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. Error Banner.fade_out.modal_overlay.modal_overlay .modal_wrapper.modal_overlay [email protected](max-width:630px)@media(max-width:630px).modal_overlay .modal_fixed_close.modal_overlay .modal_fixed_close:before.modal_overlay .modal_fixed_close:before.modal_overlay .modal_fixed_close:before.modal_overlay .modal_fixed_close:hover:before.
Such trapped Ar is not problematical when the age of the rock is in hundreds of millions of years.
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In Microsoft Excel, you can make a formula to compute an exact age in months, days, hours and even minutes.
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.
To see how we actually use this information to date rocks, consider the following: Usually, we know the amount, N, of an isotope present today, and the amount of a daughter element produced by decay, D*.