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

A. Stefansson et Sr. Gislason, Chemical weathering of basalts, Southwest Iceland: Effect of rock crystallinity and secondary minerals on chemical fluxes to the ocean, AM J SCI, 301(6), 2001, pp. 513-556
Citations number
Categorie Soggetti
Earth Sciences
Journal title
ISSN journal
0002-9599 → ACNP
Year of publication
513 - 556
SICI code
River, ground, and peat water from a basaltic catchment area, Laxa in Kjos Southwest Iceland, were sampled and analyzed for major element concentratio ns to define the effect of glassy versus crystalline basalt, the formation of secondary minerals, and runoff on fluxes of dissolved elements to the oc ean. The proportions of the dissolved solids in the water samples derived from p recipitation, air, and rock weathering were estimated. The amounts of Na, M g, Ca, E., and S originating from precipitation were calculated from the re spective solute/Cl marine ratios and the estimated aqueous CI originated fr om precipitation, which was calculated using measured B content of the wate rs and B/Cl molal marine and rock ratios. The rock contribution from weathe ring ranged from 22 to 46 percent of the total dissolved solids of the wate rs. Iron and Al showed low mobility compared to Na. This was primarily due to c onsumption of these elements by ferrihydroxides, allophane, imogolite, and clay minerals. Also, Si and to a lesser degree Ca show apparent slightly lo wer 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 oth ers, 1992). Reaction progress calculations support these findings. Accordin g to a model interaction of basaltic glass and meteoric water Al, Fe, and S i are consumed by amorphous Fe and Al hydroxides and imogolite and allophan e at low reaction progress. With increasing reaction progress, Ca-Fe-Mg sme ctite became the primary secondary minerals, limiting the mobility of Ca, F e, Mg, Si, and Al. These findings suggest that the weathering in the Laxa i n the Kjos catchment area has evolved into the beginning of the smectite we athering stage and that it is dominated by a relatively low reaction progre ss and a high water to rock ratio. In the present study the effect of runoff and rock crystallinity was quanti fied for dissolved elemental fluxes. Temperature, lithology (other than roc k crystallinity), and rock age were similar in all of the catchment areas i n this study. Silica, Ca, F, S, Al, K, Mg, and B had a very similar depende nce 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 i s 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 oth er hand, basaltic glass was observed to enhance the fluxes of more mobile e lements like Na, Si, Ca, F, and S by a factor of 2 to 5 at a constant runof f of 200 cm/yr, constant vegetative cover, and 0 to 100 percent glass conte nt of the rocks. The K and Mg fluxes were found to be independent of rock c rystallinity. However, runoff alone cannot explain the fluxes of these elem ents, and it seems that vegetative cover and seasonal variations in biomass activities influence the K and Mg fluxes.