A first look at the gallium-aluminium systematics of Early Earth’s seawater: Evidence from Neoarchean banded iron formation

Abstract

Geochemical proxies archived in banded iron formations (BIFs) can provide information on the physico-chemical conditions in the Precambrian atmosphere-hydrosphere system and the element sources to ambient Early Earth’s seawater. We here report results of the first investigation of the Ga-Al systematics of BIFs and use adjacent magnetite and metachert bands of the well-studied and very pure 2.7 Ga-old Temagami BIF in Ontario, Canada, in an attempt to test the suitability of the Ga/Al ratio as a geochemical proxy. Due to very low Ga concentrations in BIFs and multiple mass interferences on the stable isotopes 69Ga and 71Ga, we conducted comparative analyses of dissolved samples with sector-field (SF-)ICP-MS and tandem ICP-MS/MS as well as laser-ablation (LA-)SF-ICP-MS on nano-particulate pressed powder tablets (NP tablets). The unexpected results indicate uniform Ga/Al weight ratios between 2.0×10−4 and 7×10−4 for the Temagami BIF, i.e. Ga/Al ratios that are slightly above those of potential clastic detritus but well below those of modern seawater or marine hydrothermal fluids. Consistent results from solution SF- and ICP-MS/MS and a two-component mixing experiment with BIF reference material IF-G and synthetic pure quartz sand corroborate the high analytical quality of these Ga and Al data. The Ga/Al ratios of the metachert bands show the same range of Ga/Al ratios as the magnetite bands and appear to be dominated by small amounts of finely dispersed Fe oxide particles that were deposited together with the chert. The Ga/Al ratios of the magnetite bands either suggest that (in contrast to other trace elements such as the rare earth elements and Y) Ga and Al were fractionated from each other during scavenging from Archean seawater by Fe (oxyhydr)oxides or that ambient Temagami seawater had significantly lower Ga/Al ratios than modern seawater, possibly due to different weathering, riverine, and estuarine processes in the Archean. While these first results demonstrate the potential of Ga-Al systematics as an additional geochemical tool for studies of Early Earth, they also reveal the knowledge gaps that still need to be addressed in future investigations.