The third assumption is that the sample has remained in a closed system.
This is necessary due to outside influences such as heat and groundwater that can seriously alter the original material.
With the exception of Carbon-14, radiometric dating is used to date either igneous or metamorphic rocks that contain radioactive elements such as uranium, thorium, argon, etc. Now when the uranium or thorium disintegrates, the alpha particles which are emitted are slowed down by the crystals in which the grains of the uranium- or thorium-bearing minerals are embedded.
And even though various radioactive elements have been used to 'date' such rocks, for the most part, the methods are the same. This means that if you had some pure uranium-238 with no lead, that 4.5 billion years later, one half of it would have decayed into its stable daughter product (lead-206). Where these alpha particles finally stop, crystal deformation occurs (and) shows up as a discolouration or a darkening of the crystals.
In this regard it should also be pointed out that for the theory of life from non-life, and/or from amoeba to jellyfish, to man to have any chance at all of being true, then the earth must be very old. can be summarized as follows: In other words, something in the past caused a significant amount of helium to build up inside these zircons (such as from a rapid decay episode of uranium), yet, in spite of the fact that helium has been observed to leak out readily from these zircons, it has not done so: simply because it hasn't had enough time to do so -- suggesting that the zircons themselves are only a few thousand years old."There is evidence to show ...
In many cases it is quite difficult to prove whether one method is superior to another: and in this regard, the only way of doing so is to closely examine how each method works and try to find fault with it.
In regard to the radiometric dating of rocks, it is known that various different radiometric methods often yield quite discordant dates for the same rock, thus proving that they cannot all be correct.
can we be sure the U and Th have always decayed at the same rates we measure today? The second assumption is much more speculative since there is no way to verify whether or not some (or most) of the daughter element was already present when the rock solidified. However, in some cases, a few scientists are telling us that they have solved this problem.
For example, with the uranium/lead method scientists have attempted to estimate what the original ratio (of uranium-238 to lead-206) was when the Earth formed.