For example, if a rock sample was below the water table at any time, leaching would take place.For Uranium/Lead dating this means that some of the uranium that was initially present would be "leached" out of the rock.It is also a fact that those who believe the theory of evolution are the same people who believe in an old earth.In addition, some who believe in God, also claim to believe in evolution; however, if they were honest, they would admit that they actually believe in (some form of) slow-Creation, as opposed to evolution -- apart from the influence of a Creator.When scientists at the University of Regina, Saskatchewan were asked what the results would be if these roots were dated by Potassium Argon method.Their response was that the results: Two well-documented examples of "heat contamination" are the 18 eruptions from two Hawaiian volcanoes.For a more detailed discussion of this and why a Creator must have been intimately involved with the Creation of Life see: A Closer Look at the Age of the Earth.
Therefore, in virtually every case, scientists do not know what the original condition of the rock was; and, even if they did know, they don't any more due to heat contamination, mixing, and leaching. That's because radiometric dating (with the exception of Carbon 14) is almost always performed on igneous rocks (i.e. Also because, when different substances are in a liquid state, something known as mixing almost always takes place: meaning that whenever a liquid (or molten) rock is erupted out of the earth, both the mother and daughter elements will be "mixed" together, thus making it virtually impossible to determine the time that an eruption took place.
In fact, about 98% of "common" lead is "radiogenic" (containing lead 206, 207,208) and only 2% non-radiogenic.
The third assumption is that the sample has remained in a closed system.
Another problem that calls into question the credibility of radiometric dating is heat contamination.
For example, In 1973, in Alberta, Canada (near the town of Grand Prarie) a high voltage line fell which caused nearby tree roots to fossilize almost instantly.