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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.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.Our role, goals and objectives are defined and contacts for the co-ordinators are available in the relevant section of the website.Please feel free to e-mail a co-ordinator with any comments or offers to become involved.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 actinide elements U and Th are among the heaviest naturally-occurring elements.We hope you find these pages a useful resource, and will support this community effort through active participation in planned activities.We will be happy to receive any comments and suggestions you would like to make that would improve the site and direct the efforts of the community towards its goals.They are radioactive and decay by alpha-decay down a complex decay series to Pb, and also by fission.U has two main radioactive isotopes, 235U and 238U, that decay to isotopes of Pb, 207Pb and 206Pb respectively.