This lower limit is at least concordant with the independently derived figure of 4.55 billion years for the Earth's actual age.The most direct means for calculating the Earth's age is a Pb/Pb isochron age, derived from samples of the Earth and meteorites.However, the test for these assumptions is the plot of the data itself.
Below is a table of radiometric ages derived from groups of meteorites: As shown in the table, there is excellent agreement on about 4.5 billion years, between several meteorites and by several different dating methods.
Note that young-Earthers cannot accuse us of selective use of data -- the above table includes a significant fraction of all meteorites on which isotope dating has been attempted. 286) , less than 100 meteorites have been subjected to isotope dating, and of those about 70 yield ages with low analytical error.
This causes the data points to separate from each other.
The higher the uranium-to-lead ratio of a rock, the more the Pb-206/Pb-204 and Pb-207/Pb-204 values will change with time.
Rocks of this age are relatively rare, however rocks that are at least 3.5 billion years in age have been found on North America, Greenland, Australia, Africa, and Asia.
While these values do not compute an age for the Earth, they do establish a lower limit (the Earth must be at least as old as any formation on it).Young-Earthers have several methods which they claim to give "upper limits" to the age of the Earth, much lower than the age calculated above (usually in the thousands of years).Those which appear the most frequently in talk.origins are reproduced below: Note that these aren't necessarily the "best" or most difficult to refute of young-Earth arguments.This involves measurement of three isotopes of lead (Pb-206, Pb-207, and either Pb-208 or Pb-204).A plot is constructed of Pb-206/Pb-204 versus Pb-207/Pb-204.If the solar system formed from a common pool of matter, which was uniformly distributed in terms of Pb isotope ratios, then the initial plots for all objects from that pool of matter would fall on a single point.