Mass balance of major elements in relation to weathering in soils developed on igneous rocks in a semiarid region, northwestern Iran

This study was conducted to evaluate the weathering intensity of the major soils developed on igneous rocks in semiarid region of northwestern Iran. Eight parent materials were selected including monzodiorite, alkali granite, granodiorite, syenite, pyroxene diorite, hornblende andesite, pyroxene andesite, and dacite. Representative soil profiles were described and soil samples were collected and analyzed for selected chemical and physical properties and total concentrations of major elements and Zr, V, Ti and Y. Bulk densities as well as Ti, Zr and V concentrations were used to estimate the strain factors and mass balance equations were used to quantify the net result of pedogenic weathering, i.e. elemental loss and gain. The results of clay content and pedogenic iron variability as well as index of compositional variability (ICV), chemical index of alteration (CIA) and, A-CN-K and MFW ternary plots showed that the soils developed on volcanic rocks (hornblende andesite> pyroxene andesite> dacite) were more weathered than those on the plutonic parent rocks (alkali granite, granodiorite, monzodiorite, syenite, pyroxene diorite). The results of mass balance calculations based on the strain factors revealed that the Ca and Na depleted during weathering progress mostly from plagioclase grains. In the semiarid regions Ca is precipitated as pedogenic calcite in the soil horizons. K and Mg depletion is less than Ca and Na especially in the profiles on the hornblende andesite with the highest clay and LOI content. The results of this study clearly suggest that the behavior of K and Mg during the weathering cannot only be explained by the disintegration of the primary minerals, since they are fixed on the secondary clay minerals. Iron did not change in the soils compared to the parent material and was precipitated as the pedogenic iron and conserved in the soil horizons. Overall, the results on the weathering indicators and major elements mass balance enrichment/depletion in the study area confirmed that the soil profiles developed on volcanic rocks are more weathered than those on the plutonic igneous rocks.