1. Calculate the atomic 87Rb/86Sr ratio of a rock sample containing Rb = 125 mg/
ID: 590892 • Letter: 1
Question
1. Calculate the atomic 87Rb/86Sr ratio of a rock sample containing Rb = 125 mg/g and Sr = 275 mg/g. Atomic weights: Rb = 85.46, Sr = 87.62; isotopic abun- dances: 87Rb = 27.83%, 86Sr = 9.90%. (Answer: 1.31) 2. Calculate a K-Ar date for a sample of biotite from the Amitsoq gneiss containing K,0 = 9.00%, 40Ar = 2.424 ug/g. Decay constants: a = 0.581 X 10-16 a), a = 5.543 X 10-10 a; isotopic abundance of 40K = 0.01167%; atomic weights: K = 39.098, O = 15.999, 40Ar = 39.96. (Answer: 2308 x 106 a) 3. Calculate a Rb-Sr date for the same sample of|| biotite from the Amitsoq gneiss given that || 87Rb/86Sr = 107.1, 87Sr/86Sr = 3.093. Decay constant: A = 1.42 x 10-11 a-1; (87Sr /86Sr), = 0.7030. (Answer: 1554 x 106 a) 4. Calculate the slope and intercept of a Rb-Sr || isochron by the method of least squares using data for || micas from the Amitsoq gneiss.Explanation / Answer
considering 1g;
amount of K2O= 9 X 10-3g/ 94.195g mol-1 =9.5 x 10-5 mol
amount of 40K= 2 X 9.5 x 10-5 mol X 0.01167/100 = 2.217 X 10-8mol
amount of 40Ar=2.424 x 10-3/39.96 g mol-1 =6.07 x 10-5mol
t1/2 of 40K = ln 2/ Ie=
t1/2 of 40K =ln2/0.581 x 10-10
= 1.193 x 1010
date time t= (t1/2 /ln2) x (Kf +Arf /0.109) / Kf
=( 1.193 x 1010 / ln 2) x ( 2.217 X 10-8 + 6.07 x 10-5/0.019) / 2.217 X 10-8
= 247.99 x 1013
in this equation
t=date time
t1/2= half life of 40K
KF=AMOUNT OF 40K remaining
Arf =amount of 40Ar remaining
NOTE - I have no idea about the a-1 units expressed in decay constant, it is not clear
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