Transcript Steve`s Absolute Time Notes - Science at East Lee Campus

Do you believe in radiation?
Do you believe in radiation?
Some early history
1895
Roentgen discovers x rays
1896
Becquerel discovers uranium salts emit
invisible rays
1898
Curies note thorium emit radiation, call it
radioactivity
1902
Rutherford and Soddy use experiments to
formulate a model for decay
1906 Rutherford
• Used uranium (parent) and helium (related
to daughter) of a mineral to measure an
age of
• 497 million years!!!!
• 1906 Strutt, same method
• Age = 2.4 billion years
Basic Atomic
Structure
The atomic number is the number of protons in an atom of
an element. Carbon's atomic number is 6. This tells us that
an atom of carbon has 6 protons in its nucleus.
Isotopes
Atoms that have the same number of
protons but different numbers of neutrons
are called isotopes.
Radioactive decay
• Radioactive decay involves the spontaneous
transformation of one element into another. The only
way that this can happen is by changing the number of
protons in the nucleus (an element is defined by its
number of protons). There are a number of ways that
this can happen and when it does, the atom is forever
changed. There is no going back -- the process is
irreversible.
Types of
decay:
alpha
particle
electron
(Beta)
loss
electron
capture
U-238
to Pb206
Clair Patterson 1950s Noble Prize
Half-life
• The time required for one-half of the
radioactive (parent) isotopes in a
sample to decay to radiogenic
(daughter) isotopes.
Useful Parent/Daughter
Parent
Daughter
Half-life
Material
Uranium238
Uranium235
Rubidium87
Potassium40
C-14
Lead-206
4.5 billion
Zircon
Lead-207
713 million
Zircon
Strontium87
Argon-40
47 billion
Feldspars, mica,
whole rock
1.3 billion
Biotite, mica,
volcanic rock
N-14
5700
Wood, plants, shells,
limestone, organic
mat’l, bone, cloth,
cave deposits
To date geologic materials need
• 1. half life/decay rate
• 2. amount parent and daughter
Early time scale
U-Pb ages
Age
Carboniferous
Devonian
Pre-Carbonif.
Pre-Carbonif.
Silurian
Precambrian
Precambrian
Precambrian
Precambrian
Boltwood, 1907
410
510
460
535
1,300
1,700
1,800
2,200
Holmes, 1911
370
370
410
430
1,025
1,270
1,310
1,640
Love the Isochron
An isochron to love:
Age of the Earth
Multiple Methods = Same Age
• Radiometric Ages on the Amitsoq Gneisses, Western
Greenland. Data from Baadsgaard (10), Moorbath et al. (89),
Pettingill and Patchett (106)
• Method
Age (billion years)
• Rb - Sr isochron 3.70 ± 0.14
• Lu - Hf isochron 3.55 ± 0.22
• Pb - Pb isochron 3.80 ± 0.12
• U - Pb discordia 3.65 ± 0.05
• Th - Pb discordia 3.65 ± 0.08
weighted mean age 3.67 ± 0.06
Carbon -14
Carbon in:
plants,
animals,
and minerals.
Libby 1960
Carbon dating
14N + n => 14C + p
Looking closer:
Which of these two rocks is
Relative geologic time
older?
Which of these two rocks is older?
•
•
•
4. The global amount of radiocarbon (formed by the cosmic-ray
bombardment of atoms in the upper atmosphere and then dissolved in rain
water and seawater) is constant. Decaying carbon-14 is continuously
replaced in organisms while they are alive. When an organism dies, the
amount of its carbon-14 decreases as it decays to nitrogen-14.
The carbon in a buried peat bed has about 6% of the carbon-14 of modern
shells. What is the age of the peat bed? Explain.
In sampling the peat bed you must be careful to avoid any young plant roots
or old limestone. Why?
More lab homework.
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•
5. Layers of sand on a Michigan beach contain zircon crystals.
Could the zircon crystals be used to date exactly when the layers of sand
were deposited? Explain.
•
Suggest a rule that geologists should follow when they date rocks according
to radiometric ages of crystals inside the rocks.
yielded U-Pb ages up to 4.404 billion years
Sources
• education.jlab.org/qa/atom_model.html
• earthguide.ucsd.edu/.../ThreeCarbonIsoto
pes.jpg
• www.ocrwm.doe.gov/.../unit2/lesson2re
ading.shtml
• http://www.talkorigins.org/faqs/dalrymple/r
adiometric_dating.html