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?