Jökull - 01.01.2010, Side 165

Reviewed research article
Low-temperature alteration of basalts – the effects of
temperature, acids and extent of reaction on mineralization and
water chemistry
Andri Stefánsson
Institute of Earth Sciences, Science Institute, University of Iceland, Sturlugata 7, 101 Reykjavík, Iceland as@hi.is
Abstract — The weathering and low-temperature alteration of basaltic glass was studied by geochemical
modelling in order to gain an insight into the effects of temperature, acid supply and extent of reaction on
the secondary mineralogy and water chemistry. Basaltic glass was dissolved in dilute water at 10–150◦C in
a closed system and secondary minerals commonly observed in nature allowed to precipitate when saturated.
The weathering of basalts in the presence of CO2 was observed to go through three stages; initially simple
insoluble Al and Fe hydroxides were formed. Upon progressive basaltic glass dissolution imogolite, allophane
and/or kaolinite and Ca-Mg-Fe smectites predominated, decreasing the mobility of Al, Fe, Si, Ca and Mg
whereas extensive weathering and alkaline pH values resulted in the formation of smectites, zeolites, calcite
and SiO2 minerals. Under low-temperature geothermal conditions and in strong H2SO4 acid solutions (pH<4)
amorphous silica, kaolinite, Al-Fe oxyhydroxides and sulphur-containing minerals were most important and
most cations like Na, K, Ca and Mg were observed to be mobile. Under mildly acid conditions in CO2 en-
riched waters (pH 5-7) kaolinite, chalcedony, Ca-Mg-Fe smectites and Mg-Fe-Ca carbonates predominated
and Fe, Al and Si were found to be immobile, whereas Mg and Ca mobility depended on the mass of carbonate
formed and water pH. Under alkaline conditions (pH >8) that resulted from a low acid supply and/or extensive
basaltic glass dissolution chalcedony, celadonite, Ca-Mg-Fe smectites, zeolites and calcite were found to form,
greatly reducing the mobility of most dissolved elements. The dominant factor determining the weathering and
low-temperature alteration of basaltic glass and the associated elemental mobility is the pH of the water. In
turn, the pH value is determined by the input and type of acid and their ionization constant and quantity of
basaltic glass dissolution and secondary minerals formed that increases with the extent of the reaction and the
temperature. The weathering mineralogy observed associated with basaltic glass in Iceland is typical of low to
moderate degree of alteration under low (atmospheric) CO2 conditions. The low-temperature regional geother-
mal alteration commonly observed in Iceland is also typical for low CO2 alteration and the reaction order of
celadonite and chalcedony, followed by mixed clays and chlorite and eventually zeolites and calcite, indicate
increased water-rock interaction at pH >8 with temperature being less important on the overall reaction path.
INTRODUCTION
The geothermal alteration of basalts and the formation
of secondary minerals at low-temperatures are com-
monly dominated by phyllosilicates, zeolites, oxides
and hydroxides and sometimes carbonates (e.g. Krist-
mannsdóttir and Tómasson, 1978; Kristmannsdóttir,
1979; Mehegan et al., 1982; Schiffman and Fridleifs-
son, 1991; Neuhoff et al., 1999). Typically, depth
zonal distribution of various mineral assemblages in
the regional low grade metamorphosed basalt lava
piles in Iceland and other locations have been ob-
served. These have generally been interpreted in
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