The chemistry of natural waters - Geochemistry: An Introduction - page 150

50

Seawater

0

Tropical

Canaries

-50

The Geysers

Temperate

-100

Steamboat Spring

-150

Polar

Yellowstone

-200
-25

-20 -15 -10 -5 0 5 10

18 O (per mil)

δ

Figure 7.10

18 O) in meteoric
waters (solid line). Liquid precipitation depletes the atmospheric water vapor in heavy isotopes as
the clouds move poleward. Most groundwaters plot along the meteoric line (

Relationship between the isotopic concentration of hydrogen ( δ D) and oxygen ( δ

18 O

10)
demonstrating their meteoric origin and negligible interaction with minerals in the first kilometer
of the crust. Many geothermal springs (dotted lines) have the same

δ D = 8 δ +

Daslocalrainwater.As
there is practically no hydrogen in the crust, this indicates their meteoric origin, with the

δ

18 O
variations reflecting exchange with crustal mineral oxygen at temperatures of about 75-350 ◦ C.

δ

The mineral contribution of sea spray to rain water smoothly decreases with distance
from the coast. This gradient involves Na + and Cl − ions above all, the readily identifiable
so-called cyclical ions. The remainder of the water infiltrates or runs off, contributing to
erosion.
Chemical erosion (weathering) can be formulated as a set of dissolution reactions.
Silicates formed at high temperature in igneous or metamorphic environments (olivine,
pyroxenes, feldspars, micas) are metastable in the low-temperature conditions of the sur-
face. They react with precipitation to form clay minerals, that are the stable form of
silicates, hydroxides of the most insoluble ions in their oxidized form (Fe 3 + ,Al 3 + ), while
the most soluble ions (Na + ,K + ,Ca 2 + ,Mg 2 + ) are released in the run-off and river water.
Silica is scarcely soluble and quartz from any origin is largely left untouched by ero-
sion, but the level of [SiO 2 ] in rivers is comparable to the concentration of other major
elements.
It is kinetic barriers that preserve high-temperature mineral assemblages such as those
of granite and gneiss over very long geological times at the Earth's surface, whereas equi-
librium thermodynamics implies that feldspars and many other silicates should have turned
into clay. Erosion studies must therefore pay due attention to dissolution rates. Most ero-
sion processes are dissolution/reprecipitation reactions resembling the albite destruction
described by (7.22) . Normally, dissolution is the rate-limiting step, and the nature of dis-
solved cations is not critical in controlling the reaction rate, with the notable exception