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Note that the mere existence of these assumptions do not render the simpler dating methods entirely useless.

In many cases, there are independent cues (such as geologic setting or the chemistry of the specimen) which can suggest that such assumptions are entirely reasonable.

Then the computed age based on the accumulation of daughter products will be incorrect (Stasson 1998).

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This amount is often unknown and is one of the downfalls of conventional radiometric dating.

However, isochron dating bypasses this assumption, as explained below. The final condition is the number of atoms of parent and daughter isotopes remaining in the rock and can easily be measured in a lab.

Isochron methods avoid the problems which can potentially result from both of the above assumptions.

Isochron dating requires a fourth measurement to be taken, which is the amount of a different isotope of the same element as the daughter product of radioactive decay.

To see how we actually use this information to date rocks, consider the following: Usually, we know the amount, N, of an isotope present today, and the amount of a daughter element produced by decay, D*.

By definition, D* = N-1) (2) Now we can calculate the age if we know the number of daughter atoms produced by decay, D* and the number of parent atoms now present, N.

(For brevity's sake, hereafter I will refer to the parent isotope as ).

In addition, it requires that these measurements be taken from several different objects which all formed at the same time from a common pool of materials.

The simplest form of isotopic age computation involves substituting three measurements into an equation of four variables, and solving for the fourth.

The equation is the one which describes radioactive decay: If one of these assumptions has been violated, the simple computation above yields an incorrect age.

However, the methods must be used with care -- and one should be cautious about investing much confidence in the resulting age...

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