A first look into the gallium-aluminium systematics of Early Earth’s seawater: Evidence from banded iron formations

We conducted the first study on Ga-Al systematics in Archaean and Palaeoproterozoic banded iron formations (BIFs). Adjacent Fe oxide, metachert and mixed-type bands were analysed comparatively with solution-based SF-ICP-MS and ICP-MS/MS and laser-ablation SF-ICP-MS on nano-particulate pressed powder tablets and polished sections. Furthermore, we conducted a matrix-matched two-component mixing experiment with the BIF reference material IF-G and pure synthetic quartz sand. Results of the three comparative analytical procedures and the two-component mixing experiment assure a high quality of our analytical data even in the trace metal-poorest (meta)chert samples. Furthermore, the results suggest that finely dispersed Fe oxide particles dominate the Ga and Al content in BIF (meta)chert bands. Regardless of the samples' mineralogy, the Ga/Al ratios of BIFs range between 2×10^-4 and 1×10^-3. A compilation of Ga/Al ratios in the investigated samples throughout time shows that during Precambrian global marine Ga/Al ratios were most likely rather constant. The BIF Ga/Al ratios are above those of potential detritus but below those of modern seawater. Two conclusions are conceivable: (i) Precambrian seawaters had lower Ga/Al ratios than modern seawater, possibly due to the reduced importance of organisms and organic compounds during weathering, riverine and estuarine processes. (ii) Ga and Al were fractionated during BIF formation, and BIFs did not preserve the original seawater Ga/Al ratios.