Common Radiometric Methods

1. Potassium-Argon Method:

Potassium-40 has a half life of 11900 million years. It is commonly found in many rocks. It decays into argon-40, and inert gas found in the sedimentary rocks. Potassium-Argon method can be used to date rocks that are millions of years old, including meteorites(age 4600 million years). However rocks younger than 0.1 million years are difficult to date because they contain very little radiogenic argon. The minerals that are suitable for K-Ar dating include biotite, muscovite, hornblene and nepheline. The potassium felspars, such as orthoclase and microcline are unsuitable for K-Ar dating because they are likely to loose argon readily at atmospheric temperatures.

2. Rubidium-Strontium Method:

Rubidium-87 has a half life of 50,000 million years. The Rb-Sr method can be applied to rocks of almost any geological age. However rocks younger than 20-30 million years may create analytical difficulties because of the minute amount of radiogenic Sr-87 present. The Rb-Sr method can be used to date such common rockforming minerals as muscovite, biotite, and all types of potassium felspars including ortho-clase and microcline.

3. Uranium-Lead Method:

The half-life of Uranium-238 is 4500 million years. uranium and Thorium frequently occur in the same mineral and it is therefore, possible to make two independent age determinations on one mineral sample.

The U-Pb method can be applied over the greater part of the geological age range, although their usefulness decrease sharply for rocks younger than 100-200 million years. This method is less commonly used than the K-Ar and Rb-Sr methods but may be used to date rare mineral in which uranium and thorium are major constituents. it is frequently applied to minerals such as zircon and sphene.

It should be noted that the U-Pb methods are of much greater value than the Th-Pb method. The latter is now only rarely used.

4. Radiocarbon Dating:

There are three isotopes of Carbon: C-12, C-13 and C-14. Of these C-14 is radioactive. Since it has a half life of only 5730 years. It can be used to date events of recent geological history. Although C-14 is only useful in dating the last small fraction of geological time (75000 years). It has become an important means for dating Ice Age and archaeological material.

Radiocarbon (C-14) is produced in the atmosphere by cosmic ray bombardment of Nitrogen-14. The C-14 combines with oxygen to form carbon dioxide which is absorbed by living organisms. A constant ratio of C-14 to C-12 is established in each organism during its life. After death no more carbon dioxide is absorbed and the C-14 decays steadily by emitting beta particles to N-14.

Comments 1

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