Most of the iron in banded iron formation (BIF) was deposited during the period 2.7-1.8 Ga. Within this period, Fe deposition rates were at least as high as 10(12) gm/y. According to new age models for the largest BIFs, peak deposition occurred between 2.4-2.3 Ca. It is likely that such high deposition rates would not be met by a solely continental source of iron unless significant quantities were remobilized in marine sediment. Estimates of global heat loss for the Late Archean-Early Proterozoic suggest a hydrothermal cycling rate approximately three times greater than at present, so hydrothermal water-rock exchanges were a more important source of Fe to the oceans than today. Hydrothermal plumes, depleted in O-2 and enriched in Fe, were generated at mid-ocean ridge crests with substantially shallower depths than present. Assuming hydrothermal effluent dilution factors of 10(4), plumes transported iron which had a dominantly hydrothermal source into the upper water column and transferred it to margins where BIFs accumulated. However, unless the quantity of iron liberated during hydrothermal cycling was significantly greater per unit volume of water than today, then either approximate to 10% of all hydrothermal iron accumulated in the five very large class BIFs, or they also received Pe from continental sources.
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