Jökull - 01.01.2010, Page 167

Low-temperature alteration of basalts
A schematic view of a conceptual model of the
geothermal alteration process of basalts is shown in
Figure 1. The reaction mechanism adopted in the
present work assumes it to be a stoichiometric dis-
solution of basaltic glass in accordance with a given
dissolution rate. This results in an increase of so-
lute concentration with time until secondary mineral
saturation is reached. The minerals considered in-
cluded those commonly observed under natural con-
ditions during weathering and low-temperature meta-
morphism. The precipitation kinetics were assumed
to be instantaneous, i.e. the rates determining step-
controlling mass fluxes are the primary mineral dis-
solution rates.
Geochemical modelling
Sets of geochemical model calculations were carried
out to gain insight into the water-basalt interaction un-
der weathering and low-temperature geothermal con-
ditions, at 10 and 50–150◦C. This involved calcula-
tion of aquifer speciation and mineral saturation states
and reaction path modelling (e.g. Helgeson, 1968;
Denbigh, 1971). The calculations were carried out
using the WATCH (Bjarnason, 1994) and PHREEQC
(Parkhurst and Appelo, 1999) programs with ap-
propriate updates of the thermodynamic datasets for
aqueous species, gas and mineral solubilities (see
Gysi and Stefánsson, 2010).
For gases, the thermodynamic values reported
by Fernández-Prini et al. (2003) were used. Sec-
ondary mineral solubility was further updated in the
present work. The apparent Gibbs energy of forma-
tion (∆Gappi,f,T,Psat ) at 0–150
◦C at water vapour satu-
ration pressures of selected key aqueous species and
the minerals necessary to calculate the mineral solu-
Figure 1. Conceptual model of water-rock interaction. Basaltic glass is dissolved followed by secondary mineral
formation in a closed system. – Líkan af samspili vatns og bergs þar sem basaltgler leysist upp og ummyndun-
arsteindir falla út.
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