An experimental study of crystalline basalt dissolution from 2 6 pH 6 11 and temperatures from 5 to 75°C.

Steady-state element release rates from crystalline basalt dissolution at far-from-equilibrium were measured at pH from 2

to 11 and temperatures from 5 to 75 C in mixed-flow reactors. Steady-state Si and Ca release rates exhibit a U-shaped variation

with pH where rates decrease with increasing pH at acid condition but increase with increasing pH at alkaline conditions.

Silicon release rates from crystalline basalt are comparable to Si release rates from basaltic glass of the same chemical

composition at low pH and temperatures P25 C but slower at alkaline pH and temperatures P50 C. In contrast, Mg and

Fe release rates decrease continuously with increasing pH at all temperatures. This behaviour is interpreted to stem from the

contrasting dissolution behaviours of the three major minerals comprising the basalt: plagioclase, pyroxene, and olivine. Calcium

is primarily present in plagioclase, which exhibits a U-shaped dissolution rate dependence on pH. In contrast, Mg and

Fe are contained in pyroxene and olivine, minerals whose dissolution rates decrease monotonically with pH. As a result, crystalline

basalt preferentially releases Mg and Fe relative to Ca at acidic conditions. The injection of acidic CO2-charged fluids

into crystalline basaltic terrain may, therefore, favour the formation of Mg and Fe carbonates rather than calcite. Element

release rates estimated from the sum of the volume fraction normalized dissolution rates of plagioclase, pyroxene, and olivine

are within one order of magnitude of those measured in this study.

2011 Published by Elsevier Ltd./p>