Geochronology - are independent of time You can therefore...

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Geochronology When a radio-active element decays we have a parent (P) producing a daughter element (D) with a half-life T 1/2 , itself at the same time obviously losing the same number of atoms. To use this for dating one needs T 1/2 to be a suitable length of time, measurable quantities of the daughter element, and of the parent. You need also to be able to estimate the amount of the daughter element originally there. There should be no loss or gain of the parent and daughter substances through, for example, diffusion , recrystallization, metamorphosis or contamination (the latter including during analysis). There are several suitable candidate elements: P(arent) D(aughter) T 1/2 (x10 9 yrs) U 238 Pb 206 4.7 U 235 Pb 207 0.71 Th 232 Pb 208 13.9 Rb 87 Sr 87 49.8 K 40 Ar 40 11.8 also Ca 40 1.47 The Lead decay has the added complication that it is usually added to the Lead that is already there. Fortunately non-radiogenic lead contains Pb 204 and the proportions: Pb 208 + Pb 207 + Pb 206 + Pb 204 26% 21% 52% 1.4%
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Unformatted text preview: are independent of time. You can therefore use the amount of Pb 204 to estimate the amount of non-radiogenic Lead. in the Rubidium-Strontium decay the SR 87 is added to non-radiogenic Sr 87 , but we can get this latter from: (Sr 87 /Sr 86 ) non-radiogenic = 0.7 Similarly the Ar 40 in the Potassium-Argon decay will be added to the Ar 40 in the air, but, for the non-radiogenic component: (Ar 36 /Ar 40 ) air = 0.00337 We need also the fact that (K 40 /K total ) = 0.000119 Decay to a single daughter element When a simple one-to-one decay takes place we have daughter element D produced to the extent: D = P- P e-Lt = -P (e-Lt-1) where D=amount of daughter element, P of parent element and P is the original amount of the parent. L here is lambda, the decay rate. Of course by this definition L = ln2/T 1/2 . if P n (= P now) = P e-Lt then D = P n (e Lt-1) so t = (1/L)[/SUP> -1) so t = (1/L)[ln(1 + D/P n )]...
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