Dating a rock
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.(For brevity's sake, hereafter I will refer to the parent isotope as ).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.However, the methods must be used with care -- and one should be cautious about investing much confidence in the resulting age...A routine statistical operation on the set of data yields both a slope of the best-fit line (an age) and a variance in the slope (an uncertainty in the age).The better the fit of the data to the line, the lower the uncertainty.
An additional nice feature of isochron ages is that an "uncertainty" in the age is automatically computed from the fit of the data to a line.
(The range of uncertainty varies, and may be as much as an order of magnitude different from the approximate value above.
It depends on the accuracy of the measurements and the fit of the data to the line in each individual case.) For example, with Rb/Sr isochron dating, any age less than a few tens of millions of years is usually indistinguishable from zero.
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.
(Rocks which include several different minerals are excellent for this.) Each group of measurements is plotted as a data point on a graph.