Gallium-Aluminium Ratios as a proxy for Archean seawater: First Insights from the 2.7 Ga old Temagami Banded-Iron Formation, Canada

Banded iron formations (BIFs) are robust geochemical archives used for the reconstruction of the evolution of the Precambrian hydrosphere-atmosphere system. Here, we present preliminary results on the gallium and aluminium concentrations in detritus-free samples from adjacent metachert and magnetite bands in the Neoarchean Algoma-type Temagami BIF, Canada. Gallium and Al are geochemical partners with rather coherent behaviour during magmatic and clastic sedimentary processes. However, Ga and Al fractionate in hydrous environments, leading to different Ga/Al ratios in soils, river water, seawater, and hydrothermal fluids. Therefore, the Ga/Al ratio may be used as a source proxy, similar to the better-constrained Ge-Si systematics in Precambrian BIF. Ga-Al systematics may yield information on the major sources to seawater during deposition of Fe oxide and metachert BIF bands.

Measurements of adjacent Temagami BIF bands with solid (i.e., laser-ablation) and solution (i.e., acid-digested sample) ICP-MS show distinct differences for metachert. Gallium is highly susceptible to interferences in mass spectrometric analyses, and thus high-resolution ICP-MS or QQQ-ICP-MS needs to be used. Our preliminary results suggest that, in marked contrast to Ge and Si, Ga and Al do not show large differences between magnetite and metachert bands in the Temagami BIF. However, our preliminary Ga/Al ratios for the Temagami BIF are generally lower than those of modern seawater, possibly indicating Ga-Al fractionation between BIFs and their seawater source and/or lower Ga/Al ratios in ambient Temagami seawater.