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Without this knowledge, he argued that, "As for the future, we may say, with equal certainty, that inhabitants of the Earth cannot continue to enjoy the light and heat essential to their life, for many million years longer, unless sources now unknown to us are prepared in the great storehouse of creation."The same is true of the basis of Kelvin's estimate of the age of the Earth.It was based on the idea that no significant source of novel heat energy was affecting the Earth.Of course there seem to me to be fairly reasonable explanations for this observation which may allow for more slowly forming granitic rocks.For instance, polonium radiohalos are sometimes associated with polonium bands generated by the polonium being transported by hydrothermal fluids along fractures.Other factors and basic assumptions must also be considered.
However, if one does assume a constant decay rate, and if one starts with an originally pure sample of “parent element,” then the proportion of parent to daughter tells us the number of half-lives, which has been used to find the supposed age of igneous rocks. The conclusions of Renne and his team read as follows: Ar can be identified in volcanic sanidine, and while perhaps negligible in pre-Holocene rocks, it has important consequences for sample at the limit of the method’s applicability.
Chamberlain (1899) pointed out that Kelvin's calculations were only as good as the assumptions on which they were based.
"The fascinating impressiveness of rigorous mathematical analyses, with its atmosphere of precision and elegance, should not blind us to the defects of the premises that condition the whole process.
These first “geochronology studies” yielded the first “absolute ages” from geologic material, which seemed to indicate that parts of the Earth's crust were hundreds of millions of years old. There is, of course, one radiometric dating method that appears to overcome the vital "zero date problem".
The isochron dating method theoretically overcomes the need to know the initial ratio of parent and daughter isotopes. For now, we will look at those methods that do fall under the above assumptions.