Geoscience Reference
In-Depth Information
10
Ca
Nb
Eu
U
W
Hf
Ti
Nd
Yb
Ni
Co
Mo
Fe
Re
Cr
Zr
Ir
La
Ce
Al
Sc
Lu
Th
Os
Pt
Au
V
Mg
Si
Er
Sm
P
1
Mn
Refractory elements
Li
As
Cu
Na
Sb
K
Ge
Ga
Ag
F
Rb
Sn
Se
Zn
0.1
Volatile elements
S
Cl
0.01
1800
1600
1400
1200
1000
800
600
50% Condensation temperature (K)
Figure 12.11
Depletion of the Bulk Earth with respect to the CI carbonaceous chondrites. For most elements
except H and He, these chondrites have a composition similar to that of the Sun and therefore
represent the most primitive solid material in the Solar System. The
x
-axis represents the
temperature at which 50% of the initial element inventory has been taken up by solid phases in a
standard condensation model. The Earth is strongly depleted in the most volatile elements such
as K, Zn, S. Refractory elements such as Zr, La, Th, in contrast, have not been lost. They are slightly
more abundant in the Earth than in CI because of volatile element depletion.
is close to the composition assumed for the primitive mantle. The mantle and chondrites
each define a trend. It is proposed that one trend corresponds to planetary petrological dif-
ferentiation and the other to cosmochemical differentiation (fractional condensation) in the
proto-planetary nebula. If the primitive material of the Earth belongs to the chondrite fam-
ily it should lie at the intersection of these two alignments. Of course, we have no evidence
that the alignment of mantle compositions amenable to sampling passes through the prim-
itive mantle. If a significant reservoir of one of these elements is not correctly identified,
the evaluation is incorrect. This is probably the case for the core: its density, estimated
from the speeds of elastic waves, implies that it contains, in addition to Fe and Ni, light
elements such as C and S, and maybe even a substantial fraction of Si, which biases the
compositional models based on mass balance. In spite of these limitations, this method is
still the most reasonable to date.
Once the composition of the primitive mantle is obtained, the terrestrial concentration
of the lithophile elements - which do not contribute to core composition - can be eas-
ily evaluated. It then becomes apparent (
Fig. 12.11
) that the Earth is depleted in the most
volatile elements, such as K and Na. By comparison with ordinary chondrites, which also
went through intense devolatilization, and using as a guide a scale of condensation tem-
peratures, such as the temperatures obtained from the models described in the previous
section, we can infer with some confidence the composition of our planet. To the best
