Chemical weathering of basalts, southwest Iceland: effect of rock crystallinity and secondary minerals on chemical fluxes to the ocean

Abstract

River, ground, and peat water from a basaltic catchment area, Laxa in Kjos Southwest Iceland, were sampled and analyzed for major element concentrations to define the effect of glassy versus crystalline basalt, the formation of secondary minerals, and runoff on fluxes of dissolved elements to the ocean. The proportions of the dissolved solids in the water samples derived from precipitation, air, and rock weathering were estimated. The amounts of Na, Mg, Ca, K, and S originating from precipitation were calculated from the respective solute/Cl marine ratios and the estimated aqueous Cl originated from precipitation, which was calculated using measured B content of the waters and B/Cl molal marine and rock ratios. The rock contribution from weathering ranged from 22 to 46 percent of the total dissolved solids of the waters. Iron and Al showed low mobility compared to Na. This was primarily due to consumption of these elements by ferrihydroxides, allophane, imogolite, and clay minerals. Also, Si and to a lesser degree Ca show apparent slightly lower mobility than Na. These results are in good agreement with these phases being dominant in the soils of the study area (Wada and others, 1992) and the main secondary minerals of basaltic glass in Iceland (Crovisier and others, 1992). Reaction progress calculations support these findings. According to a model interaction of basaltic glass and meteoric water Al, Fe, and Si are consumed by amorphous Fe and Al hydroxides and imogolite and allophane at low reaction progress. With increasing reaction progress, Ca-Fe-Mg smectite became the primary secondary minerals, limiting the mobility of Ca, Fe, Mg, Si, and Al. These findings suggest that the weathering in the Laxa in the Kjos catchment area has evolved into the beginning of the smectite weathering stage and that it is dominated by a relatively low reaction progress and a high water to rock ratio. In the present study the effect of runoff and rock crystallinity was quantified for dissolved elemental fluxes. Temperature, lithology (other than rock crystallinity), and rock age were similar in all of the catchment areas in this study. Silica, Ca, F, S, Al, K, Mg, and B had a very similar dependence on runoff. On the other hand, Na fluxes were found to be less dependent on runoff, and Fe fluxes were found to be independent of runoff. Basaltic glass dissolves faster than fully crystallized basaltic rocks (Gislason and Eugster, 1987a). Therefore, increased glass content of the primary rocks is thought to increase the leaching and elemental fluxes at constant runoff. However, because some of the solutes, particularly Al and Fe, were highly influenced or even controlled by the formation of secondary minerals, this increasing dissolution with increasing glass content was hidden. On the other hand, basaltic glass was observed to enhance the fluxes of more mobile elements like Na, Si, Ca, F, and S by a factor of 2 to 5 at a constant runoff of 200 cm/yr, constant vegetative cover, and 0 to 100 percent glass content of the rocks. The K and Mg fluxes were found to be independent of rock crystallinity. However, runoff alone cannot explain the fluxes of these elements, and it seems that vegetative cover and seasonal variations in biomass activities influence the K and Mg fluxes.