Geoscience Reference
In-Depth Information
When rock melts, some elements (Na, K, Al, Ca, Si) are fusible, whereas others (Mg
and to a lesser extent Fe) are more refractory; magmatic melting therefore contributes to
geochemical fractionation among reservoirs. In the same way, some elements (Na, K, Ca,
Mg) are more soluble in water than others, inducing further geochemical fractionation
during erosion and sedimentation.
When the composition of the mantle is compared with that of the Earth as a whole
it can be seen to have a high refractory-element content, especially of Mg and Cr, and
a lower content of fusible elements, especially Na, K, Al, Ca, and Si, demonstrating its
residual character with regard to melting. As might be expected, olivine and pyroxene
are predominant in the mineralogy of the upper mantle: peridotite is the ubiquitous rock
forming the upper mantle. The continental crust, on the other hand, is enriched in fusible
elements (accommodated mainly in feldspar, quartz, and clay minerals) and exhibits a melt
“liquid” character in contrast to the residual mantle. The oceans are obviously enriched
in soluble Na, K, and Ca cations and anions (Cl ,SO 2 4 ), while elements that are both
insoluble and fusible (Si, Fe, and Al) accumulate in clastic sedimentary rocks (clays).
The composition of the Sun, the Earth's crust and mantle, etc., is given in Appendix A .
It is not always easy to determine these compositions; while observation of the solar spec-
trum and analysis of meteorites, of seawater, and of river water yield data that can be
tabulated fairly directly, determining the composition of the Earth's crust calls for discus-
sion of the nature of the lower crust, the lower mantle, the core, and of the Earth as a whole.
The mechanisms responsible for forming these major, but not directly observable, geolog-
ical reservoirs must therefore be reasonably well understood before their compositions can
be estimated. This will be covered briefly in Chapters 11 and 12 .
1.6 The nucleus and radioactivity
Although the forces holding the nucleus together are extremely powerful, observation
shows that some nuclei are unstable, i.e. radioactive. Radioactivity is a property of
the nucleus and involves gigantic energies of the order of a few MeV per nucleon
(1 eV/molecule
96.5 kJ mol 1 ) ( Table 1.2 ). The tem-
peratures, which are a measurement of atomic and molecular excitation, required to break
a nuclear bond are orders of magnitude higher than those required to exchange electrons
(chemical reactions) or even to remove an electron from an atomic orbital (ionization
energy) and only occur in stellar interiors like that of the Sun. These enormous energies
explain the extreme efficiency of nuclear energy with respect to any other alternative energy
form. Nuclear bonds are not dependent on the atom's electron suite and therefore not on
any chemical reaction or mineralogical phase change which take place at much lower ener-
gies: removing the first electron from an atom typically requires only a few eV per atom
(ionization potential). Mineralogical, temperature, and pressure processes involve even
smaller energies. Radioactivity is therefore independent of the chemical and mineralogical
environment of the element, of temperature and of pressure, which affects only the electron
shells. For example, the rubidium-87 or uranium-238 decay probability per unit of time is
10 19
10 23
=
1.6
×
×
6.022
×
=
 
 
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