CHEMICAL EVOLUTION OF THE ATMOSPHERE AND OCEANS: CONSTRAINTS FROM BANDED IRON-FORMATIONS

BIFs may be classified into: (I) those underlain by submarine volcanic rocks (±VMS deposits) and (II) those underlain by sedimentary rocks (shales, carbonates, sandstones, or conglomerates). Contrary to a popular belief that BIFs formed mostly before ~1.8 Ga ago, BIFs of all types have formed throughout geologic time. The environments and processes for formation for (I) were essentially the same as those for VMS deposits. They were formed by mixing of hydrothermal fluids, which were discharged in submarine depressions (basins), with local basin water. The relative concentrations of dissolved O2, bicarbonate, and H2S of the local basin water and the Fe2+ content of hydrothermal fluid determine the formation of oxide-, siderite-, or pyrite-type BIFs. The mineralogy and chemistry of (I) suggest that the Archean atmosphere was oxygenated (pO2 > ~0.5 PAL), rich in CO2 (>100 PAL), and poor in CH4 (<10 PAL). The Archean oceans were fully oxygenated, except for local/regional anoxic basins, and sulfate rich, similar to today.

The ~2.5 Ga Brockman IFs in the Hamersley Basin (Western Australia), and most other large sediment-footwall BIFs (e.g., Gunflint and Kuruman BIFs), were most likely formed in local/regional anoxic basins. The Hamersley Basin was created by the intermittent rifting of a thick continental crust caused by mantle upwelling, and became a stratified, land-locked sea, like the Black Sea. The surface water of the Hamersley Basin was oxygenated seawater that was diluted by river water, much like the Black Sea. While the basin bottom water was primarily comprised of anoxic seawater, it was periodically replaced by hot, metalliferous brine that discharged from the basin floor, similar to the metalliferous brine pools of the Red Sea. Gentle mixing of the metalliferous brine with the seaward-flowing surface water was probably the main cause for the basin-wide, simultaneous nucleation of iron (hydr)oxides and silica.

Comparisons of the temporal distributions of BIFs, VMS deposits, mantle plumes, and rift systems suggest the major cause for the changes in frequency and size with geologic time was not related to the atmospheric O2 level, but to the changes in thermal structure and dynamics of the mantle.