Geoscience Reference
In-Depth Information
Table 6.4 An informal compilation of closure temperatures
Closure temperature ( C)
Mineral
Potassium-argon
Hornblende
530
±
40
Biotite
280
±
40
Muscovite
350
Microcline
130 ± 15 (plateau segment)
110 (0% release intercept)
Uranium-lead
Zircon
> 1000
Monazite
> 650
Sphene
> 600
Allanite
> 600
Apatite
∼350
Rubidium-strontium
Biotite
300
Muscovite
> 400-500
Apatite, feldspar
∼350
Fission tracks
Zircon
175-225
Sphene
290 ± 40
Apatite
105
±
10
Source : After Ghent et al .(1988).
where T c is the closure temperature,
the characteristic dimension of the crystal,
A a geometrical factor that depends on the geometry of the system and
α
t T = T c
the rate of change of temperature at the closure temperature (the slope of the
upper graph at T c ). The solution of this equation can be found only by substitu-
tion; it cannot be inverted to give an explicit relation between T c and the other
variables.
To interpret the isotopic systematics of a mineral, all of these factors have to be
taken into account, which is not always an easy matter. Isotope measurements can
be made very accurately on very small samples. Advancing technology means
that, whereas earliest measurements had to be made using 'whole-rock' samples,
now individual mineral grains and even regions within mineral grains can be
used. 7 In this way it has become possible to attempt to unravel the timing of the
metamorphic history of a rock. In the following sections, we discuss ways to
avoid some of the problems, and how to use some of them to obtain information
T
/∂
7
The SHRIMP permits isotope measurements to be made on very small zones within individual
minerals and so can aid determination of the metamorphic history of the mineral.
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