Jökull - 01.01.2010, Síða 170

A. Stefánsson
WATER-BASALT INTERACTION
Weathering of basalts
The results of the reaction path simulations of basalt
glass reaction with unpolluted precipitation from
Langjökull Glacier are shown in Figure 2, both in
terms of pH changes and the secondary mineralogy
after reaction progress. The reaction progress (ξ) is a
measure of the mass of basalt dissolved per kg of wa-
ter. It can be related to reaction time given that the re-
active surface area of the basalt is known. The surface
area in turn is a function of the reaction progress as the
mineral grains dissolve over time. Two general con-
clusions can be drawn from the calculations. Firstly,
in a closed system, the water-rock process is described
by an acid-base reaction with the basalt being the base
and the acid initially supplied to the system, in this
case as CO2. At a particular time, the system is in
a steady state determined predominantly by the in-
put of acid and the respective ionization constants
and the quantity of basalt and secondary minerals
formed. This is clearly demonstrated for the two sets
of calculations shown in Figure 2, open and closed
to atmospheric CO2. Secondly, the secondary miner-
als formed depend on the extent of the reaction. The
weathering sequence can be divided into three stages.
Stage I is characterized by insignificant basalt dissolu-
tion and the formation of simple insoluble hydroxides,
mainly ferrihydrite. Upon progressive basaltic glass
dissolution and secondary mineral formation, stage II
is reached, characterized by the formation of simple
Al-Si phases like imogolite, allophane and/or kaolin-
ite and Ca-Mg-Fe smectites, decreasing the mobility
of Al, Fe, Si, Ca and Mg. With extensive weathering,
stage III may be reached with formation of smectites,
zeolites, calcite and SiO2 (opal or chalcedony) be-
ing the dominant alteration product. These results are
in very good agreement with observations of natural
secondary mineralogy formed during weathering in
Iceland (Arnalds, 1990; Crovisier et al., 1992; Wada
et al., 1992; Arnalds et al., 1995; Sigfússon et al.,
2008) and are similar to those proposed by Stefáns-
son and Gíslason (2001). The calculations suggest
that the predominant factors in determining the weath-
ering minerals is the extent of the reaction (reaction
time per specific reactive surface area) and the sup-
ply of acid to the system and the respective ionization
constants.
Low temperature alteration of basalts
According to the conceptual model for basalt alter-
ation, three factors are of importance in determining
the secondary mineralogy and water composition in-
cluding temperature, extent of reaction and the supply
of acids. In order to gain insight into the three factors
possibly influencing low-temperature geothermal al-
teration of basalts, sets of closed system calculations
were carried out on the basalt-water interaction. The
results of the closed system calculations at 100◦C and
1, 10 and 100 mmol/kg CO2 are shown in Figures 3
and 4. The progressive water-basalt interaction was
found to result in an increase in pH. At low initial
CO2 concentration, the basalt dissolution quickly in-
creases to pH >8. However, with increasing initial
CO2 concentration, more basalt dissolution is needed
to increase the pH of the water. At 100 mmol/kg CO2,
about 0.3 moles of basalt are needed per litre of water
to raise the pH above 6 and about 1 mol of basalt is
needed to reach pH values of >8.
The extent of the reaction and the pH together
affect secondary mineral stabilities and the resulting
water composition (Figure 4). Under mildly acid con-
ditions (pH <6.5) that are characterized either by high
initial acid supply and a low to high extent of the re-
action or a low acid supply and low extent of reac-
tion, most elements except Si, Al and Fe are relatively
mobile, resulting in formation of minerals including
kaolinite, chalcedony and simple Fe and Al oxyhy-
droxides. At pH above 8, various clays, chalcedony
and zeolites become the predominant alteration prod-
uct, in addition to carbonates at high CO2 concentra-
tions. This alkaline pH may be reached for low acid
systems at insignificant basalt dissolution whereas at
a high initial acid supply, considerable basalt dissolu-
tion is required. Based on this, one can conclude that
the very fine variations in pH at a particular tempera-
ture constitute the dominant parameter in determining
secondary mineral composition and mass as well as
the various elemental mobilities.
Further calculations on the water-basalt interac-
tion were carried out using a strong acid or H2SO4.
The results are described in detail by Markússon and
170 JÖKULL No. 60