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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.
Furthermore, the half life of the parent isotope is much longer than any of the intermediary daughter isotopes, thus fulfilling the requirements for secular equilibrium (Section 2.4).
We can therefore assume that the Some igneous minerals (notably zircon) conveniently incorporate lots of U and virtually no Pb upon crystallisation.
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.
This assumption cannot be made for other minerals, young ages, and high precision geochronology.If a zircon crystal originally crystallizes from a magma and remains a closed system (no loss or gain of U or Pb) from the time of crystallization to the present, then the Discordant dates will not fall on the Concordia curve.