Lecture _1 Principles of Isotopic Dating

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Lecture _1 Principles of Isotopic Dating Powered By Docstoc
					An Introduction to Isotopic Dating and
  its Importance to Geologic History
                        Class Aims
   After this class students should be able to….
 1. Explain what will be taught over the next 3 weeks in the
      lectures and the labs
 2. Get your geochronology geek on and get excited about dating
     rocks!!

3. Understand the process of radioactive decay and the various
methods that can be used to date rocks/organic material


4. Appreciate the wide variety of applications that isotopic dating
has in geology and what it can reveal about geological history
    Course Content for the next 3 weeks!
- Introduction to isotopic dating; principles and applications
- U-Pb dating and the wonders of Zircon!!
- U-Series, K-Ar, Ar-Ar, Rb-Sr and Sr-Sr dating and applications
- Dating the dead – Radiocarbon dating
- Fission Track dating; determining the thermal history of
orogenic terranes + other applications
- All the other isotopes! (U-Th/He, Sm-Nd, Re-Os etc.)
- Integrating geochronological techniques
- Geochronology review session
- Final lecture review of course, exam hints – PARTY TIME!!!!
       Clicker Practise Question
What is the name of the Springfield baseball
team?

a) Springfield Atoms
b) Springfield Isotopes
c) Springfield Electrons
d) Springfield Neutrons
             Geological Time Scale




HOW DO WE KNOW THE AGE OF THE
       EARTH/ROCKS???
    Geological Timescale
Represents a combination
of absolute dating and
identification of key fossil
types (relative)
The divisions in the scale itself
were based on the
appearance/disappearance of
fossil-types, more precise
absolute ages added after the
advent of radiometric dating
Unlocking the past; Relative vs. Absolute Time
Relative time
      The order of events or objects from first (oldest) to last
      (youngest)
      i.e. she is older than he is; she was born first and he was
      born last

Absolute time
      The age of events or objects expressed numerically
      i.e. she is 21 and he is 19
  The study of the timing of geologic events and processes is called
                         geochronology
  Examples of NZ rocks that have been dated
Eruption of Taupo
   U/Pb Zircon

    Banks Peninsula
       Volcanics
   K/Ar of Plagioclase
       Age of
  metamorphism of
    the Torlesse
 Rb/Sr of whole-rock

Granites & Gneisses
   in Fiordland
  U/Pb of Zircon
Other cool things that have been dated in NZ
What is an atom?
       • An atom is the smallest
         particle of an element that
         still retains the characteristics
         of the element.
       • An atom has a nucleus
         surrounded by negatively
         charged electrons.
       • The nucleus consists of
         protons (positively charged)
         and neutrons.
       • Protons and neutrons have
         about the same mass and are
         much larger than electron’s.
      Atomic number and isotope
• The number of protons (the atomic number) determines which
  element it belongs to.
• Atoms with the same atomic number but different number of
  neutrons are called isotopes.
              What is an Isotope?
“One of two or more atoms with the same atomic number but with
different numbers of neutrons”




         1H                 2H                 3H
Why are some isotopes unstable (& therefore decay)?

The arrangement of neutrons and protons in the nucleus will predispose
 some isotopes to undergo radioactive decay, while others will remain
                               stable




STABLE                STABLE                UNSTABLE
           Radioactivity
               • What happens during
                 radioactive decay?
                  – An atomic nucleus
                    undergoing radioactive
                    decay is said to be the
                    parent isotope.
                  – This process will
Unstable            continue until a stable
isotopes            daughter isotope is
                    formed.

                 The rate at which this process occurs is
                  measured in terms of the “half life”.
           The Discovery of Radioactivity (1896)
   Antoine Henri Becquerel




                                     Marie and Pierre Curie
In 1896, the discovery of radioactivity provided the needed method
             to measure the age of the Earth accurately.
            Uraninite - Uranium Ore




They discovered that a photographic film placed next to a piece of
uraninite could develop as a result of radiation
       - discovered the radioactive chemical elements Polonium
       and Radium
How does radioactive decay work?
            The transformation process of turning an
            unstable nucleus into a stable nucleus.

            U238 Parent - unstable nucleus undergoing
            radioactive decay.

            P206 Daughter - stable nucleus arising
            from radioactive decay

             Other Examples:

             PARENT                DAUGHTER
             14C                   14N
             238U                  206Pb
    Radioactive Decay: Alpha (α)




 Loss of alpha particle from the nucleus
 Alpha particle includes 2 protons, 2 neutrons
 Atomic number decreases by 2, mass decreases by 4
       Radioactive Decay: Beta (β)




 Loss of an electron from a neutron, not from outer most
energy level
 Atomic number increases by 1
Radioactive Decay: Electron Capture




Proton captures an electron from outside source
Atomic number decreases by 1
Example: Potassium-Argon Decay
 Alpha Decay:
Atomic number
decreases by 2,
mass decreases by
4


 Beta Decay:
Atomic number
increases by 1
Dating & Radioactive Decay
Exponential decay: never goes to zero




   exponential                linear
                         Popcorn Analogy
- Each corn kernel in a pan on the stove has the
potential to pop, but they only do it one at a time

