Geoscience Reference
In-Depth Information
206
Pb/
204
Pb
207
Pb/
204
Pb
Meteorite sample
Nuevo Laredo
50.28
34.86
Forest City
19.27
15.95
Modoc
19.48
15.76
Henbury
9.55
10.38
Canyon Diablo
9.46
10.34
Table 4.12
Sr isotopic data for six samples of
modern shales
87
Rb/
86
Sr
87
Sr/
86
Sr
Sample
a
6.2
0.7121
b
2.7
0.7102
c
0.5
0.7083
d
52.5
0.7111
e
13.1
0.7081
f
21.4
0.7096
of the iron sulfide of the Canyon Diablo meteorite. We can safely assume a constant
molar weight for Pb (207.2) and U (238.0) and the modern
238
U/
235
U ratio is 137.8.
What is the range of
207
Pb
∗
/
206
Pb
∗
ratios of the mantle source of modern basalts (see
Section 11.2
)? Why is the age of the upper intercept of the concordia close to the age
of the Solar System? Discuss where the pitfall is. (Hint: from the Pb concentrations,
calculate the moles of initial
204
Pb per mass unit present in each sample and use the
Canyon Diablo values to derive the same parameter for the other isotopes).
ratios. By trial and error, find Clair Patterson's estimate for the age of the last Pb
isotopic homogenization of the Solar System as a whole.
14. Isochrons improve with age.
Table 4.12
lists the Sr isotopic data for six samples of
modern shales. Using your favorite spreadsheet or any convenient software, calculate
the
87
Sr/
86
Sr ratios of each sample 100 My, 2700 My, and 4560 My into the future.
Plot the isochrons for today and for each of these future dates. Use your software to
calculate the slope and the correlation coefficients and discuss the results.
15. Isochron or mixing line? A series of modern lavas are speculated to have formed by
mixing (hybridization) of mingling basaltic and rhyolitic magmas.
Table 4.13
lists
the Nd isotopic data of two end-member magmas. Calculate the
147
Sm/
144
Nd and
143
Nd/
144
Nd ratios of hybrid rocks formed by 20-80, 40-60, 60-40, and 80-20 % of
