Geoscience Reference
In-Depth Information
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4.1 Gy old zircons from Jack Hills (Australia) crystallized, contained rocks modified by
low-temperature processes, thereby hinting at the early presence of a hydrosphere.
5.2.2 Other applications
The diffusion equation, with or without an advection term, is involved in the description of
many other geochemical processes. A very important case is that of early diagenesis, a set
of reactions occurring a few meters beneath the sea floor whereby mud is transformed into
sediment. An observer in a fixed position at the water-sediment interface “watches” the
sediment sink at the rate of sedimentation and must describe the chemical processes in an
advective context. The diffusion of elements in interstitial water and the reactions between
that water and the sedimented particles control the nature of mineralogical transformations
and the flux back into seawater or diagenetic liquids as the sediments become compacted.
Of particular importance are the reduction reactions of nitrates to nitrogen and of sulfates
to sulfides by which many micro-organisms oxidize sedimented organic matter. These pro-
cesses lie behind the dissolution of manganese nodules and the formation of sedimentary
pyrite (fool's gold), in particular. They will be discussed in more detail in
Chapter 10
.
Diffusion is also involved in the growth of minerals from solutions or magmas: an olivine
crystal that grows from a basalt is surrounded by a boundary layer depleted in the essen-
tial components of this mineral, notably Mg. This layer can often be observed under the
microscope. The diffusion of these components allows re-supply and, consequently, the
continued growth of the mineral. In the mantle, homogenization of compositions at dis-
tances greater than about ten meters is thought to be achieved by mechanical stirring caused
by convection. Conversely, diffusion ensures homogenization over short distances, in the
order of a few centimeters.
5.3 Chromatography
Imagine, on a hot summer day, a parade stretching along a long avenue lined with cafes and
shady terraces. It is likely that some of the participants from time to time will succumb to
the temptation of enjoying a few minutes of refreshment before joining the parade again,
and that some will even take a liking to these halts. We can guess what our parade will
look like after a few hours: the head of the procession will be reduced to the virtuous
and sober participants; while behind, the atmosphere will be enlivened by all those who
have made frequent stop-offs at the terraces. This type of mechanism is the essence of
chromatographic separation.
In analytical chemistry, chromatography is a technique of element separation that
involves the percolation of a liquid (the eluent or mobile phase) through a porous matrix
(the stationary phase composed, for example, of ion-exchange resin) and exchange ele-
ments with this matrix. The variable affinity of the stationary phase for the elements lets
each of them percolate through the matrix with a different velocity, which eventually
