Geoscience Reference
In-Depth Information
0.80
0.79
0.78
0.77
0.76
0.75
0.74
Carbonaceous chondrites
Ordinary chondrites
0.73
0.72
Enstatite chondrites
Earth
0.71
0.70
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
87 Rb/ 86 Sr
Figure 12.9
The loss of volatile Rb by the Earth. Comparison of the Earth's position with respect to chondrites
in the 87 Rb/ 86 Sr- 87 Sr/ 86 Sr isochron diagram. The 87 Sr/ 86 Sr ratio in the Earth is an average of
mantle and crust values. In spite of the scatter created by low-temperature processes on the
parent bodies of chondrites, terrestrial strontium is much less radiogenic than Sr in chondrites and
in the solar photosphere. Such a substantial deficit of Rb with respect to the more refractory Sr is a
strong indication that the Earth is strongly depleted in all volatiles.
12.4 The composition of the Earth and its core,
and the origin of seawater
The very difficult question of the Earth's composition has puzzled the geological com-
munity for over 40 years. Assembling the elemental inventory of the continental crust,
the mantle, and the core is not an easy task since we have no robust estimate for the
composition of the lower mantle and the core. We will see later that the Earth is essen-
tially contemporaneous with the other planetary bodies of the Solar System for which
we have samples: the Moon, the parent bodies of meteorites, and Mars. The major deep-
seated fractionation processes, particularly the segregation of iron and nickel to form the
core, which enable our planet to have a magnetic field, or the transfer of magmas to the
surface, produced a strongly layered solid Earth. Closer to the surface, weathering, sedi-
mentation, and metamorphism are other remarkable processes of chemical differentiation.
The intense processing and the geochemical variety of terrestrial samples accessible to
observation together with the existence of inaccessible domains in the deep Earth, such
as the lower mantle and core, do not allow us to build up a verifiable picture of the mean
composition of the Earth.
It is thought that the composition of the bulk Earth must resemble that of the least differ-
entiated bodies of the Solar System. The Sun and carbonaceous chondrites CI, of which one
of the best known representatives is the Orgueil meteorite, have compositions that differ
 
 
Search WWH ::




Custom Search