Localized geochemical variability produced by depositional and diagenetic processes in a 2.8 Ga Ca-carbonate system: A cautionary paradigm

A plethora of proxies has been developed over the preceding two decades in attempts to investigate the geochemistry of the Archean ocean–atmosphere system, and in particular oxygen levels. Unfortunately the necessary parallel investigations of the effects that localized ocean chemistry and diagenesis can have on Archean sediments have commonly not kept pace. We used micro-analytical techniques (LA-ICP-MS and XRF scanning), to distinguish the effects of changes in water composition during precipitation and diagenesis on marine limestone precipitates at the margin and interior of a 2.8 Ga carbonate platform (Mosher Carbonate, Steep Rock Group) in western Superior Province, Canada. Platform margin meter-scale hybrid giant domes consist of centimetric interlayered couplets of (1) Sr-rich crystal fan fabric and cuspate fenestral microbialite, both with pronounced negative Ce-anomalies, and (2) net-like fenestral microbialite, rich in diagenetic cement with diminished concentrations of Sr and lacking negative Ce anomalies. The elevated Sr in the crystal fan fabric and cuspate fenestral microbialite is a general sign of less diagenetic alteration, as is preservation of millimeter-scale chemical differences. XRF mapping revealed that samples that otherwise appear pristine from a second site on the platform rim, near a zone of alteration in the limestone, have ferroan dolomite-filled micro-fractures with Mn flooding of the surrounding calcite and lack Ce anomalies. Platform interior silicified and ferroan dolomitized columnar stromatolites have some calcite laminae, but exceptionally low Sr contents indicate that they formed by dedolomitization. In several horizons REE patterns identical to those of offshore iron formation, replacement of calcite by iron carbonate, and the presence of iron oxides all suggest that short term flooding of the platform by offshore seawater episodically introduced ferroan dolomitizing fluids. Overall, these data indicate a restricted marine environment subject to periodic flooding by offshore waters that caused seafloor diagenetic alteration and precipitation of iron hydroxides. In this system the least altered limestone was the lithotype most likely to retain evidence of free oxygen. Samples with abundant phreatic cement and/or Mn alteration associated with micro-fractures were liable to have experienced REE mobilization. Detailed studies that integrate both depositional and diagenetic information are critical for the correct interpretation of geochemical data from sedimentary rocks.