 0.07
                             Moama            3 isochron    Sm-Nd   4.46 +/- 0.03

The meteorite ages:          Y-75011
                                              4 isochron
                                              9 isochron
                                                            Sm-Nd
                                                            Rb-Sr
                                                                    4.52 +/- 0.05
                                                                    4.50 +/- 0.05
                                              7 isochron    Sm-Nd   4.52 +/- 0.16
                                              5 isochron    Rb-Sr   4.46 +/- 0.06
                                              4 isochron    Sm-Nd   4.52 +/- 0.33
                             Angra dos Reis   7 isochron    Sm-Nd   4.55 +/- 0.04
                                              3 isochron    Sm-Nd   4.56 +/- 0.04
                             Mundrabrilla     silicates     Ar-Ar   4.57 +/- 0.06
                                              olivine       Ar-Ar   4.54 +/- 0.04
                                              plagioclase   Ar-Ar   4.50 +/- 0.04
                             Weekeroo Station 4 isochron    Rb-Sr   4.39 +/- 0.07
                                              silicates     Ar-Ar   4.54 +/- 0.03
Solar system accretion




                  What are the younger
                  meteorite ages telling us?
Solar system accretion
                               The youngest lunar and
                               meteorite ages:
                               Ca. 3.8 Gyr the dense accretionary
                               nebula has largely disappeared.


            Description                Sample # Technique   Age (in Ga)
            Apollo 11 - High-K basalt 10072 40Ar/39Ar       whole rock    3.49 +- 0.05
            Apollo 11 - High-K basalt 10072 40Ar/39Ar       whole rock    3.52 +- 0.04
            Apollo 11 - High-K basalt 10072 40Ar/39Ar       plagioclase   3.57 +- 0.05
            Apollo 11 - High-K basalt 10072 40Ar/39Ar       plagioclase   3.56 +- 0.06
            Apollo 11 - High-K basalt 10072 40Ar/39Ar       ilmenite      3.58 +- 0.05
            Apollo 11 - High-K basalt 10072 40Ar/39Ar       pyroxene      3.55 +- 0.05
            Apollo 11 - High-K basalt 10072 Rb-Sr           isochron      3.57 +- 0.05
            Apollo 11 - High-K basalt 10072 Sm-Nd           isochron      3.57 +- 0.03
  Solar system accretion
Planetary crusts, evidence from Earth and Moon
Moon ages: Rb-Sr, Sm-Nd: 4.40 –4.55 Ma
Mission               Technique Age (in billions of years)
Apollo 17 Rb-Sr       isochron   4.55 +- 0.1
Apollo 17 Rb-Sr       isochron   4.60 +- 0.1
Apollo 17 Rb-Sr       isochron   4.49
Apollo 17 Rb-Sr       isochron   4.43 +- 0.05
Apollo 17 Sm-Nd       isochron   4.23 +- 0.05
Apollo 17 Sm-Nd       isochron   4.34 +- 0.05
Apollo 16 40Ar/39Ar              4.47
Apollo 16 40Ar/39Ar               4.42
Solar system accretion




When did this happen:
Pb-Pb age Allende CAI        4.568 ± 0.003 Ga
            Angra dos Reis   4.5578 ± 0.0004 Ga
Solar system accretion




     Planetary crusts, evidence from Earth, Mars and
     Moon
           Earth: U/Pb zircon        4.404 ± 0.008 Ga
           Mars: Rb/Sr, Sm/Nd        4.50 ± 0.13 Ga
Accretion: the isotope record
                      Rates of processes:
                      •Accretion,
                      •Chemical separation of
                       reservoirs: core, mantle,
                      crust


Isotopes with short
half-life's:
Accretion: the isotope record
                             Allende Ca-Al inclusions




Isotopes with short
half-life's:

53
 Cr=53Cr0 + 53Mn(1 – e-lt)
Accretion: the isotope record
                               Carbonaceous chondrites




Isotopes with short
half-life's:

129
  Xe=129Xe0 + 129I(1 – e-lt)
Accretion: the isotope record
                                Carbonaceous chondrites




Isotopes with short
half-life's:

107
  Ag=107Ag0 + 107Pd(1 – e-lt)
Accretion: the isotope record
                             Carbonaceous chondrites




Isotopes with short
half-life's:

26
 Mg=26Mg0 + 26Al(1 – e-lt)
Solar system accretion




The short lived radioactive isotope record of the
Meteorites are telling us that the step from nova
To accretion disk was very fast: ca. 10 Ma (26Al-26Mg)!
The iron meteorites are telling us that segregation processes
occurred in the first 10’s of Myrs.
The terrestrial rock record



Acasca gneiss
Slave Craton
4.031 Ga

                          Manfred Complex
                          Yilgarn Craton
       Isua North         3.75 Ga
       Atllantic Craton
       3.9 Ga
The terrestrial rock record




           Mt Narryer
           Jack Hills
           Yilgarn Craton
           4.2 – 4.4 Ga
The Terrestrial rock record
The terrestrial rock record
Evidence from detrital zircons for
the existence of continental crust
and oceans on the Earth 4.4 Gyr ago
Simon A. Wilde, John W. Valley,
William H. Peck, Colin M. Graham
NATURE, 409 175-178. 2001
The Terrestrial rock record




           Jack Hills
           NW Yilgarn
           W Australia
The Terrestrial rock record




            Jack Hills
            NW Yilgarn
            W Australia
The Terrestrial rock record

				
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posted:6/24/2012
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