Transcript Absolute Dating - Cal State LA

Absolute Dating
Using Radioactivity
Absolute

Age dating was major advance in
Historical Geology
– More accurate reconstruction of geologic
events

Mostly based on radioactivity
– Defined as spontaneous decay of one
element to another
– Pierre and Marie Curry -1903- decay
produces heat
What Are Absolute Dates
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The age of a rock, fossil,
or geologic event
expressed in units, such
as years
For example, your
birthday . . .
– You were born on a
specific day, month and
year

Absolute age can be
determined by
radioactive decay
The Atom
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
Fundamental unit of matter
Made up of components called subatomic particles
– Protons (positive charge)
– Neutron (no electrical charge)
– Electron (negative charge
Radioactive Decay – Stable Atoms
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An atom is generally
stable if the number of
protons equals the
number of neutrons in the
nucleus
Atomic Number
– An element’s identifying #
– Equals # of protons in
atom’s nucleus

Mass Number
– Equals # of protons and
neutrons in an atom’s
nucleus
Radioactive Decay - Isotopes
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Variant of the same
parent atom
Differ in the # of neutrons
Result in different mass #
than parent
For example:
– Carbon-14 (C-14)
– Types of carbon
• C-12 (stable)
• C-13 (unstable)
• C-14 (unstable)
Radioactive Decay
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The nucleus of an atom
(decays) changes into a new
element.
The proton number (atomic
number) changes
14
C  14N
6
7
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Parent – unstable radioactive
isotope
Daughter – isotopes resulting
from decay of parent
Types of Radioactive Decay
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Alpha emission
– Emission of 2 p+ and 2
n (α particle)
– Mass # ↓ by 4
– Atomic # ↓ by 2

Beta emission
– N emission of e- (β
particle)
– Mass # remains
unchanged
– Atomic # ↑ by 1

Electron Capture
– An e- captured by p
– e- + p = n
– Mass # remains
unchanged
– Atomic # ↓ by 1
Radioactive Isotopes—Determine Decay Type
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Alpha emission
– Emission of 2 p+
and 2 n (α particle)
– Mass # ↓ by 4
– Atomic # ↓ by 2

Beta emission
– N emission of e- (β
particle)
– Mass # remains
unchanged
– Atomic # ↑ by 1

Electron Capture
– An e- captured by p
– e- + p = n
– Mass # remains
unchanged
– Atomic # ↓ by 1
 U238 Radioactive Decay Series
How Long Does Radioactive Decay Take?
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Half Life - time required for one-half of radioactive nuclei
in a sample to decay
–
The half life of C-14 is 5,730 years
1. If start with 10,000 atoms of Carbon-14, how many will you
have after 5 half-lives?
2. How old is sample?
Carbon-14 Dating – The Carbon Cycle
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Carbon dating is
common
Only for young
samples
DECAY PROCESS
FOR CARBON IS
DIFFERENT FROM
OTHER ISOTOPES!
When Does the Clock Start?—Carbon Dating
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When a plant or
animal dies, the clock
starts.
Organism
dies No more
C-14 intake
C-14 begins
to decay
How the Carbon Clock Works
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There are two types of carbon used in the dating process
– C-12 (stable  does not decay)
– C-14 (radioactive  decays)
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When an organism is alive, it has the same ratio (C-12 to
C-14) that is found in the atmosphere (1 trillion to 1)
A living starfish has the same
ratio as the atmosphere
A fossilized starfish has a
different ratio
How the C-12:C-14 Ratio Works
Amount of
stable C-12
Amount of
unstable C-14
Ratio
Years dead
# of half-lives
100 trillion
100
1-T to 1
0
0
100 trillion
50
2-T to 1
5,730
1
100 trillion
25
4-T to 1
11,460
2
100 trillion
12.5
8-T to 1
17,190
3
100 trillion
6
16-T to 1
22,920
4
100 trillion
3
32-T to 1
28,650
5
 Carbon-14 can date elements up to approximately
100,000 years
– Used to date very recent events
– Important tool for anthropologists, archeologists and
geologists
Principles of Radioactive Dating
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Percentage of radioactive atoms that
decay during one half-life is always the
same (50 %)
However, the actual # of atoms that
decays continually decreases
Comparing the ratio of parent to
daughter yields the age of the sample
Radioactive Isotopes
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Frequently used isotopes in Radiometric
Dating
Radioactive Isotopes
 U238 Radioactive Decay Series
Magma Crystallization and Clock Begins
Radiometric Dating
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Sources of Error
– A closed system is
required
– Only fresh, non-weathered,
unaltered or non-deformed
rock samples should be
used
– For example,
Metamorphism
• Reheats samples
• Sample measures younger
than should be
 Can cross check age
using other isotopes
Isotope Used For Dating
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U-Pb & Th-Pb most common
– Used for ancient samples—instrusives, lunar
rocks, meteroites
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Rb-Sr used for oldest rocks
K-Ar used for fine grained volcanic rocks
– Ar is gas so sample must be fresh
– Can also be used for metamorphic rocks
Importance of Radiometric Dating
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A complex procedure that requires
precise measurement
Rocks from several localities have been
dated at more than 3 billion years
Confirms the idea that geologic time is
immense
Radiometric Dating
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Dating Sedimentary Strata
How old are Dakota Sandstone, Mancos Shale
and Mesaver?