Geoscience Reference
In-Depth Information
3. Figure 6.11 shows a rubidium-strontium isochron for some unmetamorphosed sedi-
ments and clay minerals, namely illite and chlorite, found in the sediment. Calculate
an age from this isochron and comment on its meaning.
4. (a) Given that
N
=
N
0
e
−
λ
t
,where
N
is the number of surviving radioactive atoms
at time
t
,
N
0
the initial number and
λ
the decay constant, and given that today
235
U
/
0.8
238
U
=
0.007 257 in the Earth, Moon and meteorites, estimate the date of
sudden nucleosynthesis for the two estimated production ratios of
235
U
/
238
U
=
1.5
and 2.0. Show all steps in your work clearly.
Note
:
λ
(
235
U)
=
9.8485
×
10
−
10
yr
−
1
and
λ
(
238
U)
=
1.55 125
×
10
−
10
0.7
0
2
4
6
8
87
Rb
86
Sr
yr
−
1
.
87
Sr
/
86
Sr ratio of 0.699 4.6 Ga ago, what
(b) Given that the solar system had an
Figure 6.11.
Rubidium-strontium
isochron for an
unmetamorphosed
sediment: open circles,
whole rock; and solid
circles, illite and chlorite.
(Data from Gorokhov
et al
.(1981).)
86
Sr ratio of the earliest continental crust that formed at
3.8 Ga be, given that the mantle had an
87
Sr
/
should the initial
86
Sr ratio of 0.09 at that time
and that
λ
(
87
Rb)
=
1.42
×
10
−
11
yr
−
1
? Show all steps in your work clearly.
(Cambridge University Natural Science Tripos IB, 1980.)
5. Agranite was extracted from the mantle 2500 Ma ago. Later, at 500 Ma, this granite
was remelted. What is the present-day strontium isotope ratio (
87
Sr
/
87
Rb
/
86
Sr) in the young
pluton? Assume that the granites have gross rubidium-strontium ratios of 0.5 and 0.7,
respectively.
6. A sample is being dated by the
40
Ar
39
Ar-ratio method. Would a delay of one month
between the irradiation of the sample and the spectrographic measurements adversely
affect the results?
7. Discuss which radioactive dating methods are most appropriate for dating basalt, gran-
ite, shale and ultramafic samples (use Table 6.3).
8. Four mineral samples from a meteorite have neodymium and samarium-neodynium
isotope ratios as follows.
/
143
Nd
/
144
Nd
147
Sm
/
144
Nd
0.5105
0.12
0.5122
0.18
0.5141
0.24
0.5153
0.28
(a) Find the age of the meteorite.
(b) Find the initial
143
Nd
/
144
Nd ratio for this meteorite.
(c) Discuss the relevance of the initial
143
Nd
/
144
Nd ratio of meteorites.
9. Fission-track dating was performed on two minerals in a sample. The track date from
garnet was 700 Ma and the date from muscovite was 540 Ma.
(a) Determine the cooling rate for these minerals in
◦
CMa
−
1
.
(b) Determine the closure temperatures for these minerals.
(c) Assuming that cooling continued at this rate, calculate the track date that would be
given by apatite and its closure temperature.
(d) Would a biotite fission-track date be useful?
