Geology Reference
In-Depth Information
The initial Sr isotope ratio - characterizing
a magma's origins
A geochemist studying an igneous complex like
Grønnedal-Íka (Figure 10.5) is interested not just in its
age
, but also wants information on the
source of the par-
ent magma
from which it formed: was it derived directly
from the mantle, from the continental crust, or by some
process involving both of these potential sources? Here
the initial Sr isotope ratio becomes relevant. To see how,
it helps to rewrite the isochron equation in a simplified
form. Conveniently it happens that the exponential
term
e
(a)
48°5
′
W
48°W
0
1
2km
61°15
′
N
27170
27205
27168
126706
27193
27179
27088
27107
58215
Grønnedal
31863
27074
(
1 can be quite closely approximated sim-
ply by
λ
Rb
t
. To a first approximation, we can also ignore
the slight change of
87
Rb/
86
Sr with time, and therefore
delete the suffix
t
. The resulting simplified equation is:
λ
Rb
−
t
39763A
Nepheline Syenites:
Porphyritic, xenolithic
Upper Series
Lower Series
87
≈
87
+
87
Sr
Sr
Sr
Sr
Rb
Sr
(10.5)
λ
t
mainly
foyaites
Rb
86
86
86
t
0
Carbonatite, etc.
Olivine dolerite dikes
Gneiss
Treating this as a straight-line equation of the form
y
=
c
+
m.x
, we can plot the evolution of
87
Sr/
86
Sr ratio
(e.g. of a magma source region) as a function of time,
61°11
′
N
Fault
Moraine, etc.
λ
.
Figure 10.6a illustrates such a 'Sr growth diagram'.
Point
m
represents the
87
Sr/
86
Sr ratio of the primordial
Earth's mantle 4.55 Ga ago (the value of 0.69900 has
been determined from meteorite studies). The gently
rising line
m-n
represents the 'growth' in radiogenic
87
Sr relative to
86
Sr in the primitive mantle. Mantle
rocks contain much less Rb than Sr (Table 10.2), there-
fore the limited amount of radiogenic
87
Sr formed in
a given time will raise mantle
87
Sr by a only small
amount relative to the
87
Sr already present. The low
gradient between
m
and
n
therefore reflects the low
Rb/Sr ratio (=0.03) of primitive mantle peridotite. If
extended to the present day (thick dashed line), this
Rb/Sr would lead to a mantle
87
Sr/
86
Sr value of around
0.7045, consistent with the average ratio measured in
many young basalts.
But suppose that a parcel of mantle underwent
par-
tial melting
around 2.5 Ga ago (
n
) and, after ascent to
crustal depths, the melt solidified to form rocks broadly
similar in composition to continental crust. These rocks
inherited the
87
Sr/
86
Sr ratio of the mantle source at this
time, but because Rb is more
incompatible
in mantle
minerals than Sr (Figure 9.1.1),
6
the melt formed by
87
Rb
Sr
with a gradient equal to
Rb
86
(b)
0.80
Grønnedal-Íka
0.78
27170
27088
0.76
12670
ε
27168
58315
0.74
27205
27193
0.72
Age 1299
±
17m.y.
Init. ratio 0.7032
±
0.0004
27179
27107
27074
31863
0.70
0
1
2
3
4
87
Rb/
86
Sr
Figure 10.5
(a) Geological map of the Grønnedal-Íka
intrusion, South Greenland, showing the distribution of
recognized petrological units. (b) An
isochron
plot of
Sr isotope data from the Grønnedal-Íka intrusion. The same
symbols are used in (a) and (b) to correlate Rb-Sr compositions
with map units.
5
Note the expanded scale of the
87
Sr/
86
Sr
axis in (b). (Source: Blaxland
et al.
(1978). Reproduced
with permission of the Geological Society of America.)
5
The original age of 1327 Ma calculated by these authors was
based on a
87
Rb decay constant value of 1.39 × 10
−11
year
−1
. This
age has been corrected to 1299 Ma to conform to the currently
accepted decay constant of 1.42 × 10
−11
year
−1
. One discrepant
data point has been omitted for clarity.
6
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