Recent chemical weathering of basalts

Abstract

Relative bulk leach rates of the major elements Na, Ca, Mg, Fe, Al, Ti, and Si from studied basaltic weathering profiles are not greatly affected by primary mineralogy or by bulk composition of the parent basalt, probably because major silicate phases are weathered at grossly similar rates. By contrast, trace element leach rates may be effectively controlled by their mineralogical siting in parent basalts. Release rates of trace elements, including potentially toxic heavy metals, are confidently predicted only where bulk compositions, mineralogical and modal analyses of the parent basalt are available. Ti is the most conservative major element in the basaltic profiles studied. Fe and Al are leached from soil zones and from weathering profiles subjected to extreme chemical weathering. Both Fe and Al, however, are effectively conserved in recent, aerated profiles that have suffered moderate to extensive (but not extreme) weathering. Generally, Al is more extensively leached than Fe(III), an observation supported both by bulk compositional changes of profiles and by compositions of associated weathering solutions. Climatic conditions do not affect greatly the gross, relative release rates of major elements from weathered basalts. By implication mechanisms of chemical weathering are similar regardless of climate. Climate, however, affects dramatically the relative rates of chemical and physical weathering (that is, leaching and erosion). The diverse clay mineral suites observed in soils of various climates may result more from establishment of different steady state conditions in each climatic regime (chemical weathering versus erosion) than from different weathering-reaction mechanisms. During weathering of the Baynton Basalt, the order of susceptibility of primary phases is olivine \> glass \> plagioclase \> clinopyroxene \> Fe-Ti-oxide. Glass, plagioclase, and clinopyroxene are weathered at grossly similar rates. Field observations, local equilibrium, and kinetic considerations indicate that there is no fixed order of mineral susceptibility to weathering and no fixed leaching order of elements from basalts. Only after classification of a basalt according to bulk composition and model proportions can generalizations be made in these regards.