Naslov
On the occurrence and origin of eogenetic Fe-Mn nodules in the post-Würmian Alpine sediments of Geneva basin, Western Switzerland

Autori
Šegvić, Branimir ; Girardclos, S ; Zanoni, G ; Arbiol Gonzalez, C ; Steimer-Herbert, T ; Besse, M

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Skup
XVI international Clay conference

Mjesto i datum
Granada, Španjolska, 17.07.2017. - 21.07.2017

Vrsta sudjelovanja
Predavanje

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
Fe-Mn nodules ; Clay minerals ; Surficial geology ; Mid-Holocene Paleoclimatology ; The Alps

Sažetak
The soils and surface sediments of Geneva basin located in western Switzerland are intrinsically tied to the last Alpine glaciation that reached its maximum around 20 to 22 ky BP. Substantial areas have thereafter become free of ice due to the global warming and subsequent glacier retreat [1]. Colluvial and alluvial processes are considered parental to the young Holocene sediments featured by some remarkable eogenetic phenomena like the formation of ferromanganese nodules, which is discussed in this contribution. Namely, along the hillside of Grand-Saconnex, a locality situated within the Geneva city limits, recent archaeological excavations have revealed a nicely developed ~1.5 m thick exposure of post- glacial sediments. The sequence consists of reworked glacial till, preceded by several distinct layers of clayey to sandy deposits [2]. At the interface of two colluvial units, the upper very fine grained and the lower marked by the brown-orange alterations, a cm- thin zone rich in Fe-Mn nodules has been reported. A detailed X-ray elemental mapping coupled with XRD and XRF investigations has been performed in order to understand occurrence modes as well as the origin of such mineralization. The nodules host layer mainly consists of illite, chlorite, several mica varieties, mixed-layer illite-smectite, quartz and amorphous matter. In addition, myriad ferromanganese concretional forms are present, attaining up to 1.5 mm in diameter size. Compared to adjacent layers, 10 to 15 times higher concentrations of Fe and Mn were reported in nodule-rich domains. The nodules are spherical to subangular, with an onion-like quasi-layered internal architecture marked by selective enrichments of Fe and Mn. Iron is preferentially encountered along the rims, whilst Mn is mostly concentrated in the nodules’ cores. An increasing porosity gradient has been reported toward the centre of the nodules, which points to the secondary pore filling that must have commenced tangentially to the nodules’ rim. Mineralogy of nodules reflects the composition of the host sediment with an exception of very low crystalline, practically amorphous, Fe-Mn aggregates present solely within the studied concretional forms. In comparison to the neighbouring colluvial layers, the nodule-rich sediment level is 10 to 100 times enriched in igneous and metamorphic phases such as olivine, Cr-spinel, clinopyroxene, amphibole, garnet, chlorite and biotite. Furthermore, this layer shows different clay mineral content, which is devoid of hydroxyl-interlayered minerals typical for the rest of the colluvium. The above-mentioned Fe-rich mineral suite of magmatic and metamorphic origin, reported in the nodule-rich layer, is believed to have served as a feedstock of Fe and Mn. Most probably, the morphology of terrain receptive to water retention, caused favourable pH-Eh conditions that enhanced mobilization of Fe and Mn in their soluble forms eventually leading to their dispersion within the host sediment. Electron microscopy identified exfoliated chlorite, and presumably metamorphic vermiculite (mixed 10-14 Å sheet silicates), as the most evident sources of iron. Oscillating redox conditions triggered the precipitation of Fe and Mn in the form of low-crystalline aggregates attached to the outer surfaces of illite crystallites. This process ultimately saw an end with the formation of nodules thoroughly cemented by Fe- Mn mineralization.

Izvorni jezik
Engleski

Znanstvena područja
Geologija

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