15 April, 2016

April reading: isotopes, bentonite, dolomite, clay minerals and zeolites.

Several interesting articles appeared that I find worth and fun to read. The articles are focused on radiogenic and stable isotope compositions of bentonites as well as on dolomite formation associated with clays and zeolites in volcanic ash and soils. The research is nicely situated in the vicinity of my doctoral studies and own interests. So it is a good practise for reading and writing! I hope you enjoy the short and personal list!

Warr et al. (2016) Constraining the alteration history of a Late Cretaceous Patagonian volcaniclastic bentonite - ash - mudstone sequence using K-Ar and 40Ar/39Ar isotopes. Link 

Smectites are often considered unsuitable for radiometric age dating because they only incorporate tiny amounts dateable elements, most of which are easily exchangeable. Warr et al. explore the K-Ar and 40Ar/39Ar dating of smectite from sodium bentonite of Lago Pellegrini in Northern Patagonia, Argentina. They demonstrate that it is possible to date smectites if radiogenic Ar is retained in low-permeability rocks such as bentonites. Results tentatively indicate that a fairly long time (13-17 Ma) was needed for complete alteration. I try something similar using the Rb-Sr system using smectites from bentonites in Southern Germany. Maybe this can be combined...

Bauer et al. (2016) Stable isotope composition of bentonites from the Swiss and Bavarian Freshwater molasse as a proxy for paleoprecipitation. Link

Smectites might be problematic to use in radiometric dating but they are good for reconstructing the stable isotope composition of e.g. the water they formed in. This is especially true for smectite formed in-situ from the alteration of volcanic ash and preserved as bentonite. The isotope fractionation of oxygen and hydrogen is easily affected by temperature, evaporation, or other factors. Bauer et al. utilise this for paleoclimate and -topography reconstructions in the Swiss and German Foreland basin using bentonites from Switzerland and Germany, including some from my own research area in Bavaria. Again an exciting read in my opinion!

Alonso-Zarza et al. (2016) Chabazite and dolomite formation in a dolocrete profile: An example of a complex alkaline paragenesis in Lanzarote, Canary Islands. Link

Zeolites and dolomite are rare minerals in ancient and modern soils that require special conditions of formation. Alonso-Zarza et al. investigated a chabazite- and dolomite-bearing soil profile formed on basaltic rocks of Lanzarote Island with to elucidate its formation conditions, e.g. vadose vs phreatic, Mg distribution, and water sources. The lithostratigraphic distribution of dolomite in the lower and calcite in the upper parts of the profile together with the stable (C and O) and radiogenic (Sr) isotope composition nicely illustrate how carbonate formation was brought about by Mg from local basalt and meteoric water; with chabazite formed during drier, more alkaline periods.

Cuadros et al. (2016) Chemical and textural controls on the formation of sepiolite, palygorskite and dolomite in volcanic soils. Link

This study is about the formation of dolomite and Mg-bearing clay minerals such as palygorskite and sepiolite, and some smectite, in volcanic soil on Gran Canaria Island. Cuadros et al. examined a profile with predominantly sepiolite and calcite towards the top, and palygorskite, dolomite and some smectite towards the base. The dolomite formed related to sepiolite/palygorksite and volcanic ash particles while calcite is located in inter-particle pore-spaces. The study emphasises, among other issues, the role of Mg and Si transported by solutions within the profile but also within volcanic grains. It is a nice addition to the growing number of papers on dolomite formation in soils and terrestrial settings.

Note: Summaries are neither complete article reviews nor guaranteed to be free of misunderstandings. 

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