Jökull - 01.01.2010, Page 176

A. Stefánsson
of reaction. Under these conditions, the mass of smec-
tites formed was insignificant and zeolites were found
to be unstable. This resulted in apparent high mobil-
ity of Mg2+, Ca2+ and Fe2+. The carbonates formed
under these conditions were predominantly siderite,
magnesite and Fe-Mg carbonate solid solutions. With
increasing pH, Ca-Mg-Fe containing smectites and
Ca-Na containing zeolites become increasingly im-
portant, reducing the mobility of Mg and Fe. This
lack of Mg and Fe in solution resulted in calcite be-
ing the stable carbonate at pH >8. This pattern is
in good agreement with natural carbonate mineralogy
under elevated and depleted CO2 conditions (Neuhoff
et al., 2006; Rogers et al., 2006; Gysi and Stefánsson,
2010).
The well known zeolite zonation with depth has
been interpreted in terms of formation temperatures
caused by variable burial depth. The reaction path
calculations of basaltic glass alteration failed to some
degree in simulating the very detailed compositional
variations of zeolites observed in nature. Usually, a
given zeolite pair was formed independent of the ex-
tent of reaction and temperature. However, some pre-
liminary calculations indicated that varying the basalt
composition, different zeolite pairs may be formed. In
addition, small changes in the clay solid solution com-
position were found to change the zeolite assemblage
formed. The results are not presented in detail here
as they are more speculative and much more work is
needed; however, they suggest that primary rock com-
position, incongruent dissolution of primary minerals
of basalts and composition of phyllosilicates, as well
as temperature, may play important roles in determin-
ing the composition of zeolites. The above factors in
turn are again dependent on acid supply and extent of
reaction.
The conclusions that may be drawn from the
above geochemical model calculations are that the ob-
served low-temperature secondary mineralogy is de-
termined predominantly by the pH of the water inter-
acting with the basalt and extent of reaction. Tem-
perature is probably less important than the other two
factors. The pH of the water, on the other hand, is de-
termined by the nature and concentration of acid into
the system, the respective ionization constants and the
extent of reaction that is reflected by increased reac-
tion time. At a pH above 8, the well known mineral
sequence of celadonite, simple oxides (chalcedony)
and oxyhydroxides forming first, followed by Ca-Mg-
Fe smectites, chlorites and eventually zeolites is ob-
served at all temperatures between 50 and 150◦C. The
mass of carbonates formed depend on the CO2 con-
centration of the system.
The overall reaction pattern of basalt alteration
and the associated elemental mobilities are summa-
rized in Figure 7. The reactions and secondary min-
eral assemblages formed depended primarily on the
water pH. The pH in turn was controlled by the nature
of the acid (strong H2SO4 or weak CO2) and extent
of reaction. Three categories may be defined:
(1) Low pH H2SO4 alteration (pH <4) dominated
by amorphous silica, kaolinite, Al and Fe oxyhydrox-
ides and sulphur-containing minerals
(2) Mildly acid CO2 alteration (pH 5–7) domi-
nated by kaolinite, chalcedony, Ca-Mg-Fe smectites
and Mg-Fe carbonates
(3) Alkaline water alteration (pH >8) dominated,
in order of appearance with extent of reaction, by
chalcedony and celadonite, Ca-Mg-Fe smectites, ze-
olites and calcite.
Reaction path modelling and natural low-
temperature geothermal waters
The effect of temperature, pH and extent of reaction
on basalt alteration may be analysed by comparison
with the results of the geochemical model calcula-
tions with natural low-temperature water. The water-
rock reaction can be viewed as an acid-base titration
whereas the dissolution of basalt results in consump-
tion of the acid (H+) and release of the cations. This
results in changes in the cation to proton ratio with in-
creased extent of reaction as well as an increase in the
pH value of the water. In the calculations, chemical
equilibrium was assumed between the water and the
secondary minerals formed to control the composition
of the solution. It follows that the slope of the cation
to proton ratio as a function of pH is controlled by the
mineral saturation and the overall reaction stoichiom-
etry of the basalt alteration reaction under particular
conditions.
176 JÖKULL No. 60