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i o p ere
(cru and upper
mo olid man le)
Man le
ru 0 100 m
ic
eno p ere
Man le
ru
2 00 m
i uid
ore
u er core
5 100 m
Solid
nner core
6 3 m
Fig. 5.1 Cutaway of the Earth. Redrawn from USGS (Watson, 1999)
5.5.1 The chemical composition of the upper continental crust
There are two basic methods employed to determine the composition of the upper
crust (Rudnick, 1995):
•
establishing weighted averages of the compositions of rocks exposed at the sur-
face and,
•
determining averages of the composition of insoluble elements in fine-grained
clastic sedimentary rocks or glacial deposits and using these to infer upper-crust
composition.
The determination of the major element composition of the upper continental
crust relies on the first method. It has been used by a variety of authors starting
with Clarke (1889) and continuing with Ronov and Yaroshevsky (1969), Shaw et al.
(1967, 1976), Eade and Fahring (1973), Condie (1993) and Gao et al. (1998). The
results of these independent studies show a very similar composition for most major
element averages except for rare earths (REE) which show significant differences.
Estimates of the trace element composition of the upper crust rely on the natural
sampling processes of sedimentation and glaciation. This method was suggested by
Goldschmidt (1937, 1954), based upon the fact that glacial clays are compositionally
representative of the crust from which they were derived. Another way of estimating
crustal average compositions is taking advantage of the fact that elements that are
insoluble during weathering are transported from the site of erosion to that of
deposition. The concentration of these elements in sedimentary rocks may provide
robust estimates of the average composition of their source regions. Relevant studies
which analyse the composition of the crust through such methods are: Taylor and
McLennan (1985, 1995), Plank and Langmuir (1998) and McLennan (2001).
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