1. Absolute Dating

2. Absolute Dating Methods
Uses Radioactive Decay Sequences
1. Radioactive Isotopes change (decay) into daughter isotopes at known rates.
2. Rates vary with the types of isotopes: Uranium, Carbon, etc.
3. Act as an “atomic clock”. We see the clock at the end of its cycle—like watching a stopwatch count down.
4. Allows for the assignment of Numerical Dates to Rocks and Fossils.

3. What is radiation and what does it do?

4. Radiometric dating
Uses continuous decay to measure time since rock formed
Only possible since late 1890’s -- radioactivity discovered in 1896
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, rock contains less parent and more daughter

5. Chemistry Review: What is a Radioactive Isotope?
Atom
model
# protrons = atomic #,
defines the element
electrons
nucleus
protrons
neutrons
# neutrons can vary:
“isotopes”

6. Radioactive isotopes 1. have nuclei that spontaneously decay
2. emit or capture subatomic particles
3. parent: decaying radioactive isotope
4. daughter: decay daughter
parent
daughter
loss or gain
5. The loss or gain of neutron converts parent to daughter of same element
6. The loss or gain of proton changes parent into entirely new daughter

7. particle has 2 neutrons and 2 protons U238 Th234 92 protons 90 protons
3 primary ways of decay
alpha decay (Z ≥ 58)
particle has 2 neutrons and 2 protons
U238 Th234
92 protons 90 protons
beta decay (n0 = p+ + e-)
breakdown of neutron into an
electron and a proton and loss
of the electron to leave a proton
(result is gain of one proton)
K40 Ca40
19 protons 20 protons
electron capture (e- + p+ = n0)
capture of an electron by a proton
and change of proton to neutron
(result is loss of proton)
K40 Ar40
19 protons 18 protons

8. What about the RATE?
(i.e. How Fast does an element decay?)
To use the rate to determine age, we must understand the concept of half-life.

9. Half-life
Amount of time it takes for half the atoms of the parent isotope to decay
Regardless of isotope, the ratio of parent to daughter atoms is predictable at each half-life.

10. Rate of Decay t All atoms are parent isotope or some1 3
All atoms are parent isotope or some
known ratio of parent to daughter
1 half-life period has elapsed, half of the
material has changed to a daughter
isotope (6 parent: 6 daughter) 2
2 half-lives elapsed, half of the parent
remaining is transformed into a daughter
isotope (3 parent: 9 daughter)
3 half-lives elapsed, half of the parent
isotope (1.5 parent: 10.5 daughter)
We would see the rock at this point.

11. Radioactive Decay Rates are Exponential! (not Linear…)
Exponential decay: never goes to zero

12. Radioactive Isotopes analogous to sand in an hour glass
As more sand flows out, you have less “parent” and more “daughter” sand. Only here each set of sand is a different element.
100
Parent
Parent
% parent remaining
Daughter
Daughter 50 25 13
time >

13. Five Common Radioactive Isotope Pairs Five Radioactive Isotope Pairs
Effective
Dating Range
Minerals and
Isotopes
Half-Life
of Parent
(Years)
Rocks That Can
Parent
Daughter
(Years)
Be Dated
Uranium 238
Lead 206
4.5 billion
10 million to
Zircon
4.6 billion
Uraninite
Uranium 235
Lead 207
704 million
Muscovite
Thorium 232
Lead 208
14 billion
48.8 billion
Biotite
Potassium feldspar
Rubidium 87
Strontium 87
4.6 billion
10 million to
Whole metamorphic
4.6 billion
or igneous rock
Potassium 40
Argon 40
1.3 billion
100,000 to
Glauconite
4.6 billion
Muscovite
Biotite
Hornblende
Whole volcanic rock

14. Example: Uranium 238 decay to Lead 206 (stable)

15. Most common Radioactive dating systems
•1. uranium-thorium-lead dating (previous example)
U-238, U-235, Th-232
each of these decays through a series of steps to Pb
U-238 to Pb-206 half-life = 4.5 by
U-235 to Pb-207 half-life = 713 my
Th-232 to Pb-208 half-life = 14.1 my
•2. potassium-argon dating
…argon is a gas--may escape
(ages too young--daughter missing)
K-40 to Ar-40 half-life = 1.3 by
•3. rubidium-strontium dating
Rb-87 to Sr-87 half-life = 47 by

16. 4. What about carbon dating?

17. Radiocarbon Dating
Carbon-14 dating is based on the ratio of C-14 to C-12 in an organic sample.
Living things take in three isotopes of carbon
Carbon 12 and 13 are stable, but carbon 14 is not
Carbon 14 has a half-life of 5730 years
When the organism dies, the “clock” starts. (C-14 begins to decay to N-14)
Valid only for samples that are less than 70,000 years old (not useful for most rocks)
Method can be validated by cross-checking with ice cores and tree rings

18. Carbon 14 Cycle

19. Correlation: Cross-checking dating techniques
How do we “check” our readings and correlate from different sources?
Absolute Dating Techniques are cross-checked using:
1. Ice Cores—up to 70,000 years old
2. Dendrochronology—about 14,000 years (recent samples only)
3. Stratigraphy—rock layers of known formation rates and types

20. Ice Cores?

21. Some Ice core drill sites in Greenland:

22. How are ice cores sampled?
Samples are examined using various light sources for particle types and ice dynamics

23. What do we get from ice cores?
1. Climate information: Paleoclimatology.
Temperature changes
Precipitation changes
Air Particulates counts/types—including carbon, dust, bacteria, algae
2. Atmospheric Composition (Paleoatmospheric studies)—what gases were in the atmosphere in the past and at what levels?
3. Solar Activity (i.e. by the algae, bacteria and particulate types)

24. More recent events? Use Dendrochronology

25. Tree Ring Dating Method

26. Cross-checking: Stratigraphy of Vertical Layers
Stratigraphy—large undisturbed layers of rocks/fossils are laid down in sequential order. (Law of Superposition)
Some Layers of iron-bearing igneous rocks can be correlated by their magnetic properties based on the Earth’s periodic reversal of magnetic poles! (like Seafloor Spreading)

27. What about Horizontal or Diagonal section layers? We use Walther's Law•
The vertical sequence is repeated by the horizontal
sequence -
walking from A to B to C to the Coast you would encounter the
rocks that would be encountered by drilling a core into the
earth at any point (A, B, or C)

28. Basic Geochronological assumptions
1. Decay rates are constant through geological time
* Good reasons to believe that this is correct from nuclear physics
* Measurements of decay sequences in ancient supernovae
yield the same values as modern lab measurements
2. The system is closed to adding or subtracting of parent/daughter isotopes
* Isotopic system and type of mineral are important.
* Careful procedure is essential to correct analysis
The type of rock used for dating is well known.
* Igneous rocks are most reliable for absolute dating.
* Metamorphism may cause loss of daughter products
* Sedimentary rocks will give ages of the source rocks