Geoscience Reference
In-Depth Information
1.5 The different reservoirs and their compositions
In solids, elements are sequestered in minerals, minerals assemble as rocks, and rocks
constitute the main geodynamic units of the mantle and the crust. Some elements occur
in particularly large abundances in seawater or in the atmosphere (H, O, N, Ar). Terres-
trial rocks fall into three main categories: igneous rocks, such as basalts and granites,
produced by magmatic processes, i.e. from the melting of rocks; sedimentary rocks,
formed by the accumulation of clastic and biological particles or by chemical precipi-
tation on the floors of the oceans and other bodies of water; and metamorphic rocks,
produced by “firing” existing rocks at high temperature, under pressure, and, for most
of them, in the presence of aqueous or carbonic fluids. Extra-terrestrial rocks cannot
be classified in quite the same way. Igneous rocks occur on the Moon, most probably
on Mars, and as meteorites (achondrites). Another type of meteorite, known as a chon-
drite, has no terrestrial equivalent. Chondrites were formed by condensation of gases from
the solar nebula and by droplets of silicate liquids called chondrules, from which their
name is derived. Metamorphic transformations may affect planetary rocks and meteorites
as well.
A reservoir is a loosely defined term referring to a very large body of rock (mantle,
crust), water (ocean), or gas (atmosphere) whose mean composition stands in sharp con-
trast to the composition of other reservoirs. It may contain a variety of components, but
its composition is normally very different from the composition of components present in
other reservoirs: for instance, the Si-rich rocks making up the continental-crust reservoir
are easily differentiated from the Mg-rich rocks making up the mantle. A reservoir may be
spatially continuous (e.g. the ocean) or scattered over large distances in the Earth (e.g. recy-
cled oceanic crust in the deep mantle). The most abundant chemical elements in each of the
main terrestrial reservoirs are listed in
Appendix A
.
However, a clearer understanding of
these chemical distributions requires some idea of which minerals contain these elements
(see next chapter). Oxygen is found just about everywhere, whereas silicon is confined to
the silicates, which are by far the most abundant minerals. The silicon content of minerals
is a particularly significant parameter because the SiO
2
(silica) concentration is a measure
of the “acidity” of rocks: this obsolete term, which dates from the days when silicates were
viewed as silicic acid salts, still pervades the literature. The silica concentration of miner-
als increases from olivine, pyroxene, amphibole, and mica through to feldspar and quartz.
Magnesium and iron are particularly abundant in olivine, in pyroxene of igneous rocks,
in amphibole, and in the sheet minerals (biotite, chlorite, and serpentine) of metamorphic
rocks. A felsic (acidic) rock, such as a granite or a rhyolite, is rich in silicon and poor in
magnesium and iron; a mafic rock, the archetype of which is basalt, has a high Mg and
Fe content. Calcium is found, above all, in igneous pyroxene and calcitic feldspar (pla-
gioclase), in sedimentary carbonate, and in metamorphic amphibole. Aluminum has many
carriers: in igneous rocks it is located, by increasing order of pressure, in plagioclase, spinel
(oxides), and garnet; it concentrates in the clay minerals of sedimentary rocks and in mica
of metamorphic rocks (biotite and muscovite). Potassium and sodium are scarcely found
outside mica and feldspar.
