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increase with time, while a closed-system playa with de-
clining water volumes, as implied by a drying climate,
will increase the concentration of solutes but leave the
mass constant (Yechieli and Wood, 2002). Other fac-
tors influencing precipitation include the catchment area,
pan surface area and the presence of aquatic vegetation
(Russell, 2008).
(a)
Water table
(b)
Evaporation
15.2.2
Geochemical processes and
mineral precipitation
No evaporites
Water table
Increasing near-surface salinity, resulting from either cli-
matic or hydrological factors, can result in evolution from
clay-floored pans to salt pans containing evaporites. As
already noted, the chemistry of inflowing water is largely
dependent on solute output from the catchment; the sub-
sequent evolution of evaporites will be dependent on the
ratios of solutes present and the precipitation gradient of
the salts involved. From an understanding of fractional
crystallisation of mineral phases resulting from the evap-
oration of seawater (e.g. a sequence with increased evapo-
ration of calcite, anhydrite/gypsum and halite followed by
epsomite, sylvite, kainite, carnellite and borates/celestite;
see Valyashko, 1972), we can gain some idea of the rel-
ative type and proportion of evaporite phases that may
be present in playa basins. However, given their extreme
chemical variability, most nonmarine saline waters do not
follow this template. Hardie, Smoot and Eugster (1978)
identified four main brine types resulting from a series of
evaporative concentration steps on undersaturated inflow
for nonmarine brines, a model that has been subsequently
modified by others (see Jankowski and Jacobson, 1989;
Rosen, 1994). These represent an accepted set of geo-
chemical pathways along which most nonmarine evapor-
ites develop within playa basins (Figure 15.8(a)).
The three pathways outlined by Eugster and Hardie
(1978) use the relationship between the molar content
of bicarbonate (HCO
3
) relative to molar Mg
(c)
Evaporite minerals
Evaporation
Capillary fringe zone
Water table
(d)
Evaporation
Subaqueous and
displacive evaporites
Figure 15.7
Relationship of sustained groundwater flow to
evaporite mineral accumulation to all possible groundwater
configurations (modified after Rosen, 1994 and Reynolds
et al.
,
2007). (a) When the water table is far below the ground surface
only flow through conditions can occur and no evaporites will
accumulate. (b) When water is transported or precipitated in
this type of situation the depression acts as a recharge zone
and leaks water to the subsurface. No significant evaporites will
accumulate, although a thin crust may develop when the final
solution evaporates to dryness. This crust would likely be de-
flated. (c) When the water table intersects the ground surface
in a hydrologically closed basin (a playa situation), displacive
evaporites may form, but significant accumulations of subaque-
ous evaporites cannot. Those that can accumulate after a rain
event will likely be deflated when the lake dries out. However,
significant accumulations of displaced evaporites may occur.
(d) Only when the groundwater table is above the surface of
the deepest part of a closed basin playa, so that groundwater
input is constant, can subaqueous evaporates accumulate in
a hydrologically closed basin. Although slightly more compli-
cated for throughflow basins, this model also applies to these
Ca for
all inflow waters to determine the ultimate brine type
and mineral assemblage that may result from evapora-
tion. In all, these authors identify five end-member brine
types for nonmarine waters and a number of key min-
eral phases that are associated with their evaporation
(Figure 15.8(b)). In this scheme, initial evaporation and
degassing (Eugster and Kelts, 1983) leads to the pro-
gressive precipitation of low-Mg calcite, aragonite and
high-Mg calcite (protodolomite), followed by gypsum
(CaSO
4
ยท
+
2H
2
O) at concentrations of 40-100 g/L (Bowler,
1986), dependent on the type and duration of processes in
the evaporation zone. Gypsum precipitation is also depen-
