Geoscience Reference
In-Depth Information
and this mineral does not precipitate until a late stage in the magmatic differentiation. Alu-
minum is therefore useful in assessing the depth of differentiation of basaltic series. In
mid-ocean ridge and continental flood basalts, Al concentrations do not vary much with
fractionation because they are buffered by plagioclase removal: these lavas are differenti-
ated at low pressure in the plagioclase stability field. In contrast, Al concentrations increase
steadily with fractionation of Hawaiian basalts: these rocks evolve at higher pressure in
the absence of plagioclase. Typical concentrations of Al 2 O 3 in basaltic and granitic melts
average 15 wt %.
Aluminum solubility in hydrous fluids is low, except at very high temperature and high
pH. This solubility is controlled by the stability of different complexes with the hydroxyl
ion OH and the solubility of clay minerals in equilibrium with the solution, e.g. kaolinite.
The minimum in solubility at pH
6 reflects the amphoteric character of this element. Dur-
ing weathering, Al solubility is controlled by kaolinite reactions of alkalinity production
(7.22 ). The destruction of feldspars by CO 2 -rich fresh water leaves Al in clays: this element
is most efficiently transported by rivers to the sea in the suspended load. In seawater, Al sol-
ubility is controlled by the input of airborne clay particles transported from deserts, which
gives Al distribution in the water column an unusual profile with concentration decreasing
down the thermocline ( Fig. 7.15 ) .
13.3 Potassium
Most common form: K +
Ionic radius: 1.51 Å (octahedral) and 1.64 Å (dodecahedral)
Stable isotopes: 39 (93.26%), 40 (0.011%), 41 (6.73%)
Atomic weight: 39.098
Long-lived isotope: 40 ( T 2
10 9 a)
=
1.25
×
Condensation temperature: 1000 K
No significant complex in waters
Residence time in seawater: 12
10 6 years
×
Potassium is an alkali element, i.e. both volatile and lithophile. Its concentration
in the Earth is therefore poorly constrained. As for other volatile elements, K is depleted in
the Earth with respect to carbonaceous chondrites and enriched with respect to Mars and
the Moon. The dual decay of the radioactive isotope 40 K into either 40 Ca by
β emission
or 40 Ar by electron capture makes this element, after U and Th, one of the three significant
sources of heat in the Earth and accounts for about 20% of the radioactive heat production.
As discussed in Chapter 11 , it is not known whether K enters into the core.
The mineral phases that host K in the mantle are not well understood. Under upper
mantle conditions, traces of high-K mineral phases, notably the Mg-rich mica phlogopite
and occasionally alkali feldspar, may be present in those parts of the mantle that have
been contaminated by subducted sediments or by fluids produced by the dehydration of
the subducted oceanic crust. In the lower mantle, K may at least in part reside in the
mineral hollandite, a high-pressure equivalent of alkali feldspar. The prime K repositories
 
 
Search WWH ::




Custom Search