Jökull - 01.01.2010, Side 181

Low-temperature alteration of basalts
However, though not commonly discussed, problems
with many other geothermometers, particularly those
based on the Na/K and Na/K/Ca ratios, do not often
compare with the silica geothermometry temperatures
at low temperatures (<150◦C); indeed the concentra-
tions of these elements seem to be controlled by sev-
eral factors including temperature, water pH and ex-
tent of reaction.
There are still many unanswered questions on the
detailed reaction mechanism of basalt alteration un-
der low-temperature geothermal conditions. The ap-
proach in the present study is a very simplified view
of the actual system, assuming it to be closed and
that the basalt dissolves stoichiometrically. The ef-
fects of continuous fluid flux, similar to what occurs
in fractured rocks, need to be investigated, including
what alteration mineral formations coexist in time and
space for such systems. Moreover, systematic effects
of rock composition and primary mineral composition
and abundance are needed. The dissolution rate of the
basalt is the predominant factor in determining the
overall mass fluxes in the system. Recent work has
demonstrated that the dissolution rate of crystalline
basalt is different to that of basaltic glass as well as
being non-stoichiometric with respect to bulk compo-
sition (Gudbrandsson et al., 2010) and depends on the
pH of the waters.
SUMMARY AND CONCLUSIONS
The low-temperature alteration of basaltic glass un-
der weathering and low-temperature geothermal con-
ditions was studied using reaction path modelling. In
particular, the effect of initial fluid acidity, extent of
reaction and temperature was investigated.
The weathering of basalts in the presence of at-
mospheric and lower CO2 pressures was found to
go through three stages. Stage I is characterized as
insignificant basalt dissolution and the formation of
simple insoluble hydroxides, mainly ferrihydrite, thus
decreasing the mobility of Fe and Al. Stage II was
reached upon progressive basaltic glass dissolution
and resulted in the formation of simple Al-Si phases
like imogolite, allophane and/or kaolinite and Ca-Mg-
Fe smectites, decreasing the mobility of Al, Fe, Si, Ca
and Mg. Upon extensive weathering and increase in
the pH of water, stage III may be reached with the for-
mation of smectites, zeolites, calcite and SiO2 (opal
or chalcedony). These results are in very good agree-
ment with observations of the weathering mineralogy
of basalts in Iceland (Arnalds, 1990; Crovisier et al.,
1992; Wada et al., 1992; Arnalds et al., 1995; Sigfús-
son et al., 2008). The primary factors in determining
the weathering of basaltic glass was the water pH. The
pH in turn was found to be controlled by the supply
of CO2 and the extent of basaltic glass dissolution or
reaction time and reflected steady state conditions be-
tween the extent of reaction, CO2 input and the mass
and composition of weathering minerals formed.
Under low-temperature geothermal conditions
(50–150◦C) the pH of the water and the extent of re-
action were to be the predominant factor determining
the alteration mineralogy. Under strongly acid con-
ditions (pH <4), for example in acid sulphate wa-
ters, amorphous silica, kaolinite, Al-Fe oxyhydrox-
ides and sulphur-containing minerals were most im-
portant. Sodium, K, Ca and Mg were observed to
be mobile whereas Si, Al and Fe were less mobile or
immobile. Under mildly acid conditions (pH 5–7),
for example in CO2 rich waters, the alteration min-
eralogy was dominated by kaolinite, chalcedony, Ca-
Mg-Fe smectites and Mg-Fe carbonates. Iron, Al and
Si were found to be immobile whereas Mg and Ca
mobility depended on the mass of carbonate formed
and the pH of the water. Under alkaline conditions
(pH >8) resulting from a low acid supply and/or ex-
tensive basaltic glass dissolution, the following sec-
ondary minerals formed in order of appearance: chal-
cedony, celadonite, Ca-Mg-Fe smectites, zeolites and
calcite, thus greatly reducing the mobility of most dis-
solved elements.
Based on comparison between the geochemical
models, naturally observed low-temperature alter-
ation mineralogy and water chemistry, the predomi-
nant factor controlling the alteration process was the
pH of the water. The various secondary mineral as-
semblages formed were strongly dependent on pH,
which in turn determined the various elemental mo-
bilities. The pH value reflected steady state condi-
tions between the supply and type of acid and quan-
tity of basaltic glass dissolved (extent of reaction) or
JÖKULL No. 60 181