UNIVERSITY OF DAR ES SALAAM
GEOLOGY DEPARTMENT
GY 100 ASSIGNMENT NO. 4
RADIOMETRIC DATING;
Use the following in solving the problem.
87Sr/86Sr
= (87Sr/86Sr) i + 87Rb/86Sr (e λ t – 1)
λ (87Rb) =1.42× 10-11 y-1
N = Noe-tλ , Half-life of carbon 14 = 5730 years
1.The following data were obtained for three minerals from a pegmatite.
Muscovite
Rb,
ppm
238.4
87
Sr/86Sr
Sr,
ppm
1.8
1.4125
Biotite
1080.9
12.8
1.1400
k-feldspar
121.9
75.5
0.7502
i.
ii.
iii.
Plot the three minerals on the isochron diagram using a graph paper. Calculate the slope
and deduce the intercept of the isochron.
Calculate the date from the slope of the isochron.
What is the specific use of the initial strontium ratio (87Sr/86Sr) i concentration above?
2. How old is a piece of wood in which the Carbon-14 is 3.12% of that in the wood formed
today?
3. A fossilized tree killed by a volcano was studied. It had 6.25% of the amount of Carbon-14
found in the sample of the same size from a tree that is alive today. When did the volcanic
eruption take place?