Jökull - 01.01.2010, Page 166

A. Stefánsson
terms of burial depth of the lavas and increased tem-
perature with depth (e.g. Walker, 1960; Sukheswala
et al., 1974; Kristmannsdóttir and Tómasson, 1978;
Mehegan et al., 1982; Jørgensen, 1984; Murata et
al., 1987; Neuhoff et al., 1999, 2006; Weisenberger
and Selbekk, 2009). As many as five depth-controlled
zeolite zones have been described. These tempera-
ture dependences in turn have been extensively used
as thermobarometers for evaluation of crustal temper-
atures and burial depth of basaltic crust. Moreover,
systematic changes in phyllosilicate composition and
sequence of the appearance of the alteration miner-
als have been observed, often with celadonite and
SiO2 minerals (chalcedony) followed by mixed clays
and chlorites and eventually zeolites in the pore space
(Neuhoff et al., 1999; Weisenberger and Selbekk,
2009). Similar changes have been reported with rock
age and geochemical modelling during weathering
of basaltic glass with simple Fe and Al oxyhydrox-
ides and allophane forming initially, which is then re-
placed by Ca-Mg rich smectites with time (Crovisier
et al., 1992; Stefánsson and Gíslason, 2001). The ef-
fect of rock and fluid composition has also been in-
vestigated (e.g. Kristmannsdóttir, 1984, 1985).
The alteration of basalts at low-temperature in-
volves the dissolution of primary minerals and pri-
mary glasses and the formation of secondary minerals
and dissolved solutes in water. In a closed system of
fixed composition, the overall reaction is incongruent
and is affected by temperature, rock and fluid compo-
sition and extent of reaction. Moreover, for systems
containing more than one phase, the extent of reac-
tion will further result in changes in mass between
the various phases. The result is that the process of
alteration of a chemical system of fixed composition
(rock and fluid) must be influenced by several factors
including temperature, reaction progress and reaction
mechanism. However, the contribution of the various
factors to basalt alteration is somewhat unclear.
To answer these questions one needs to separately
study the effects of temperature, initial fluid composi-
tion including acidity, and the extent of reaction. The
mass fluxes in the system are controlled by mineral
solubilities and their respective dissolution and pre-
cipitation kinetics. Fluid-rock reaction simulations
provide a unique tool to approach this problem. How-
ever, such calculations have to be treated with care
and compared with experimental and field observa-
tions. In the present study, water-basalt interaction
modelling for closed systems was carried out under
weathering and low-temperature geothermal condi-
tions and the results used to gain insight into the role
of temperature, water composition, and time on low-
temperature geothermal alteration of basalts.
METHODS
A conceptual model of low-temperature geother-
mal alteration
Let’s assume a closed system of fixed composition
and mass, like basalt and water. The system contains
two phases, liquid and solid. Initially, the water is
undersaturated with respect to all minerals and will
dissolve the basalt. This continues until the water be-
comes supersaturated with respect to a given second-
ary mineral or mineral assemblage. The secondary
minerals formed generally have a different relative
composition than the primary minerals, resulting in
changes in mass ratio of elements (components) be-
tween the two phases along the reaction path. This
phenomenon may be called water-rock diffraction as
an analogue of magma diffraction. This result is that
the composition of the two phases, the solid and liq-
uid, changes with time and is a function of the extent
of the reaction, yet the total system mass is conserved.
The extensive variables acting on the system are then
temperature and pressure (Helgeson, 1968; Denbigh,
1971). Under weathering and low-temperature condi-
tions pressure plays a small role and may be ignored.
In an open flow-through system equivalent to a per-
meable fracture, the conservation of mass also may
not hold. The overall status of the system must there-
fore reflect steady state conditions of fluid supply (wa-
ter and acids and their ionization equilibria) and the
extent of water-rock reaction at a particular tempera-
ture. This results in a conceptual model of geothermal
alteration with several important factors affecting the
water-rock process including temperature, water and
acid supply, and extent of reaction.
166 JÖKULL No. 60