Geoscience Reference
In-Depth Information
In silicates, the Na
+
ion normally occupies octahedral sites. It is a moderately incompati-
ble element during mantle melting and the early stages of basalt differentiation. Plagioclase
saturation in basalts differentiated at low pressure such as mid-ocean ridge and continental
flood basalts (see
Section 10.2
)
and in more felsic melts turns Na into a moderately compat-
ible element. Basalts typically contain 2-3 wt% Na
2
O and granitic melts 3-4 wt % Na
2
O.
Although albite may be stable under near-surface conditions, it rarely reaches saturation in
hydrous fluids at ambient temperature. Authigenic albite is known in sediments, but is not
ubiquitous. Sodium is therefore preferentially leached by low-temperature hydrothermal
fluids in contrast to K, which dominates in higher-temperature fluids. Alteration of sea-
floor basalts by warm percolating fluids rich in sodium and silica leads to the replacement
of calcic plagioclase (anorthite) by albite:
6Na
+
+
3Ca
2
+
3CaAl
2
Si
2
O
8
+
12SiO
2
⇔
6NaAlSi
3
O
8
+
Basalts altered on the sea floor are therefore notably enriched in Si and Na with respect
to their unaltered precursor. There is a stark contrast between K and Na behavior under
surface conditions imposed by the lack of a major stable low-temperature Na-rich phase.
Sodium ions set free by the weathering of feldspars are transported by rivers into the sea in
the dissolved load. Sodium is the second most abundant element in seawater and the most
abundant cation. Its concentration is limited mostly by interaction of seawater with the sea
floor through diagenetic and hydrothermal fluids. Its concentration is constant throughout
the water column. In warm landlocked or lagoonar environments, NaCl saturation may be
reached and evaporitic halite becomes a prominent sedimentary deposit.
13.5 Magnesium
Most common form: Mg
2
+
Ionic radius: 0.72 Å (octahedral)
Stable isotopes: 24 (78.99%), 25 (10.00%), 26 (11.01%)
Atomic weight: 24.305
Condensation temperature: 1340 K
Complexes in water: hydroxides, carbonates, sulfates
Reactions limiting solubility in water:
Ca
2
+
+
Mg
2
+
+
2CO
2
3
CaMg(CO
3
)
2
⇔
(
−
log
K
=
1.70) (dolomite)
10
6
years
Residence time in seawater: 13
×
Magnesium is a refractory and lithophile alkaline-earth element. It does not enter core
composition in substantial concentration. After oxygen, magnesium is the second most
abundant element in the mantle where it is hosted in most major minerals (olivine,
pyroxenes, garnet, spinel, ringwoodite, etc.). Its concentration in the average crust is rela-
tively low: as a consequence, amphiboles, micas, Mg-rich clays (smectite), and carbonates
(dolomite) normally remain minor mineral phases. Magnesium-rich calcite is a major form
of carbonate precipitated by marine organisms. After Na, Mg is the second most abundant
cation of seawater in which it is partly complexed by carbonate and sulfate ions.
