Geoscience Reference
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particular sample or set of samples can be determined by measuring dates in
various minerals.
For the two minerals discussed above, ages determined from fission tracks
in sphene are always greater than ages determined from fission tracks in apatite.
The ages are interpreted as representing the last time the mineral cooled below its
closure temperature (which depends on the cooling rate!). The differences among
the closure temperatures of minerals do not, however, depend on the cooling rate.
Thus, the difference between the sphene and apatite ages indicates the length of
time taken for the sample to cool between the two closure temperatures, and so
the cooling rate can be determined. Closure temperatures for fission tracks in a
wide range of minerals cooled at various rates are shown in Fig. 6.8.
The temperature dependence of fission tracks provides an excellent method of
determining the details of the cooling history of rock samples. This method has
been used in the analysis of the erosional history of sedimentary basins.
6.10 The age of the Earth
Some of the early estimates of the age of the Earth were discussed in Section 6.1.
Radioactivity provided the tool with which accurate estimates of the Earth's age
could be made as well as providing an 'unknown' source of heat that helped to
make sense of the early thermal models (see Section 7.4).
The first radioactive dating method used to limit the age of the Earth was the
accumulation of
particles (helium nuclei) in minerals as the result of the decay
of uranium. In 1905, Rutherford obtained ages of around 500 Ma for the uranium
mineral he tested. Also in 1905, Boltwood, as a result of an idea of Rutherford,
used the relative proportion of lead and uranium in a rock sample to obtain a date.
Measurements on a variety of samples gave dates of between 92 and 570 Ma with
the radioactive production rates then available. (This first attempt at U-Pb dating
was hampered by the fact that in 1905 neither isotopes nor the thorium-lead decay
were understood.) Now, almost a century later, we have a detailed knowledge of
the age of rocks and of the Earth based on a variety of radiogenic methods.
The oldest rocks on the surface of the Earth are to be found in the ancient cra-
tons which form the hearts of the continents. The oldest known rocks on earth are
the Acasta gneisses in the Slave province of northwestern Canada. U-Pb measure-
ments on zircon grains indicate that the original granitoid parent to this metamor-
phosed gneiss crystallized at 3962
3 Ma. Amongst the most ancient rocks are the
deformed and metamorphosed Isua supracrustal rocks in Greenland, for which the
igneous activity has been dated at 3770
±
42 Ma by Sm-Nd data and at 3769 + 11
±
8
Ma by U-Pb work on zircons. Felsic volcanic rocks from the Duffer Formation
of the Pilbara supergroup in Western Australia have been dated at 3452
±
16 Ma
by U-Pb work on zircons; and, most interesting of all, dates of 4408
8Mahave
been obtained for some detrital zircons from the Jack Hills region of the Yilgarn
block in the south of Western Australia (the source of the zircons is unknown).
The conclusion to be drawn from all these ancient rocks is that, although they
±
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