Geoscience Reference
In-Depth Information
Whole
rock 4
Whole
rock 3
e
λ
147Sm
T
0
−
1
Garnet 2
e
λ
147Sm
T
1
−
1
Pyroxene 2
Whole
rock 2
Whole
rock 1
143
Nd
144
Nd
0
147
Sm/
144
Nd
Figure 4.9
During a metamorphic event, large samples (whole rock) will remain virtually closed (see
minerals of rock 2 (pyroxene, garnet) will have exchanged their isotopes and their
143
Nd
144
Nd
isotope ratios will be in equilibrium with that of the mean ambient value, i.e. that of the rock.
The “internal” isochron therefore gives the age
T
1
of the disturbance.
/
This pattern of exchange suggests the sampling strategy (
Fig. 4.9
). To determine the age
of any metamorphic perturbation, the minerals of a rock are separated, here garnet and
pyroxene, whose small size ensures that they will have been returned to isotopic equilib-
rium during metamorphism. The age indicated by what is termed the internal (or mineral)
isochron will be that of the metamorphic event. Conversely, to obtain the age of forma-
tion of the rock, the largest possible samples must be taken so as to minimize the effect of
exchanges with the exterior. The whole rock isochron will be obtained from small fractions
of large samples that have been thoroughly ground and mixed.
There are many methods of dating using isochrons. Historically,
87
Rb-
87
Sr chronology
has yielded many ages for the emplacement of granites, whereas the dating of white mica
(muscovite) or black mica (biotite), whose high
87
Rb/
86
Sr ratio ensures good age preci-
sion, have dominated metamorphic geochronology alongside
40
K-
40
Ar ages. The mobility
of rubidium, an alkali element, and of strontium, an alkaline-earth element, in metamor-
phic and hydrothermal fluids unfortunately often disturbs this chronometer.
147
Sm-
143
Nd
chronology is valuable for dating old basaltic rocks. Samarium and neodymium, two of the
rare-earths, are much less mobile, but the method has also been known to go awry. Dat-
ing high-temperature and high-pressure metamorphism, especially with pyroxene-garnet
internal isochrons, has yielded good results by this method, as has, more recently, the
176
Lu-
176
Hf system. Dating of basaltic rocks and peridotites, but also of sulfide ore
deposits and petroleum by the
187
Re-
187
Os method, two highly siderophile elements, has
also proved to be very successful.
