The Uranium Method

The Uranium Method - ta by taking the ratios for U 238 and...

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The Uranium Method The main problem with the Uranium method is that Uranium is not very commonly occurring. It does not substitute for the common ions in a mineral and so is "left out" when crystallization occurs, and is amongst the last of the rocks to crystallise. This means crystallisation takes place over a long time period, and so the crystals will be large. (Don't, therefore, look for U in a fine- grained area!) To find Uranium we look for coarse grained granites and giant-grained granite(Pegmatite). Uranium and Thorium are found together as they substitute for each other and Zirconium. Uraninite is UO 2 and Zircon is ZrSiO 4 on 4 one further complication is that lead diffuses so D obs = e ta D produced , where e ta is the proportion that has not diffused away. You can eliminate e
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Unformatted text preview: ta by taking the ratios for U 238 and U 235 decay: so: Pb 207 /Pb 206 = [U 235 (e L235t-1)]/[U 238 (e L238t-1)] This gives t. Compare this with the values from U 238--> Pb 206 and U 235--> Pb 207 . If they are the same (i.e. same ages) they they are said to be 'concordant' and of high accuracy. Usually they are 'discordant'. Usually Thorium in the same rock gives a further estimate - i.e Th 232 /Pb 208 is also used. Oldest rocks found The oldest rocks found are from Russia (3.5 10 9 years and South Africa (3.2 10 9 years). Many rocks are 1.0-1.8 10 9 years old. There have been "quiet" times for rock formation as rocks between 0.5-0.9 10 9 and 2.0-2.3 10 9 are infrequent....
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