Geoscience Reference
In-Depth Information
Erosion
Continents
Ocean
Sediments
Magmatism
(basalts and
andesites)
Subduction
Figure 11.19
Crustal composition is the net result of an input - crustal growth out of magmas erupted from the
mantle - and an ouput - erosion and subduction of sediments and oceanic crust. The composition
of the continental crust does not therefore reflect that of any magmatism feeding it.
12
Modern
depleted
mantle
8
4
0
0
1.0
2.0
3.0
4.0
Age (Ga)
Figure 11.20
Isotopic evolution of neodymium (shown in units of
ε
Nd
) in rock from the mantle over geologic
time. The slope of the curve of evolution is proportional to the Sm/Nd ratio. The existence of
radiogenic Nd 3.8 Gy ago suggests that the Earth probably underwent intense fractionation of this
ratio between 4.5 and 3.8 Gy ago (probably during its primordial differentiation). The Sm/Nd
ratio remained low up to 2 Gy ago, then increased to its present-day value. Data compiled by
it can be seen that these compositions varied little, with slightly positive
ε
Nd
(
T
) and
ally continuous growth of the radiogenic character of Nd and Hf from 2.0 billion years
to the present. From the definition of the notation of
given by
(4.11)
and of evolution
given by
(4.33)
,
the plot of
Fig. 11.20
is very similar to that of
Fig. 4.12
:
foraclosed
system, the slope of the curve is proportional to the
147
Sm/
144
Nd ratio of the source man-
tle. As the oldest known basaltic rocks (3.85 Ga from west Greenland) already contain
radiogenic Nd, it can be deduced that the
147
Sm/
144
Nd ratio of their source mantle at
the very beginning of the Earth's history (
ε
4 Ga or Hadean) was higher than the chon-
dritic ratio. Rocks rich in incompatible elements were therefore extracted from the Earth's
mantle very early in its history, probably forming a continental proto-crust. This proto-
crust was since reworked into more recent crust or engulfed by subduction into the lower
mantle.
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