Radiometric Dating of Rocks

1896-1898

Radioactivity discovered by:

Henri Becquerel and Marie and Pierre Curie.

Phenomenon of Radioactivity

Some elements, such as uranium (U) and thorium (Th), are unstable:

They decay spontaneously.

The Atom

Uranium Nucleus

spontaneously emits a particle from its nucleus called an alpha particle (2 protons + 2 neutrons).   

Alpha Particle

emits a particle from its nucleus called an alpha particle (2 protons + 2 neutrons).   

Uranium - Thorium Decay

U He + Th238

92

4

2

234

90

spontaneous

decay

“parent” “daughter product”

alpha particle =2 protons + 2 neutrons = positively charged ion of Helium

Thorium:90 protons + 144 neutrons

Beta Particle Emission

But, Th is also unstable, and it emits a beta particle …

234

90

Th + Pa234

90

234

91

Thorium - Protactinium Decay

beta particle

beta particle = an electron discharged from the nucleus when a neutron splits into a proton and an electron

Protactinium:91 protons + 143 neutrons

U PbSeries

This process is called radioactive decay, and eventually uranium (parent) decays to lead (daughter product).

U PbSeries

The rate at which this process occurs is measured in terms of the “half life”.

Half Life

Half Life = Number of years for 1/2 of the original number of atoms to decay from U to Pb

The phenomenon of radioactivity relates to our story about the age of the Earth in two ways:  

(1) As radioactive elements decay in the Earth, they heat up the surrounding rocks.

(2) Radiometric Dating

Radiometric Dating

Assume: Rate of decay (half life) of long-lived elements has not varied since the Earth was formed.

This is an example of uniformitarianism.

The Principle of Uniformitarianism

The physical, chemical and biological laws that operate today have also operated in the geologic past.

To estimate the age of a rock:

D = amount of daughter product. P = amount of parent.  

For a particular radioactive element in a rock, determine the present ratio = D/P.

D

P

Rate of decay (from theory and measurement)

Make assumptions about original ratios (from theory of geochemistry).

As time passes, the amount of parent decreases, and the amount of daughter product increases. This provides a way of estimating the amount of time since the "clock" got started (i.e., since the rock solidified).

D

P

D

P

Half-Lives of Radioactive Isotpes

Examples of Very Old Rocks Found on Earth:

3.4 billion year old granites - South Africa.

3.7 billion year old granites - Greenland.

Oldest terrestrial rocks > 4.6 billion years.Central Canada and Central Australia

Oldest terrestrial rocks (4.0 billion years?) are deep in the interiors of continents.

Oldest terrestrial rocks (4.0 billion years?) are deep in the interiors of continents.