Modern rather than Mesoarchaean oxidative weathering responsible for the heavy stable Cr isotopic signatures of the 2.95 Ga old Ijzermijn iron formation (South Africa)

Abstract

Previously reported stable Cr isotopic fractionation in Archaean paleosols and iron formations (IFs) have been interpreted as a signature of oxidative weathering of Cr(III) to Cr(VI) in soils, and delivery of isotopically heavy Cr(VI) into the oceans. One of the oldest reported fingerprints of this process is isotopically heavy Cr preserved in the 2.95 Ga old Ijzermijn IF, Sinqeni Formation of the Mozaan Group (Pongola Supergroup), South Africa and could suggest that atmospheric free oxygen was present ca. 600 million years earlier than the Great Oxidation Event (GOE). However, fractionated stable Cr isotopic signatures have only been found to date in surface outcrop samples of the White Mfolozi Inlier exposed along the White Mfolozi River Gorge. In this study, the latter outcrop was resampled along with two drill cores of the Ijzermijn IF and a drill core of the Scotts Hill IF to represent multiple exposures of Mozaan Group IFs with different states of preservation. A detailed geochemical comparison on bulk samples of different units was undertaken using stable Cr isotopes coupled with trace and major elements. Outcrop iron-rich mudstones (Fe - lutites) show very low LOI [wt] %, and very low Fe(II)/Fetot ratios, and lower Ca and Mg relative to equivalent facies in drill cores, indicating the effects that oxidative recent surface weathering had on Fe/Mn-rich carbonate minerals of the IF. Overall rare earth element and yttrium (REE + Y) mixing models agree well with previous studies, confirming that they were minimally disturbed by weathering and are consistent with a high magnitude of continental solutes delivered in a near–shore depositional environment, with a minor contribution of hydrothermally derived fluids that upwelled into shallower depositional setting. Importantly, all drill core samples of this study revealed δ53/52Cr values within the igneous inventory, despite variable amounts of detrital Cr input that includes nearly detritus-free, chert/jasper-rich units. By contrast, a specific group of Fe-lutite samples near the base of White Mfolozi River outcrop bear fractionated Cr isotopic signatures with δ53/52Cr values up to 0.418‰. These outcrop samples also display unusually high U/Th ratios (max. 12.6) as well as enrichments of other elements (W, Tl, As, MREE) that far exceed that observed in correlative drill core units. These observations together with the lack of Cr isotopic fractionation in drill core samples lead us to propose that the heavy δ53/52Cr values of Fe-lutites from outcrop Ijzermijn IF samples reported here and in a previous study are the product of modern oxidative weathering rather than an indicator for Mesoarchaean oxidative weathering at ca. 2.95 Ga.