Determining the source of Fe for Banded Iron Formation (BIF) and reconstructing conditions that lead to their deposition is crucial to our understanding of the early evolution of the geochemical and biogeochemical processes that have come to dominate the modern Earth. The majority of BIF was precipitated between 2.8 and 1.7 Ga during a time of anoxia and low sulfate conditions in seawater. Fe-rich plumes of hydrothermal origin are thought to be the main source of Fe for BIF formation during this time, though estimates of Fe-flux needed to form these deposits suggest higher hydrothermal heat fluxes and/or fluid-rock reaction temperatures greater than indicated by modern vent systems. pH and dissolved Cl in hydrothermal systems, however, have been shown to greatly influence Fe solubility. Estimates from Archean fluid inclusion data indicate that dissolved Cl and Ca may have been higher than modern seawater by factors of approximately two and twenty, respectively. A relatively high dissolved Ca concentration could buffer pH during hydrothermal alteration at low values owing to constraints imposed by the solubility of Ca-bearing hydrous silicates. Thermodynamic data indicate that the effect of this is even greater at relatively low pressureóa condition that can be inferred from geophysical models for Archean hydrothermal systems. Thus, the combination of high Ca and Cl, with low hydrostatic pressure at the base of ancient MOR-related hydrothermal systems could have provided dissolved Fe fluxes in great excess of modern hydrothermal systems.

We conducted a series of experiments to test the solubility of Fe in Archean-like seawater coexisting with basalt at high-temperature (350-400º C), low pressure (300 bars) hydrothermal conditions. Based on estimates of Archean ocean composition (De Ronde et al., 1997), we prepared Archean-like artificial seawater composed of Na (789.0 mmol/kg), K (19.0), Mg (51), and Ca (232) with a total of 1374 mmol/kg Cl. Experiments were performed in a flexible gold reaction cell, which permitted sampling at elevated temperatures and pressures and monitoring of fluid chemistry with reaction progress. Fluid samples at 350º C indicated an average of 5.14 mmol/kg of Fe in solution, considerably higher than for modern systems at similar temperatures. When temperature was increased to 400º C, however, dissolved Fe increased by nearly an order of magnitude (40.1 mmol/kg). We interpret this dramatic increase in dissolved Fe to be chiefly the result of pH lowering induced by Ca-metasomatism associated with the formation of epidote and/or Ca-amphibole minerals after plagioclase. In effect, loss of Ca generates acidity, which is then balanced largely by an increase in Fe (Ca-Fe exchange). H2S concentrations revealed only a 2-fold increase to a maximum value of 12.0 mmol/kg, when temperature was increased from 350 to 400º C. The absence of dissolved sulfate in the starting fluid precludes additional H2S formation from sulfate reduction during basalt alteration. Thus, Archean vent fluids were likely characterized by high Fe/ H2S ratios, which would greatly limit precipitation of Fe as sulfides near vent sources, enhancing delivery of Fe to the ancient ocean. Our results showing high Fe solubility combined with relatively low H2S concentrations confirm that hydrothermal plumes can be an unusually effective source of Fe for BIF deposition during the middle Precambrian.
 
 

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