Jökull - 01.01.2010, Page 172

A. Stefánsson
Figure 3. The results of the geochemical
modelling for pH as a function of extent
of reaction at 100◦C and 1, 10 and 100
mmol/kg initial CO2. – Niðurstöður líkan-
reikninga á samspili vatns og bergs við
pH-gildi vatnsins við 100◦C og 1, 10 og
100 mmol/kg CO2.
Stefánsson (2010). Strongly acid geothermal waters
are commonly formed upon steam-heating of surface
waters and oxidation of H2S to H2SO4 by atmo-
spheric O2. This results in intensive surface leaching
and the mud pots commonly observed at the surface
of active high-temperature geothermal systems. The
results indicate that at pH <4 most elements are mo-
bile with the exception of Si, Al and Fe as well as S,
forming amorphous silica, kaolinite, simple Al and Fe
oxides and hydroxides, sulphates and elemental sul-
phur (Markússon and Stefánsson, 2010).
The effect of temperature at various reaction pro-
gresses on the overall secondary mineralogy and for
waters initially containing 10 mmol/kg CO2 is shown
in Figure 5. At a low reaction progress (ξ = 0.01), the
secondary mineralogy is very dependent on tempera-
ture with chalcedony and kaolinite predominating as
well as carbonates, the mass depending on the initial
CO2 concentration. At 100–120◦C, there is a sharp
change to zeolites and chlorites becoming the domi-
nant Al-Si containing minerals. On the other hand,
at a higher reaction progress (ξ >0.1), the secondary
mineralogy becomes almost independent of tempera-
ture with smectites, zeolites and chlorite being impor-
tant alteration minerals, whereas chalcedony becomes
less important. The composition of phyllosilicates
and the reaction order of Al-Si containing minerals
were also observed to be dependent upon the extent
of reaction at a particular temperature. The first phyl-
losilicate to precipitate was generally celadonite, fol-
lowed by Ca-Mg-Fe rich smectites, chlorite and even-
tually Ca-Na containing zeolites. This was the case
independent of temperatures between 50 and 150◦C
when the pH of the waters was above 7 to 8. These re-
sults are in excellent agreement with observed miner-
alogy in pores from Teigarhorn and Hvalfjördur, Ice-
land (Neuhoff et al., 1999; Weisenberger and Sel-
bekk, 2009) (Figure 6). Neuhoff et al. (1999) inter-
preted this trend as the consequence of increased time
and temperature caused by increased burial of the lava
piles. The results shown in Figures 3 to 5 clearly indi-
cate that the factors predominating in the temperature
range from 50 to 150◦C are the pH of the water and
the extent of reaction. The pH in turn reflects attain-
ment of steady state conditions between acid supply
and extent of reaction, although this may not always
be the case in natural open systems. The primary role
of temperature seems to be related to increased disso-
lution rates of basaltic glass with increasing tempera-
ture (Gíslason and Oelkers, 2003), thus enhancing the
mass flux and shortening the alteration time.
The composition of carbonates was further found
to depend on pH. In mildly acid solutions, the pH was
controlled by the concentration of CO2 and the extent
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