Geology Reference
In-Depth Information
Radioactive alkali metals
(Box 8.2). Heating such minerals leads to dehydration
(Figure 2.3), releasing H 2 O-rich fluids that are essential
agents in regional metamorphism. The presence of
hydrous fluid also lowers the temperature (the sol-
idus) at which a rock begins to melt, and such fluids
therefore play an important role in bringing about
melting in the mantle and crust above subduction
zones, where the hydrous fluid originates by dehydr-
ation of hydrous minerals in the altered oceanic crust
of the downgoing slab.
Water, as well as accelerating many geochemical
reactions (Chapter  3), exerts a powerful influence on
the physical properties of melts (Box  8.3). The expl-
osive expansion of steam escaping from ascending
magma, as it experiences depressurization close to the
surface, provides the energy for the most destructive
volcanic eruptions on Earth.
The isotopes of hydrogen are discussed in Chapter 10.
Two of the alkali metals have radioactive isotopes of
great significance to the geoscientist: 40 K and 87 Rb. Both
have long half-lives (1.25 × 10 9 and 48.8 × 10 9 years
respectively), of similar order of magnitude to the age
of the Earth (4.55 × 10 9 years). The amounts of these
isotopes in the Earth are decreasing with time, but too
little time has elapsed since the last episode of element-
formation (Chapter 11) for them to have decayed away
completely. The rates of decay are accurately known
from laboratory measurement, and 40 K and 87 Rb pro-
vide isotopic clocks that can be used for dating geo-
logical events (Chapter 10).
Radioactive decay generates heat (Box  11.2). Much
of the heat currently escaping from the Earth's interior
is due to the decay of the radioactive isotopes of potas-
sium (K), thorium (Th) and uranium (U) in the crust
and mantle (Box 10.1). 40 K is thought to be responsible
for about 15% of the heat generated in the crust. Owing
to its low abundance and slow rate of decay (long half-
life), 87 Rb does not make a significant contribution to
the Earth's heat flow.
Alkaline earth metals
Relations between the divalent alkaline earth elements
bear some resemblance to those between the alkali
metals. Beryllium (Be), like lithium, has rather different
chemical properties from the other mem-
bers of its group (Box 9.2). Magnesium (Mg)
and calcium (Ca) are major elements in most
rock types, whereas strontium (Sr) and bar-
ium (Ba) are trace elements showing incom-
patible behaviour. All are strongly
electropositive, reactive metals (Figure 6.3).
The alkaline earths form stable, highly
refractory, basic oxides (Box 8.1).
The ionic radius of Mg 2+ is similar to that
of Fe 2+ (Box  7.2). Ferromagnesian minerals such as
olivine and pyroxene exhibit complete solid solution
between a magnesian end-member (such as forsterite,
Mg 2 SiO 4 ) and a ferrous end-member (fayalite, Fe 2 SiO 4 ).
Because MgO is a more refractory oxide (melting point
2800 °C), the Mg end-member of these minerals has the
higher melting temperature (e.g. Box 2.4). The crystal-
lization of ferromagnesian minerals from a magma
depletes it in MgO more rapidly than FeO, so the
FeO:MgO ratio of the residual melt increases with
advancing crystallization and provides a useful indic-
ator of the degree of fractionation of a magma.
Magnesium is an essential constituent of chlorophyll
(Box 9.5), and therefore plays a part in photosynthesis.
The position of hydrogen (H) in Group I of the Periodic
Table suggests that it should behave like the alkali met-
als, but in fact the similarity is restricted to
valency. The ionization energy of hydrogen
is much higher than the alkali metals
(Figure  6.1b) and, because the valence shell
consists of the 1 s orbital alone, hydrogen
possesses no metallic properties (except
under extreme conditions, such as within
the interior of Jupiter).
The predominant hydrogen compound on
Earth is the oxide H 2 O, which occurs in the familiar gas
(vapour), liquid and solid forms (Figure  2.2.1b). The
presence of liquid-water oceans on the Earth's surface
is unique in the Solar System, and has been essential to
the evolution of life on Earth (Chapter 11). The pecul-
iarities of the ice-water system play an important part
in regulating the Earth's climate (Box 4.1) and in driv-
ing erosion and the transport of sediment.
Within the Earth, hydrogen occurs in rocks, mainly
in the form of OH - ions in hydrous minerals such as
muscovite [KAl 3 Si 3 O 10 (OH) 2 ] and the clay minerals
K Ca
Rb Sr
Rb Sr
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