22 August, 2015

Paper: Dolomite formation in non-marine bentonite deposits


On August 6th the first of three publications that are a part of my doctoral research was finally published online in the Clay Minerals Journals at GSW (alternatively via ingentaconnect - abstract also available on RG). The study explores the role of authigenic carbonate formation during bentonitization. We used a combination of X-ray diffraction, micromorphology (based on thin-sections and SEM) and stable isotope analysis to investigate bentonite-associated dolomite and calcite formation.

Bentonites in the Upper Freshwater Molasse formed from practically Ca- and Mg-free, calc-alkaline, rhyolitic air-fall tuffites. The presence of abundant authigenic dolomite (one deposit is capped by 1 m thick dolomite horizon) was a major surprise, and we extensively used XRD to confirm the mineralogical composition. The combination of methods enabled us to distinguish both dolomite-rich and calcite-rich pedogenic, palustrine and groundwater facies within bentonite deposits. The figures taken from the paper illustrates some of the carbonate microfabrics.

The carbon and oxygen stable isotope results (figure from paper shown below) were essential in resolving formational environments. The carbon isotope ratios imply a C3-plant-dominated carbon source with small additions from dissolved carbon from groundwater and atmospheric carbon in upper "soil horizons". However, the bi-modal distribution of carbon isotope results is consistent with carbonate formation in both water-logged and non-water-logged conditions, and suggests repeated wetting and drying cycles. The oxygen isotope data the impact of evaporation and temperature on δ18OV-SMOW values of meteoric water of -7.0 to -4.8 per mil during carbonitization, and hence bentonite formation.
We concluded that dolomitization was a syngenetic to early diagenetic process - perhaps on the time-scale of soil formation. Both dolomite and bentonite formation occurred in non-saline, non-arid and repeatedly partially-oxygenated and reducing soil and groundwater environments. That actually comes as a small surprise because carbonates were so far dismissed as unrelated to bentonite formation. It also places bentonite formation into a soil and groundwater environment, and not later diagenesis. We, however, cannot rule out that smectite formation continued after blanketing of deposits with younger sediments.

References:

M. H. Köster and H. A. Gilg (2015) Pedogenic, palustrine and groundwater dolomite formation in non-marine bentonites (Bavaria, Germany) Clay Minerals, June 2015, v. 50, p. 163-183, published online August 6, 2015, doi:10.1180/claymin.2015.050.2.0

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