Abstract
Opaline silica (1– μm) from tropical and temperate soils developed in recent volcanic ash was concentrated by chemical purification and heavy liquid separation. Oxygen isotope ratios ( δ 18O SMOW) of the opaline silica samples were estimated from the isotopic mass balance of the fractions before and after selective chemical dissolution with heated 0.5M NaOH. The δ 18O values range from +24.7 to +33.6%, indicating an authigenic origin. The values are higher than those of crystalline silica polymorphs (cristobalite: +5 to + 11%, fine-grained quartz: +16%) commonly found in soils derived from volcanic ash. The contribution of biogenic processes in the formation of opaline silica is limited, as indicated by a substantial difference in the isotope values of opaline silica and plant opal in the 10–53 μm size fractions. Possible mechanisms of formation of the opaline silica are discussed. These are based on the equilibrium oxygen isotope fractionation of amorphous silica and water, and on the estimated δ 18O values of the solution from which the silica formed. We suggest that authigenic silica arises from the evaporation of water at the soil surface in tropical regions, and from the freezing of soil solutions containing monomeric silicic acid in temperate regions.
Published Version
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