A life cycle of volcanic ash from the example of micro- and macro-scale investigation of Miocene tuff layers from the Dinarides



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Tuffaceous layers are regularly preserved in Miocene carbonate and siliciclastic sediments of the broader Alpine region of southeastern and central Europe. Detailed mineralogical and geochemical analyses of 13 tuffs of known ages acquired from sedimentary successions of intramontane Dinarides basins and the southwestern Pannonian Basin were carried out to infer on plausible source areas, relative strengths of volcanism, and ash distribution patterns. Studied tuffs were altered to various degrees with illite-smectite and smectite as dominant phases while volcanic glass, carbonates and other silicates are minor constituents. Tuffs’ geochemistry ranges from andesitic through trachyandesitic to rhyolitic and trachytic. Trace-element based correlation with regional data reveal that Lower Miocene (LM) and Lower Middle Miocene (LMM) tuffs (17.0–14.0 Ma) likely originated in the Western Carpathians (the Bükkalja volcanic field) while the source areas of Upper Middle Miocene (UMM) tuffs (13.8–12.5 Ma) were in the Apuseni Mountains and/or Eastern Carpathians. The spatial relation of LM/LMM and UMM tuffs with respect to their source areas (the Bükkalja volcanic field and Apuseni Mountains, respectively) is most consistent with tropospheric easterly trade winds that carried ash hundreds of kilometers to the SW toward an azimuth of c. 200–250°. Many tuffs from the Dinarides Lake System (DLS) are affected by various extents of volcanic glass shard alteration. A selected ~14.68 Ma felsic tuff from DLS Livno-Tomislavgrad Basin that was found to exhibit volcanic glass degradation and co-genetic Mn-Fe precipitation was studied. Microbial activity took part in both reactions thus adding the interest of revealing effects of biotic and abiotic processes occurring during tuff eogenesis. X-ray diffraction and electron microbeam analysis with energy-dispersive X-ray spectroscopy revealed the pitting or granular structures developed at glass rims along with smectite flakes protruding from a degrading glass. Mn-Fe mineralization emerges in the form of Mn-Fe coatings, an initial step to micronodule formation, where traces of biogenetic influence included a high content of phases rich in structural Mn(IV) (i.e., ranciéite and jacobsite) and presence of microbial microfossils. Co-genetic ties between glass degradation and Mn-Fe precipitation were established through the report of dioctahedral smectite formed at the expense of altered glass which then served as nuclei of the ongoing biotic and abiotic Mn-Fe mineralization. These processes manifest on a continuous involvement of microbial life in the course of eogenesis of pyroclastic material in lacustrine environment. The final contribution of this doctoral dissertation reports a novel approach in the study of trace-element mobility during argillitization of volcanic glass that is based on in situ laser ablation inductively coupled plasma mass spectrometry glass analyses and that of spatially related illite-smectite collected in the form of fraction separates. The material studied originates from lacustrine sediments of the DLS that bear evidence of intensive weathering of distal tephra during the Miocene climatic optimum. Yttrium and HREE were probably mobilized from decomposing glass in the form of carbonate complexes and were consequently depleted significantly in studied clays. On the other hand, the Mg-rich illite-smectite demonstrates an elevated adsorption potential of solvated LREE complexes. This may be explained through the clay surface geochemistry controlled largely by Mg for Al octahedral substitution. This contribution highlights the role of eogenetic 2:1 clay aluminosilicates that, under favorable geological conditions, may be conducive to secondary REE enrichment and the formation of potential ion adsorption-type deposits. Finally, preliminary analysis of correlating specific surface area measurements of separated clay fractions with their corresponding REE mobility patterns is considered as a potential avenue for future work on this topic.

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geobiology, Tephra, Tuffs, Dinarides, clays, REE mobility