Principe de chronologie absolue - e-monsite

Principe de chronologie absolue - e-monsite

Principes de chronologie absolue La mthode des isochrones CHRONOLOGIE ABSOLUE Utilisation de la radioactivit naturelle La vitesse de dsintgration d un lment radioactif est proportionnel la quantit de cet lment

prsent dans l chantillon. dP/dt = - .P : Constante de radioactivit dP/dt = - .P dP/P = - dt Soit en intgrant entre l instant zro, date de fermeture du systeme, et aujourd hui:

ln (P/P0) = - . t t = - 1/ . ln (P/P0) T: priode ou demi vie. Temps qu il faut pour que la moiti des lments radioactifs soient dsintgrs. P = P0 / 2

T = ln2 / Plus la demi vie sera grande, (plus l lment chimique se dsintgre lentement), plus l lment radioactif permettra de dater des vnements anciens 14 C 14N: T = 5730 ans

40 K 40Ar: T = 11,9 . 109 ans 87 Rb 87Sr: T = 50 . 109 ans Datation au Rubidium / Strontium Rb 87Sr

87 Lors de la formation d une roche magmatique, les minraux intgrent du rubidium et du strontium dans leur rseau cristallin. La quantit dlments intgrs est variable en fonction des minraux. On ne connat pas linstant initial la quantit dlment pre ou dlment fils. Sr = 87Sr0 + (87Rb0 - 87Rb)

87 Avec 87Rb = 87Rb0. e- t Soit 87 Sr = 87Sr0 + 87Rb .(e t - 1)

Ou (87 Sr/ 86Sr) = (87Sr/ 86Sr) 0 + 87Rb / 86Sr.(et - 1) Equation du type y = a.x + b Avec a = et - 1 et b = (87Sr/ 87Sr) 0

Pente de la droite: a = et - 1 Soit t = 1/ . ln (a + 1) Exercice dapplication Exercice 10 page 162

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