REE and Nd isotope data indicate that most of the iron in banded iron formations is derived from hydrothermal sources but do not exclude a significant contribution from terrestrial sources, such as diagenetic recycling. A diagenetic model has been used to estimate the recycling of iron into overlying seawater, due to microbial reduction and dissolution at depth in anoxic sediment pore waters, followed by diffusion upward through a surface layer of sediment that contains oxygenated pore waters. Rates of iron recycling increase with higher pore-water dissolved iron concentrations, decreasing pH and temperature, and smaller thicknesses of the surface oxygenated layer. Iron can be recycled at rates of 1000–5000 µg cm −2 yr −1 from Proterozoic (pO 2 = PAL) pore waters with dissolved Fe 2+ = 1–5 µg cm −3 , pH 6.5 (and T < 65 °C), or pH 7.0 (and T < 40 °C), or pH 7.5 (and T < 20 °C), provided the thickness of the surface oxygenated layer is less than 0.1 cm. Lower pO 2 levels and more weakly oxygenated surface layers do not significantly increase the maximum recycling rates but enable these to be achieved at larger thicknesses of the surface layer, for all pH 6.5–7.5 and temperatures from 10 to 65 °C. Rates of iron supplied by diagenetic recycling can be substantially modified by the export efficiency (ϵ) from the source area and by the ratio (Source Area)/(Sink Area), which can either disperse or concentrate the recycling flux that is delivered to a sink area of banded iron formation. Banded iron formations that require maximum iron delivery rates of 22500 µg cm −2 yr −1 can be produced only by recycling rates of 5000 µg cm −2 yr −1 (and ϵ = 1) from a source area that is at least four times larger than the area of banded iron formation. Modern basins have ratios of shelf area (<200 m water depth) to deep basin area that commonly range from 0.2 to 4. Basins at either extreme have ratios of (Deep Basin Area)/(Shelf Area) or (Shelf Area)/(Deep Basin Area) that exceed 4 and are potentially able to concentrate iron either from a deep basin source area to banded iron formation on the shelf, or from a shelf source area to a banded iron formation depositing in the deep basin. However, these mass balance constraints require the existence of substantial areas of contemporaneous source sediments (or smaller areas of iron-enriched sediments) located either on the shelf or in the deep basin.

Rare earth element (REE) analyses of Precambrian banded iron formations (BIFs) show that distinct negative Ce anomalies, although rather weak or moderate (Ce/Ce* = 0.5–0.9), are commonly present in Algoma-type BIFs of the Early and Middle Archean, and even in the 3.8–3.7 Ga Isua iron formation (IF). This indicates that the seawater columns from which the BIFs precipitated were not entirely anoxic and that Ce oxidation mechanisms already existed in the 3.8–3.7 Ga oceans. The presence of pronounced negative Ce anomalies (Ce/Ce* = 0.1–0.5) in Late Archean (2.9–2.7 Ga) Algoma-type BIFs suggests that strongly oxygenated oceanic conditions like today emerged by 2.9–2.7 Ga. This suggestion is consistent with geologic evidence that small but widespread Mn deposits formed during Late Archean time. The Hamersley and Transvaal IFs (2.7–2.4 Ga in age) have less noticeable Ce anomalies (Ce/Ce* = 0.7–1.0). These BIFs were deposited on an evolving rift in a land-locked ocean that became anoxic due to intense hydrothermal activity. The 2.2–2.1 Ga Superior-type IFs exhibit distinct negative Ce anomalies (Ce/Ce* = 0.2–0.7), but the ca. 1.9 Ga IFs and the ca. 0.7 Ga IFs have less distinct Ce anomalies. These variations in Ce/Ce* values of the post-2.7 Ga BIFs may reflect the episodicity in global mantle plume activity that created locally anoxic basins. The Archean Algoma-type BIFs have distinctly positive but variable Eu anomalies (Eu/Eu* = 0.8–7). Their strong positive Eu anomalies suggest large contributions of hydrothermal fluids to the seawater involved in BIF precipitation. The large variation in Eu anomalies in Algoma-type BIFs reflects the large variation in mixing ratios of hydrothermal fluids and ambient seawaters at various depositional sites. However, the post-2.7 Ga Superior-type BIFs exhibit much lower and constant Eu/Eu* values (0.7–1.9). This implies that the REE chemistry of the basin water that hosted voluminous Superior-type BIFs was influenced by a riverine influx from the surrounding continent that grew rapidly due to global mantle plume activity, besides the intense hydrothermal influx.