- You never know which kernel is going to pop, but
if you wait long enough you know that most of them
will have popped

- The number of kernels popping at a given time is
relative to the number of kernels left in the pot

- As the number of un-popped
kernels decreases, the number
of popped kernels increases
               Rates of radioactive decay
• Each radioactive isotope has its own rate of decay ranging from
milliseconds to billions of years

• Rates are NOT affected by the chemical or physical environment

• Therefore, isotopes in an igneous magma in the interior of the
Earth will decay at the same rate as those in a sandstone at the
surface of the Earth
              SHOW ANIMATION




http://lectureonline.cl.msu.edu/~mmp/applis
t/decay/decay.htm
      Examples of common radioactive decay
       reactions used to date rocks/organics

 parent             daughter               half life (years)
  235U               207Pb                  0.71 x 109
    238U             206Pb                    4.50 x 109
    40K              40Ar                     1.25 x 109
    87Rb             87Sr                     47.0 x 109
    14C              14N                      5,730
After ~10 half lives have gone there isn’t enough parent isotope left
to measure
       Clicker Question #1
Based on the half-life of 14C, which of the
following represents the upper limit of
radiocarbon dating?

  a) 100 000 years
  b) 1.1 million years
  c) 15 000 years
  d) 60 000 years
   In 1905, Ernest Rutherford
suggested that radioactivity could
be used to study the age of rocks
          and minerals



       HOW????
He used the Alpha Decay of U ore
to calculate mineral ages of >500
           million years
   Arthur Holmes Bertram Boltwood
                                        (with help from Rutherford)




                                        1904-1907: Dated first rocks:
  1921: Earth about 4                   250 million to 1.3 billion years
  billion years old!
Holmes published the first geologic time scale based on radiometric age
determinations of rocks and minerals (& not just fossils)
      Radiometric Dating – Absolute Dating
     uses continuous decay to measure time since rock formed

- as minerals crystallize in magma;
  they trap atoms of radioactive isotopes in their crystal structures

   radioactive isotopes will decay immediately and continuously




AS TIME PASSES, THE ROCK/MINERAL CONTAINS LESS
           PARENT & MORE DAUGHTER
       GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION
• We use igneous rocks and minerals. The clock starts when radioactive
  atoms that are present in the magma are incorporated into the crystal
  lattice of minerals as they grow

• These crystals represent a “container” with parent isotopes that can begin
  decaying to form daughters.

• We can then measure the parent-daughter ratio. This is our “atomic clock”
  that records the time since the rock crystallized.
      Closure temperatures: temperature above which a
      mineral/rock no longer behaves as a closed system
                            Fig. 5.6
If a rock/mineral is
subjected to
temperatures above
their closure
temperature (blocking)
it can result in altering
the parent/daughter
ratio by;
     - Radioactive
       disintegration
     - resetting the
       isotopic clock
           Closure temperatures
Approximate closure temperatures of commonly dated
                      minerals
Hornblende           K-Ar                 500˚C
Muscovite            Rb-Sr                500˚C
Muscovite            K-Ar                 400˚C
Biotite              K-Ar                 350˚C
Biotite              Rb-Sr                350˚C



Minerals only retain radiogenic daughter products below their
                     closure temperatures
Cooling of Igneous Rock – U/Pb, Rb/Sr, K/Ar   When does a system
                                              become “Closed”?
                                              (i.e., What are you
                                                     dating?)
                                                Metamorphism – U/Pb. Rb/Sr




  Death of Organic Material – C/N
                            Fig. 5.10



    Metamorphic redistribution of daughter isotopes
1. Mineral crystallizes 1000 mya (1 billion yrs ago)
2. After 500 my (million yrs) some parent isotopes have decayed.
3. 480 mya (million yrs ago) metamorphic event redistributes daughter
   atoms out of crystal into adjacent rock
4. Dating of the mineral would now yield the age of the metamorphic
   event
5. But a whole rock age would provide the original age of the
   rock/mineral (1000 mya).
What is it that we are dating?
         Clicker Question #2
Based on the stratigraphic sequence shown, what
radiometric technique do you think is the most
appropriate to determine the age of the x-cutting felsic
dike (it contains zircon, muscovite, plag, k-spar)

   a) U-Pb             Granite Pluton
                       dated at 3.45 Ma ±
   b) K-Ar             0.2
   c) Rb-Sr
   d) C-N
              Sedimentary Rocks:
              What are we dating?




      Dating the age of
   crystallization of older     = PROVENANCE
igneous/metamorphic rocks
prior to erosion, transport &
                                   ANALYSIS
  deposition in a sediment
        Information Required for
           Radiometric Dating
•   Initial Parent Isotope Content
•   Half Life of Isotope (what system are we using?)
•   Current Parent Isotope Concentration
•   Closed System
Basic equations for radioactive decay
Basic equations for radioactive decay
The Fundamental Rule of
     Absolute Ages
The Earth is older than everything
             on or in it
         -Except its atoms
   -All ages are minimum ages

				
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posted:12/12/2011
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