## How do we determine the age of a rock?

1. Relative dating - Steno's Laws, etc.
"A is older than B"
2. Absolute dating
Quantify the date in years. Radiometric Dating

Naturally-occurring radioactive materials break down into other materials at known rates. This is known as radioactive decay.

Radioactive parent elements decay to stable daughter elements.

Radioactivity was discovered in 1896 by Henri Becquerel. In 1905, Rutherford and Boltwood used the principle of radioactive decay to measure the age of rocks and minerals (using Uranium decaying to produce Helium. In 1907, Boltwood dated a sample of urananite based on uranium/lead ratios. Amazingly, this was all done before isotopes were known, and before the decay rates were known accurately.

The invention of the MASS SPECTROMETER after World War I (post-1918) led to the discovery of more than 200 isotopes.

Many radioactive elements can be used as geologic clocks. Each radioactive element decays at its own nearly constant rate. Once this rate is known, geologists can estimate the length of time over which decay has been occurring by measuring the amount of radioactive parent element and the amount of stable daughter elements.

Examples:

### Stable Daughter

#### Nitrogen 14

In the above table, note that the number is the mass number (the total number of protons plus neutrons).
Note that the mass number may vary for an element, because of a differing number of neutrons.
Elements with various numbers of neutrons are called isotopes of that element.

Each radioactive isotope has its own unique half-life.
A half-life is the time it takes for half of the parent radioactive element to decay to a daughter product.

Examples:

### Half life

#### 5730 years

Radioactive decay occurrs at a constant exponential or geometric rate.
The rate of decay is proportional to the number of parent atoms present.

The proportion of parent to daughter tells us the number of half-lives, which we can use to find the age in years.
For example, if there are equal amounts of parent and daughter, then one half-life has passed.
If there is three times as much daughter as parent, then two half-lives have passed. (see graph, above)

Radioactive decay occurs by releasing particles and energy.

Uranium decays producing subatomic particles, energy, and lead.

As uranium-238 decays to lead, there are 13 intermediate radioactive daughter products formed (including radon, polonium, and other isotopes of uranium), and 8 alpha particles and 6 beta particles released. There are three types of subatomic particles involved:

1. Alpha particles
large, easily stopped by paper
charge = +2
mass = 4
2. Beta particles
penetrate hundreds of times farther than alpha particles, but easily stopped compared with neutrons and gamma rays.
charge = -1
mass = negligible
3. neutrons
highly penetrating
no charge
mass = 1
Gamma rays (high energy X-rays) are also produced.
Highly penetrating electromagnetic radiation. Photons (light).
No charge or mass.
Can penetrate concrete. Lead shield can be used.

## Minerals you can date

Most minerals which contain radioactive isotopes are in igneous rocks. The dates they give indicate the time elapsed since the magma cooled.
• Potassium 40 is found in:
• potassium feldspar (orthoclase)
• muscovite
• amphibole
• glauconite (greensand; found in some sedimentary rocks; rare)
• Uranium may be found in:
• zircon
• urananite
• monazite
• apatite
• sphene

Note that some elements have both radioactive and non-radioactive isotopes. Examples: carbon, potassium.

As seen in the tables above, there are three isotopes of uranium. Of these, U-238 is by far the most abundant (99.2739%).

Radioactive elements tend to become concentrated in the residual melt that forms during the crystallization of igneous rocks. More common in SIALIC rocks (granite, granite pegmatite) and continental crust.

Radioactive isotopes don't tell much about the age of sedimentary rocks (or fossils). The radioactive minerals in sedimentary rocks are derived from the weathering of igneous rocks. If the sedimentary rock were dated, the age date would be the time of cooling of the magma that formed the igneous rock. The date would not tell anything about when the sedimentary rock formed.

To date a sedimentary rock, it is necessary to isolate a few unusual minerals (if present) which formed on the seafloor as the rock was cemented. Glauconite is a good example. Glauconite contains potassium, so it can be dated using the potassium-argon technique.

## How does Carbon-14 dating work?

1. Cosmic rays from the sun strike Nitrogen 14 atoms in the atmosphere and cause them to turn into radioactive Carbon 14, which combines with oxygen to form radioactive carbon dioxide.

2. Living things are in equilibrium with the atmosphere, and the radioactive carbon dioxide is absorbed and used by plants. The radioactive carbon dioxide gets into the food chain and the carbon cycle.

3. All living things contain a constant ratio of Carbon 14 to Carbon 12. (1 in a trillion).

4. At death, Carbon 14 exchange ceases and any Carbon 14 in the tissues of the organism begins to decay to Nitrogen 14, and is not replenished by new C-14.

5. The change in the Carbon 14 to Carbon 12 ratio is the basis for dating.

6. The half-life is so short (5730 years) that this method can only be used on materials less than 70,000 years old. Archaeological dating uses this method.) Also useful for dating the Pleistocene Epoch (Ice Ages).

7. Assumes that the rate of Carbon 14 production (and hence the amount of cosmic rays striking the Earth) has been constant (through the past 70,000 years).

## Fission Track Dating

Charged particles from radioactive decay pass through mineral's crystal lattice and leave trails of damage called FISSION TRACKS. These trails are due to the spontaneous fission of uranium.

Procedure to study:

• Enlarge tracks by etching in acid (so that they may be visible with light microscope)
• See readily with electron microscope
• Count the etched tracks (or note track density in an area)

Useful in dating:

• Micas (up to 50,000 tracks per cm squared)
• Tektites
• Natural and synthetic (manmade) glass

Reheating "anneals" or heals the tracks.

The number of tracks per unit area is a function of age and uranium concentration.

Document copyright: Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA

Document created February 4, 1996
Modified April 4, 1997
Modified February 3, 1999