Radiogenic dating methods www ee dating iale ro
If the Earth’s interior were a simple and homogeneous reservoir with respect to the ratio of uranium to lead, a single sample extracted by a volcano would provide the time of extraction. No parent–daughter value for a closed system is involved, rather just a single isotopic measurement of lead viewed with respect to the expected evolution of lead in the Earth.
Unfortunately the simplifying assumption in this case is not true, and lead model ages are approximate at best.
The progressive increase in the abundance of daughter isotopes over time gains a special significance where the parent element is preferentially enriched in either the mantle or the crust. In contrast, modern volcanic rocks in the oceans imply that much of the mantle has a value between about 0.703 and 0.705.
In this case, a valid age can still be obtained, provided that they remain within the rock.
In uranium–lead (U–Pb) dating of zircon, the zircon is found to exclude initial lead almost completely.
Minerals, too, are predictable chemical compounds that can be shown to form at specific temperatures and remain closed up to certain temperatures if a rock has been reheated or altered.
When a single body of liquid rock crystallizes, parent and daughter elements may separate so that, once solid, the isotopic data would define a series of points, such as those shown as open circles designated R.
With time each would then develop additional daughter abundances in proportion to the amount of parent present.If a number of samples are analyzed and the results are shown to define a straight line within error, then a precise age is defined because this is only possible if each is a closed system and each has the same initial ratio and age.