Geological and geochemical characteristics of banded iron formations (BIFs) suggest that they formed by mixing locally (or regionally) discharged submarine hydrothermal fluids with local seawater, rather than by upwelling deep ocean water. Submarine hydrothermal fluids typically evolved from local seawater by acquiring heat, metals, and sulfur during deep circulation through a variety of rocks (e.g., volcanics, evaporites) in greenstone terranes that developed under a variety of tectonic settings. In general, when the fluids were heated above ∼350 °C, they may have produced Cu- and Zn-rich volcanogenic massive sulfide deposits (VMSDs), whereas those heated less than ∼200 °C were generally poor in H 2 S and heavy metals, except Fe, and may have subsequently produced BIFs. Depending on the salinity contrast between discharging hydrothermal fluids (evolved seawater) and local seawater, hydrothermal fluids may (1) mix rapidly with local seawater to form smoker-type BIFs or (2) create a metal- and silica-rich brine pool, mix slowly with the overlying water body, and form brine pool-type BIFs. BIFs associated with VMSDs and volcanic rocks generally belong to smoker-type BIFs; many formed at seawater depths >2.5 km. Large BIFs, including the 2.6–2.4 Ga BIFs in the Hamersley Basin, Australia, the 2.5 Ga Kuruman IF in South Africa, and the 1.87 Ga BIFs in the Lake Superior region, United States-Canada, belong to brine pool-type BIFs. The Hamersley Basin and possibly other large BIF-hosting basins were probably land-locked seas (like the Black Sea) where river waters diluted the surface water zone and the underlying water bodies were anoxic. During the accumulation of a BIF sequence, the dominant Fe mineralogy frequently changed from ferric (hydr)oxides (oxide BIFs) to siderite (carbonate BIFs) and to pyrite (sulfide BIFs). Such changes were probably caused by changes in the relative amounts of dissolved O 2 (DO), ΣCO 3 2− , and ΣS 2− in local seawater. From the Fe 2+ -O 2 mass balance calculations for the formation of iron oxides in smoker-type BIFs, and the relationship between the atmospheric pO 2 and oceanic O 2 depth profile, we conclude that the atmosphere and oceans have been fully oxygenated since ca. 3.8 Ga, except in local anoxic basins. Thermodynamic analyses of the formational conditions of siderite and analyses of the carbon isotopic composition of siderite associated with major BIFs suggest that the pre–1.8 Ga atmosphere was CO 2 -rich (pCO 2 >100 PAL) and CH 4 -poor (pCH 4 ≈ 10 ppm); therefore, CO 2 , rather than CH 4 , was the major greenhouse gas throughout geologic history. After a decline of hydrothermal fluid flux, BIF-hosting basins generally became euxinic (H 2 S-rich) because of the increased activity of sulfate-reducing bacteria (SRB) and SO 4 2− -rich seawater, and thereby accumulated organic carbon-rich and pyrite-rich black shales (sulfide-type BIFs). The SO 4 2− contents and SRB activity in the oceans have been essentially the same since ca. 3.8 Ga. The Archean oceans were most likely poor in both Fe 2+ and silica, much like modern oceans. Our study also suggests that diverse communities of organisms, including cyanobacteria, SRB, methanogens, methanotrophs, and eukaryotes, evolved very early in Earth's history, probably by the time the oldest BIFs (ca. 3.8 Ga) formed. BIFs have been found in rocks of all geologic age. Therefore, they cannot be indicators of an anoxic atmosphere and/or anoxic oceans as suggested by many previous researchers. Instead, BIFs indicate that the atmosphere and ocean chemistry have been regulated at present compositions (except pCO 2 ) through geologic history by interactions with the biosphere. The general trend of declining size and abundance of BIFs with geologic time reflects the cooling history of Earth's interior.