Geoscience Reference
In-Depth Information
(a)
(b)
0.760
0.900
0.740
0.800
0.720
intercept
0.7009 = 0.0011
A
0.700
0.700
O
0
1
2
3
4
87
Rb
86
Sr
0
4000
2000
Age (Ma)
Figure 6.2.
(a) A whole-rock rubidium-strontium isochron for the Amits oq gneiss of
the Godthaab district of western Greenland. These rocks are some of the oldest
found on the Earth. They were originally granite before undergoing metamorphism
to gneiss. The age of 3660 Ma determined by this method probably represents the
time of metamorphism. The original granite was therefore older. (After Moorbath
et al.
(1972).) (b) Strontium isotope growth curves for the mantle and crust. The
heavy line OA is the mantle growth curve. Crustal growth curves are illustrated for
rocks extracted from the mantle 3000 and 500 Ma ago. Gross rubidium-strontium
ratios for crustal rocks are 0.10 (solid lines) and 0.50 (dashed lines).
87
Sr from the decay of
87
Rb. These ratios are consistent with a gross rubidium-
strontium ratio for the mantle of about 0.03. The line OA on Fig. 6.2(b) is known
as the
mantle growth curve
. Melts, now forming crustal rocks, that were extracted
from mantle can be assumed to have the same initial strontium isotope ratio as the
mantle at the time of their extraction. Because the gross rubidium-strontium ratio
for crustal rocks is greater than that for the mantle (generally between 0.05 and
1.0, compared with 0.03), the rate of increase of the strontium isotope ratio for
crustal rocks, the
crustal growth curve
,ismuch steeper than the mantle growth
curve. Equation (6.30) enables us to calculate crustal growth curves, given the
gross rubidium-strontium ratio and assuming approximate isotopic proportions.
Figure 6.2(b) shows crustal growth curves for rocks derived from the mantle at
3000 and 500 Ma with rubidium-strontium ratios of 0.10 and 0.5. The strontium
isotope ratio measured today for these rocks would therefore be considerably
more than 0.704.
If a crustal rock is remelted, then the subsequent rock has an initial
87
Sr
86
Sr
/
ratio corresponding to the
87
Sr
86
Sr ratio of the source rock at the time of melting.
Clearly this will almost always be considerably
greater
than 0.704, often greater
than 0.710. However, rocks derived directly from the mantle have an initial ratio of
/
