Geoscience Reference
In-Depth Information
Geochemistry is walking down the last stretch of a long and difficult road. Most of the
observations that once were controversial are now largely piecing together into a unified
model. Some issues remain, but, yes, our understanding of the deep Earth looks certainly
much better than a few decades ago.
11.4 The growth of continental crust
The extraction of the substance of continents from the mantle is an essential process in the
evolution of the planet. It concentrates major elements, such as Si, Al, Na, P, and incom-
patible trace elements, such as Rb, Ba, the light rare-earths, and the radioactive elements,
U, Th, and K, almost irreversibly near the surface. These elements share the characteristic
of occurring in greater concentrations in melts than compatible elements. Proof that the
continents are being permanently extracted from the mantle and that continental material
is also being injected back into the mantle comes from isotopes. If we plot the evolution
of certain isotopic ratios, and especially the
143
Nd/
144
Nd ratio, against the age of forma-
tion of rocks, we generally obtain a graph such as in
Fig. 11.16
.
If the continents and the
mantle had separated out since the beginning of geological time and had remained isolated
from one another, the slope of the straight lines
p
c
≈
0.14 and
p
m
≈
0.22 would reflect
Basalts from
the upper
mantle
(
p
m
=
0.22)
(0.25)
(0.12)
Granites from
the remelting of
continental crust
(
p
c
=
0.14)
Today
Time
Figure 11.16
Isotopic evolution of Nd over time. The solid lines show the evolution of the
143
Nd
144
Nd ratio in
mantle (basalt) and crust (granite) samples of different ages, the ratio being evaluated for the
time the rocks were formed. The slopes of these two straight lines yield apparent
147
Sm
/
144
Nd
/
ratios of the two geologic reservoirs. Conversely, the dashed lines show the changing
143
Nd
144
Nd ratios observed in
samples of present-day mantle and crust. The difference in the slopes of the two curves over time
is proof that the mantle and crust exchange material. It reflects the crustal growth phenomenon
from the mantle and the recycling of continental crust back into the mantle.
144
Nd ratio recalculated over time using
147
Sm
144
Nd and
143
Nd
/
/
/
