Alkali basalt gravel weathering in quaternary Allier river terraces, Limagne, France

Weathering of alkali basalt pebbles was studied in three Quaternary Allier terraces of different ages. The isovolumetric method and Ti-constant method were applied to quantify compositional changes due to weathering. Bulk density of fresh gravel in the younger terraces (Weichselien and Holocene) was adequately estimated from geochemical data by a multiple linear regression model (R2 = 0.81**, significant at P = 0.01). Since no chemical and micromorphological differences could be demonstrated between fresh gravel and the unweathered cores of partly weathered gravel, the regression model was also applied to calculate the bulk density of the cores of partly weathered pebbles in the oider terraces. The decrease in bulk density of the rind, compared with that of the core, was used as a measure of weathering intensity. The chemical composition of the weathering rind revealed a relative increase in the contents of Ti, Al, Fe, and Mn and a decrease of K, Na, Ca, and Si. Changes were estimated considering Ti essentially immobile. Except for Fe and Mn, all elements tended to decrease in the rind. The relative elemental mobilities were as follows:

K > Na > Si > Mg > Ca > P > Al > Ti

No relationship was found between weathering intensity and the age of the two oldest terraces or sampling depth. Differences in weathering intensity are predominantly a function of chemical (= mineralogical) composition of the alkali basalt pebbles.