Title: Water-rock interaction of silicic rocks: an experimental and geochemical modelling study

Abstract

The water-silicic rock interaction under geothermal conditions was studied both
experimentally and using reaction path simulations to get insights into to the process of
rock alteration including secondary mineralogy, water chemistry and mass transfer as a
function of rock composition and reaction progress (ξ). The experiments and model
calculations were conducted at 240°C and water vapour saturation pressures on two glass
samples, dacite from Askja and rhyolite from Hekla, and initial non-thermal groundwater
containing ~5000 ppm NaCl, ~1600-3500 ppm CO2 and ~140 ppm H2S. The dissolution
of the silicic glasses were found to be incongruent with the formation of secondary
minerals including quartz, anhydrite, clays like montmorillonite, illite and/or mixed illitesmectite
and chlorite, zeolites like analcime and phillipsite as well as traces of anatase
and fluorite. Moreover, most of the observed minerals were found to be saturated or
supersaturated. The changes in water chemistry were characterized by a decrease in CO2,
Mg, Fe and Al concentrations, relatively steady concentrations for Na and SO4 whereas Si
initially rose followed by decrease after ~40 days. For H2S, F and Ca considerable
differences were observed depending on the starting material. The formation of secondary
minerals greatly reduced the mobility of Al, Fe, Mg and Si and to lesser extent Ca and
Na, however, K was observed to be mobile relative to B. The reaction path simulations
demonstrate that the appearance of various secondary minerals in a closed system is a
function of reaction progress, initially with the formation of clays and sulphides followed
by the appearance of quartz and zeolites. Upon considerable reaction (>1 mol rock
dissolution in 1 kg of water) other Al-Si minerals also become important and sometimes
predominant including epidote, feldspars and chlorite, this last stage closely corresponds
to the commonly observed alteration mineralogy associated with geothermal systems
hosted by silicic rocks. The exact clay mineralogy was also found to be dependent on the
initial system composition, with illite and mixed illite-smectite being more important
associated with the rhyolite and montmorillonite associated with the dacite. In addition,
comparison of the experimental results and reaction path simulations revealed that
reaction kinetics may be of potential importance in the formation of Na, K and Si
containing minerals with some profound influences on the respective elemental solution
concentrations. This in turn affected the predicted geothermometry temperatures that
were found to vary from <150 to >350°C and be a function of the extent of reaction for
the 240°C experiments.