Marine chemical sedimentary rocks, like banded iron formations (BIFs), ferromanganese (Fe-Mn) crusts and nodules, marine carbonates or cherts, are of great scientific interest because they can preserve primary information on the physico-chemical conditions of ambient seawater. Especially for research on the Precambrian, marine chemical sedimentary rocks are invaluable archives as they are the only remaining access point to the geochemical conditions of the Archaean and Palaeoproterozoic marine environment.
This PhD thesis investigates the geochemical partner couples of gallium, aluminium (Ga-Al), germanium, and silicon (Ge-Si). Those two couples show mostly coherent geochemical behaviour in igneous and clastic sedimentary processes. However, in (low-temperature) aqueous environments, both partners decouple from each other. This thesis aims to investigate the behaviour of Ga-Al and Ge-Si during the precipitation of Fe (oxyhydr)oxides in the natural environment and to elaborate on whether characteristic distributions of Ga/Al and Ge/Si ratios in marine chemical sedimentary rocks can be applied as geochemical proxies.
Applying Ga/Al and Ge/Si ratios as geochemical proxies requires sound knowledge and understanding of their geochemical behaviours in the natural environment. Few studies have been published on the coupled or decoupled geochemical behaviour of Ga-Al and Ge-Si. These studies focus on the distribution between solid and fluid phases of Ga-Al and Ge-Si during rock weathering, soil formation, in riverine and estuarine processes, and within the seawater column. However, an investigation of Ga-Al and Ge-Si during the formation of Fe-rich precipitates was missing. While BIFs are absent in the modern system, hydrogenetic Fe-Mn crusts (and nodules) might come closest to some kind of “BIF-analogue”. Although the geochemical characteristics of BIFs and Fe-Mn crusts clearly differ, both contain Fe (oxyhydr)oxides as a major phase. The comparison of Ga-Al (PhD thesis of Katharina Schier, see below) and Ge-Si (this thesis) systematics in seawater and hydrogenetic Fe-Mn crusts shows fractionation of both couples during scavenging by Fe (oxyhydr)oxides. Hydrogenetic Fe-Mn crusts from the Pacific and Atlantic show lower Ga/Al but higher Ge/Si ratios than ambient seawater, indicating preferred scavenging of Al and Ge, respectively. This observation is in accordance with data from the existing studies on Ga-Al and Ge-Si behaviour in the natural environment. It is commonly attributed to the higher particle reactivity of Al and Ge compared to Ga and Si, respectively. Particle reactivity, in general, should not have been very different during the Precambrian. Nevertheless, in the modern environment, large amounts of particles are of organic origin and metal complexation by organic ligands might also influence the behaviour of Ga, Al, Ge and Si. During the Archaean and Palaeoproterozoic, however, organic particles and ligands were probably almost absent and negligible for particle and complexation reactions in the natural systems.
Banded iron formations may preserve primary trace element signals from ambient seawater, which in turn can give insights into the contemporaneous physico-chemical conditions of the atmosphere and hydrosphere. However, essential prerequisites are purity and pristineness of the investigated BIF samples, which also applies to geochemical archives in general. The rare earth elements (REE) and yttrium (REY) are a powerful tool commonly used to assess potential detrital contamination and post-depositional alteration of marine chemical sediments. Many geochemical studies on BIFs have been conducted throughout the past decades, and many BIFs are already well-characterised. Nevertheless, there are still unstudied BIFs like the Mt. Ruker BIF from East Antarctica. The first geochemical study conducted on an Antarctic BIF shows that the ~2.5 Ga old Mt. Ruker BIF is pure, pristine, and, therefore, a promising target for further geochemical investigations. An in-depth investigation of individual Fe oxide and metachert bands shows that the Mt. Ruker BIF preserved not only a general seawater-like REY distribution (with the BIF-typical exceptions of Ce and Eu) but also the very subtle W-type lanthanide tetrad effect (LTE). This W-type LTE is a characteristic feature of (modern) seawater and indicates, unfractionated incorporation of REY into BIFs. Furthermore, the presence of W-type LTE and uniform Y/Ho ratios in BIF Fe oxide and metachert bands clearly argue for a primary origin of BIF banding. Lab experiments have shown that a diagenetic separation from a previously homogeneous Fe-Si gel would have caused Y-Ho fractionation and the opposite M-type LTE.
A compilation of published BIF REY data shows that also the 2.7 Ga Temagami BIF from Ontario, Canada, preserved the W-type LTE. The Temagami BIF is one of the most studied BIFs, and it is considered to be extremely pure and pristine. It is frequently used as a benchmark test for applying newly developed proxies and, therefore, was chosen for the first study on Ga-Al systematics in BIFs. However, the determination of Ga and Al in BIFs is challenging due to extremely low mass fractions, especially in (meta)chert bands, and due to multiple mass-interferences on Ga when measured with inductively coupled plasma mass spectrometry (ICP-MS). Therefore, comparative analyses with solution-based sector-field (SF-)ICP-MS, ICP-MS/MS and laser-ablation (LA-)ICP-MS and a two-component mixing experiment were conducted to ensure analytical quality. The high-quality Ga-Al data show that the Ga/Al ratios in Temagami BIF are below those of modern seawater but, surprisingly, comparable to those of modern Fe-Mn crusts and nodules.
Furthermore, Temagami Fe oxide and metachert bands show similar Ga/Al ratios, regardless of mineralogy. This indicates that finely dispersed Fe-oxide particles most likely dominate the Ga and Al content in metachert bands. Additional measurements of BIFs of different ages also show Ga/Al ratios similar to those of Temagami BIF, arguing for a rather uniform Ga/Al ratio range in Precambrian BIFs. At the current stage, two explanations for the observed BIF Ga/Al ratios are conceivable: (i) Precambrian seawater had lower Ga/Al ratios than modern seawater given that BIFs incorporated Ga and Al without fractionations, or (ii) Ga and Al were fractionated during BIF formation, diagenesis and/or metamorphic alteration. Future research on the Ga-Al systematics of other BIFs, but also of marine carbonates and cherts, might solve this question.