Absolute Dating

Absolute Dating MethodsUses Radioactive Decay Sequences

3. Act as an “atomic clock”. We see the clock at the end of its cycle—like watching a stopwatch count down.

4. Allows for the assignment of Numerical Dates to Rocks and Fossils.

1. Radioactive Isotopes change (decay) into daughter isotopes at known rates.2. Rates vary with the types of isotopes: Uranium, Carbon, etc.

What is radiation and what does it do?

Radiometric dating

As minerals crystallize in magma; they trap atoms of radioactive isotopes in their crystal structuresRadioactive isotopes will decay immediately and continuously

As time passes, rock contains less parent and more daughter

Uses continuous decay to measure time since rock formed

Only possible since late 1890’s -- radioactivity discovered in 1896

Atommodel

nucleus

electrons

protronsneutrons

# protrons = atomic #,defines the element

# neutrons can vary:“isotopes”

Chemistry Review: What is a Radioactive Isotope?

1. have nuclei that spontaneously decay

daughterparent

loss or gain

5. The loss or gain of neutron converts parent to daughter of same element

6. The loss or gain of proton changes parent into entirely new daughter

Radioactive isotopes

2. emit or capture subatomic particles

3. parent: decaying radioactive isotope

4. daughter: decay daughter

3 primary ways of decay

alpha decay (Z ≥ 58)

beta decay (n0 = p+ + e-)

electron capture (e- + p+ = n0)

capture of an electron by a protonand change of proton to neutron(result is loss of proton)

K40 Ar40

19 protons 18 protons

particle has 2 neutrons and 2 protonsU238 Th234

92 protons 90 protons

breakdown of neutron into anelectron and a proton and lossof the electron to leave a proton(result is gain of one proton)

K40 Ca40

19 protons 20 protons

What about the RATE? (i.e. How Fast does an element decay?)

To use the rate to determine age, we must understand the concept of half-life.

Amount of time it takes for half the atoms of the parent isotope to decayRegardless of isotope, the ratio of parent to daughter atoms is predictable at each half-life.

Half-life

Rate of DecayRate of Decaytt 00

tt 11

tt 33

All atoms are parent isotope or someAll atoms are parent isotope or someknown ratio of parent to daughterknown ratio of parent to daughter

1 half-life period has elapsed, half of the1 half-life period has elapsed, half of thematerial has changed to a daughtermaterial has changed to a daughter

isotope (6 parent: 6 daughter)isotope (6 parent: 6 daughter)

tt 22

2 half-lives elapsed, half of the parent2 half-lives elapsed, half of the parentremaining is transformed into a daughterremaining is transformed into a daughter

isotope (3 parent: 9 daughter)isotope (3 parent: 9 daughter)

3 half-lives elapsed, half of the parent3 half-lives elapsed, half of the parentremaining is transformed into a daughterremaining is transformed into a daughter

isotope (1.5 parent: 10.5 daughter)isotope (1.5 parent: 10.5 daughter)

We would see the rock at this point.We would see the rock at this point.

Exponential decay: never goes to zero

exponential linear

Radioactive Decay Rates are Exponential! (not Linear…)

Radioactive IsotopesRadioactive Isotopesanalogous to sand in an hour glass

50

100

25

13

time----------->

Parent

Daughter

Parent

Daughter

% p

aren

t rem

a ini

ng

As more sand flows out, you have less “parent” and more “daughter” sand. Only here each set of sand is a different element.

Five Radioactive Isotope PairsFive Common Radioactive Isotope Pairs

Half-LifeEffective Minerals and

Isotopes of ParentDating Range

Rocks That Can Parent Daughter

(Years)Be Dated

Uranium 238 Lead 206 4.5 billion 10 million to Zircon 4.6 billion UraniniteUranium 235 Lead 207 704 million Thorium 232 Lead 208 14 billion 48.8 billion

Rubidium 87 Strontium 87 4.6 billion 10 million to

Muscovite

Biotite

Potassium feldspar

Whole metamorphic

or igneous rock

Potassium 40 Argon 40 1.3 billion 100,000 to Glauconite 4.6 billion Muscovite Biotite Hornblende Whole volcanic rock

(Years)

4.6 billion

Example: Uranium 238 decay to Lead 206 (stable)

Most common Radioactive dating systems

•1. uranium-thorium-lead dating (previous example)U-238, U-235, Th-232

each of these decays through a series of steps to Pb

U-238 to Pb-206 half-life = 4.5 byU-235 to Pb-207 half-life = 713 myTh-232 to Pb-208 half-life = 14.1 my

•2. potassium-argon dating

K-40 to Ar-40 half-life = 1.3 by

…argon is a gas--may escape (ages too young--daughter missing)

•3. rubidium-strontium dating

Rb-87 to Sr-87 half-life = 47 by

4. What about carbon dating?

Radiocarbon Dating

Method can be validated by cross-checking with ice cores and tree ringsMethod can be validated by cross-checking with ice cores and tree rings

Carbon-14 dating is based on the ratio of C-14 to C-12 in an organic sample.

Valid only for samples that are less than 70,000 years old (not useful for most rocks)

Living things take in three isotopes of carbon

When the organism dies, the “clock” starts. (C-14 begins to decay to N-14)

Carbon 12 and 13 are stable, but carbon 14 is notCarbon 14 has a half-life of 5730 years

Carbon 14 CycleCarbon 14 Cycle

Correlation: Cross-checking dating techniques

Absolute Dating Techniques are cross-checked using:

1. Ice Cores—up to 70,000 years old

2. Dendrochronology—about 14,000 years (recent samples only)

How do we “check” our readings and correlate from different sources?

3. Stratigraphy—rock layers of known formation rates and types

Ice Cores?

Some Ice core drill sites in Greenland:

How are ice cores sampled?

Samples are examined using various light sources for particle types and ice dynamics

What do we get from ice cores?

1. Climate information: Paleoclimatology.

Temperature changesPrecipitation changes

Air Particulates counts/types—including carbon, dust, bacteria, algae2. Atmospheric Composition (Paleoatmospheric studies)—what gases were in the atmosphere in the past and at what levels?3. Solar Activity (i.e. by the algae, bacteria and particulate types)

More recent events? Use Dendrochronology

Tree Ring Dating Method

Cross-checking: Stratigraphy of Vertical Layers

Stratigraphy—large undisturbed layers of rocks/fossils are laid down in sequential order. (Law of Superposition)

Some Layers of iron-bearing igneous rocks can be correlated by their magnetic properties based on the Earth’s periodic reversal of magnetic poles! (like Seafloor Spreading)

What about Horizontal or Diagonal section layers?What about Horizontal or Diagonal section layers?

We use Walther's LawWe use Walther's Law• • The vertical sequence is repeated by the horizontalThe vertical sequence is repeated by the horizontalsequencesequence

- - walking from A to B to C to the Coast you would encounter thewalking from A to B to C to the Coast you would encounter therocks that would be encountered by drilling a core into therocks that would be encountered by drilling a core into the

earth at any point (A, B, or C)earth at any point (A, B, or C)

Basic Geochronological assumptions

1. Decay rates are constant through geological time

3. The type of rock used for dating is well known.

2. The system is closed to adding or subtracting of parent/daughter isotopes

* Good reasons to believe that this is correct from nuclear physics

* Measurements of decay sequences in ancient supernovae yield the same values as modern lab measurements

* Isotopic system and type of mineral are important.

* Careful procedure is essential to correct analysis

* Igneous rocks are most reliable for absolute dating.* Metamorphism may cause loss of daughter

products * Sedimentary rocks will give ages of the source rocks