Condensed Notes on Radioactive Dating:

RD is a way of performing absolute dating since numbers are obtained.

Some isotopes in nature are stable and some are unstable. The unstable ones are radioactive and will therefore, decay at a measured/constant rate.

Isotopes are atoms of the same element that have different mass numbers because they have different numbers of neutrons

Example is uranium. Could be:

U234

U235

U238

Radioactive Decay: the process whereby an unstable element gives off charged particles until it forms a stable element (i.e. isotopes).

The original radioactive material is called the parent material(i.e. unstable, radioactive element). The new material created (isotopes resulting from the decay of the parent) is called the decay product or daughter element.

Radioactivity provides a reliable means of calculating the ages of rocks, minerals, and bones that contain particular radioactive isotopes.

Half Life: the time it takes for a radioactive substance to decay so that 50 % (½) is remaining.

Isoptope pairs and their memorized half-life values are as follows:

U238 - Pb206Half-life = 4.51 billion years

U235 - Pb207Half-life = 713 million years

K40 - Ar40Half-life = 1.31 billion years

C14 - N14Half-life = 5730 years

Rb87 – Sr87Half-life = 47 billion years

If the half-life of a radioactive isotope is known (which is will be) and the parent/daughter ratio can be measured, then the age of the sample can be calculated. Also, if the half-life of a radioactive isotope is known (which is will be) and the number of elapsed half-lives that occurred is known, then either the amount of original parent material or amount of remaining parent material can be determined.

Always Remember and/or Know

1:1 = 1 half-life = 1/2

1:3 = 2 half-lives = 1/4

1:7 = 3 half-lives = 1/8

1:15 = 4 half-lives = 1/16

1:31 = 5 half-lives = 1/32

Notice that the percentage of radioactive atoms that decay during one half-life is always the same: 50 %. However, the actual number of atoms that decay with the passing of each half-life continually decreases. Thus, as the percentage of radioactive parent atoms declines, the proportion of stable daughter atoms rises, with the increase in daughter atoms just matching the drop in parent atoms. This fact is the key to radiometric dating.

Five radioactive isotopes have proven to be important in providing radiometric ages for ancient rocks.

Rubidium - 87

Uranium - 238

Uranium - 235

Potassium – 40

Carbon – 14

When using radioactive dating you must assume:

  1. No parent material or daughter element has been removed.

Example: leaching

  1. No parent material or daughter element has been added from outside sources.

Example: hydrothermal fluids

  1. With uranium - lead dating, you must assume that no non-radioactive lead was present in the sample.

Problems associated with radioactive dating:

C-14 can only be used on once living samples.

C-14 can only be used on samples less than 50, 000 years old.

U-238 can only be used on samples greater than 100, 000 years old.

Only rarely can sedimentary rocks be dated. Note that sedimentary rocks are composed from rock fragments, which means that it is made of various fragments of varying rock type and age (DUE TO WEATHERING AND EROSION).

Metamorphic dating is difficult due to the changes involved. High heat and pressure resets the radioactive clock.

Addition or removal of parent material or decay product will flaw results.

Argon – 40 is a noble gas and could escape over geological time causing the ratio of potassium – 40 to argon – 40 to be wrong.

NOTE THAT QUESTIONS COULD REFER TO THE RADIOACTIVE PARENT ISOTOPES IN EITHER OF THE FOLLOWING FORMS:

-FRACTION (E.G. ¼)

-PERCENT (E.G. 25 %)

-AMOUNT IN GRAMS (E.G. 360 GRAMS)

A COMMON ERROR THAT STUDENTS MAKE IN RELATION TO READIOACTIVITY PROBLEMS IS THAT THEY COUNT 100 % OF PARENT MATERIAL REMAINING TO BE 1 HALF-LIFE WHEN IN ACTUALLY IT IS O HALF-LIVES. COUNTING IT AS 1 HALF-LIFE WOULD RESULT IN THE WRONG NUMBER OF HALF-LIVES BEING DETERMINED, THEREBY RESULTING IN THE WRONG RATIO OF PARENT MATERIAL TO DAUGHTER MATERIAL AND THE WRONG AGE FOR THE SAMPLE.

NOW YOU JUST NEED TO LEARN THE VARIOUS PROBLEM TYPES THAT COULD ARISE ON THE QUIZ, UNIT TEST, OR PUBLIC EXAMINATION RELATING TO RADIOACTIVE DATING. WE WILL NEED TO PRACTICE EACH PROBLEM TYPE.