Geoscience Reference
In-Depth Information
Table 4.8
230
Th
ex
(excess) at different
depths in a ferromanganese nodule
230
Th
ex
Depth
z
(mm)
0.0
1242
0.2
788
0.4
488
0.6
285
0.8
180
1.0
119
Table 4.9
U-Pb isotopic data for four old
zircons from Jack Hills (Australia)
206
Pb/
238
U
207
Pb/
235
U
1
0.928
69.5
2
0.919
68.2
3
0.965
71.9
4
0.968
74.6
Table 4.10
U-Pb isotopic data for basaltic samples from Mauna Kea, Hawaii
Pb
(ppm)
U
(ppm)
206
Pb
/
204
Pb
207
Pb
/
204
Pb
208
Pb
/
204
Pb
Sample
SR0137-5.98
1.0
0.285
18.43
15.48
37.97
SR0157-6.25
0.593
0.192
18.44
15.48
38.00
SR0346-5.60
1.525
0.211
18.52
15.48
38.10
SR0664-5.10
0.765
0.415
18.55
15.49
38.14
SR0930-15.85
0.521
0.212
18.51
15.49
38.13
SR0967-2.75
0.695
0.319
18.49
15.48
38.11
Canyon Diablo
9.3066
10.293
29.475
Plot the concordia between 4.2 and 4.5 Ga and plot the Jack Hills data on the same
diagram. What is the probable age of these zircons?
12. A rather difficult one! In the mid 1960s, it was proposed that an independent age of the
Earth could be derived from the Pb isotope compositions of modern basalts, but this
idea later proved to be incorrect. Plot
x
206
Pb
∗
/
238
U of basaltic
samples fromMauna Kea, Hawaii, in the concordia diagram. The Pb isotope composi-
tions and the U and Pb concentrations of the samples are provided in
Table 4.10
.
Inthis
context, the
∗
superscript denotes the radiogenic isotopes accumulated since the Earth
formed. Suppose that at that time, the Pb isotope composition of the Earth was that
207
Pb
∗
/
235
U and
y
=
=
