Geology Reference
In-Depth Information
Time before present/Ga
4.5
4.0
3.0
2.0
1.0
0.0
t
0.720
Event that fractionates Rb/Sr
s
r
(b)
t
s
0.715
r
q
p
p
0.710
87
Rb/
86
Sr
pm
0.705
0.7045
G-I
dm
n
m
0.699
(a)
Primitive mantle growth curve
Continental crust growth curve
Igneous rock growth curves
Time
dm
0.5140
Event fractionating Sm/Nd
pm
0.5126
Solid
residue
0.5120
q
r
s
t
p
Intrusive
igneous
rocks
0.5100
n
0.5080
m
0.5068
(c)
0.5060
4.5
4.0
3.0
0 .0
2.0
1.0
Time before present / Ga
Figure 10.6
(a) Sketch of a 'growth diagram' for the Rb-Sr isotope system, with geological time progressing from left to right.
A mantle melting event
n
leads to the generation of continental crust with higher Rb/Sr (providing steeper
87
Sr/
86
Sr growth),
leaving a volume of depleted mantle with lower Rb/Sr. This continental crust may undergo subsequent partial melting at
p
, and
fractionation of the melt formed leads to intrusions with a range of Rb/Sr ratios.
pm
and
dm
represent the present-day
87
Sr/
86
Sr
ratios for
primitive mantle
and
depleted mantle
as defined in the text. The cross marked 'G-I' shows for reference the initial ratio and
age obtained for the Grønnedal-Íka complex (Figure 10.5b). (b) Cartoon showing the isochron that would be obtained for
intrusive rocks
r, s
and
t
in Figure 10.5a. (c) The
143
Nd/
144
Nd growth curves for the sequence of events envisaged in Figure 10.5a.
partial melting of the mantle would have had a Rb/Sr
ratio
higher
than the mantle value. The higher Rb/Sr in
continental crust formed by mantle melting causes its
87
Sr/
86
Sr ratio to increase with time at a faster rate than
in the mantle: segment
n-p
in Figure 10.6a represents
the steeper evolution of
87
Sr/
86
Sr in typical continental
crust, formed by a hypothetical mantle melting event
at 2.5 Ga before the present (BP).
What happens in the mantle during this time? Regions
of the mantle unaffected by this (or subsequent) melting
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