Stable isotope geochemistry, mineralogy, and microscopy of gypsiferous soils from central Iran

Abstract

Gypsum accumulation is one of the prominent pedogenic processes occurring in many arid regions of the world. Gypsiferous soils are common in large areas of the Iranian central plateau. There has been a long debate about the source of gypsum, mechanism(s) of gypsum deposition, and the genesis of associated silicate clay minerals in these soils. The main objectives of this thesis were to: (1) trace the source of gypsum, (2) understand the mechanism(s) of the accumulation of high amounts of gypsum, and (3) study the clay mineralogy and micromorphology of gypsiferous soils from the Isfahan region, central Iran. Soil samples from 15 pedons occurring in three different landscapes, river and rain waters, and dominant parent rock samples from the area were studied. Stable isotope techniques coupled with mineralogical, submicroscopic, and micromorphological studies provided an insight into an understanding of the pedological processes involved in the formation of the soils. The sulfur and oxygen stable isotope data of the soil gypsum and dissolved sulfate indicate that the gypsum in the soils originates from the local sedimentary rocks tying in well with the hypothesis that central Iran was part of the Tethyan geosynclinal belt which was cut off from the sea in the late Mesozoic era. Both $\delta\sp{34}S$ and $\delta\sp{18}O$ values of the soil gypsum decrease from high elevations in colluvial fans to plateaus and eventually to the lowest positions in the toposequence (alluvial plains). Dissolution-precipitation followed by the less significant process of adsorption-desorption of sulfate appear to be the possible mechanisms responsible for the restricted isotope fractionation. Palygorskite is the dominant silicate clay mineral in both fine and coarse clay fractions of the soils, and of the Oligo-Miocene limestone, a parent rock that forms low hills in the area. The mountains are underlain by Jurassic shale and Cretaceous limestone, which contain illite and chlorite with a trace amount of palygorskite. The association of extremely large amounts of well-formed palygorskite bundles with gypsum in the soils supports the hypothesis that palygorskite was mainly formed after the initial precipitation of gypsum, which created high pH and Mg/Ca ratio when central Iran was covered by the post-Tethys sea shallow hyper-saline lagoons. In addition, examination of pedogenic carbonates by scanning electron microscope, and ultra-microtomic cuts by transmission electron microscope reveal that the pedogenic calcite crystals are intimately associated with palygorskite. Evidence of illuviation in colluvial soils is the increase in the fine to total clay ratio in the argillic horizon compared to the overlying horizon in the colluvial soils and also well-developed, but considerably disrupted, argillans observed in thin sections. (Abstract shortened by UMI.)