Transcript Radiometric Dating

Radiometric Dating
Chapter 18
Geology
Absolute Dating
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•
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Gives a numerical age
Works best with igneous rocks
difficult with sedimentary rocks
Uses isotopes
– Radioactive
– Different number of neutrons (than stable
atom)
• Carbon 12 (12C)
– Atomic mass is on
periodic table
• Carbon 14 (14C)
– 2 “extra” neutrons
– radioactive
Half-Life
The time required for the
Parent material
to decrease by one-half
1200
1000
800
Activity
600
400
200
0
New
1 HalfLife
2 HalfLives
3 HalfLives
4 HalfLives
How long is a half-life?
Parent
Daughter
Half life
Potassium 40
Argon 40
1.3 by
Rubidium 87
Strontium 87
48.8 by
Uranium 235
Lead 207
.7 by
Carbon 14
Nitrogen 14
6,000 years
Sodium 22
Aluminum 27
15 hours
14 Carbon
• Only accurate for ages less than 100,000
years
• Parent gets too small to accurately
measure
Preparing a rock for mass
spectrometer
Mass spectrometer
measures D/P ratio
Half life problem
• If you begin with 80 grams of 14C after a
time, 20 grams are left. How old is the
sample?
Another half-life problem
• 1,000 grams of radioactive element is in a
rock when it is formed. The element’s
half-life is 2 million years. After a time,
125 grams of the original element remain.
How old is the rock?
Using daughter/parent ratio
• 1) D/P ratio
• 2) figure out number of half-lives
– Use graph
• 3) multiply number of half-lives by the time
of one half-live
• Example: 240 g 14N ; 8 g 14C
14C
14
N
one half life= 6000 years