Absolute DatingAbsolute 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 DatesWhat Are Absolute Dates 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 AtomThe Atom
Fundamental unit of matter Made up of components called subatomic particles
– Protons (positive charge)– Neutron (no electrical charge)– Electron (negative charge
Radioactive Decay – Stable AtomsRadioactive Decay – Stable Atoms 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 - IsotopesRadioactive Decay - Isotopes 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 DecayRadioactive Decay The nucleus of an atom
(decays) changes into a new element.
The proton number (atomic number) changes
Parent – unstable radioactive isotope
Daughter – isotopes resulting from decay of parent
14C 14N6 7
Types of Radioactive DecayTypes of Radioactive Decay 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 Radioactive Isotopes—Determine Decay TypeDecay Type 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 How Long Does Radioactive Decay Take?Take?
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 CycleCarbon-14 Dating – The Carbon Cycle
Carbon dating is common
Only for young samples
DECAY PROCESS FOR CARBON IS DIFFERENT FROM OTHER ISOTOPES!
When Does the Clock Start?—Carbon When Does the Clock Start?—Carbon DatingDating
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 WorksHow the Carbon Clock Works There are two types of carbon used in the dating process
– C-12 (stable does not decay)– C-14 (radioactive decays)
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 WorksHow the C-12:C-14 Ratio WorksAmount 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 DatingPrinciples of Radioactive Dating
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 IsotopesRadioactive Isotopes
Frequently used isotopes in Radiometric Dating
Radioactive IsotopesRadioactive Isotopes U238 Radioactive Decay Series
Magma Crystallization and Clock Magma Crystallization and Clock BeginsBegins
Radiometric DatingRadiometric Dating
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
U-Pb & Th-Pb most common– Used for ancient samples—instrusives, lunar
rocks, meteroites 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 DatingImportance of Radiometric Dating
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 DatingRadiometric Dating
Dating Sedimentary Strata How old are Dakota Sandstone, Mancos Shale
and Mesaver?
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