U leakage would cause discordant points to plot above the cocordia. Lunar rocks also lie on the Geochron, at least suggesting that the moon formed at the same time as meteorites. Pb separated from continents and thus from average crust also plots on the Geochron, and thus suggests that the Earth formed at the same time as the meteorites and moon.
But, again, exptrapolation of the discordia back to the two points where it intersects the Concordia, would give two ages - t* representing the possible metamorphic event and t and solve for t . This argument tells when the elements were formed that make up the Earth, but does not really give us the age of the Earth. Thus, our best estimate of the age of the Earth is 4.55 billion years.
Thus, if we start out with 1 gram of the parent isotope, after the passage of 1 half-life there will be 0.5 gram of the parent isotope left.
After the passage of two half-lives only 0.25 gram will remain, and after 3 half lives only 0.125 will remain etc.
Zircon has a high hardness (7.5) which makes it resistant to mechanical weathering, and it is also very resistant to chemical weathering. Chemically, zircon usually contains high amounts of U and low amounts of Pb, so that large amounts of radiogenic Pb are produced.
Other minerals that also show these properties, but are less commonly used in radiometric dating are Apatite and sphene.
U would have been stable in the crystallographic site, but the site is now occupied by by Pb.Nd ratios on several minerals with a mass spectrometer and then from the slope determine the age of the rock. If a magma cools quickly on the surface of the Earth, some of the Ar may be trapped.The initial ratio has particular importance for studying the chemical evolution of the Earth's mantle and crust, as we discussed in the section on igneous rocks. If this happens, then the date obtained will be older than the date at which the magma erupted.For example lavas dated by K-Ar that are historic in age, usually show 1 to 2 my old ages due to trapped Ar.Such trapped Ar is not problematical when the age of the rock is in hundreds of millions of years.Prior to 1905 the best and most accepted age of the Earth was that proposed by Lord Kelvin based on the amount of time necessary for the Earth to cool to its present temperature from a completely liquid state.