Hydrothermal vent fluid-seawater mixing and the origins of Archean iron formation

Nanoparticulate apatite and greenalite in oldest, well-preserved hydrothermal vent precipitates

“…To examine the behavior of dissolved phosphorus in subseafloor hydrothermal systems during the Archean, we estimated equilibrium solution compositions and thermodynamically predicted mineralogy by executing reaction path models. These models, originally developed by Tosca and Tutolo (13), were modified to include P species and minerals, as described below. The models are designed to represent three major components of subseafloor-hosted hydrothermal systems: (i) the reaction zone, where peak pressure and temperature conditions are encountered near the base of the near-axis hydrothermal system; (ii) the upflow and reaction of hydrothermal fluids through, and with, the oceanic crust; and (iii) the mixing between hydrothermal vent fluids and ambient seawater.…”

“…At the pressure/temperature conditions relevant to Archean near-axis systems, which are expected to have featured shallower magma chamber depths and lower pressures of water-rock interaction, experimental and theoretical studies indicate that fluid chemistry is dominantly controlled by fluid-mineral equilibria involving feldspar, chlorite, epidote, quartz, and NaCl-rich fluids at approximately 375° to 400°C and 300 to 500 bar, which is consistent with geophysical constraints on pressure and temperature. We examined this system under rock-buffered conditions at 400°C and at 400 and 500 bar, which are taken to be broadly representative of Archean subseafloor reaction zone conditions on the basis of considerations discussed above (13).…”

“…The second component of the reaction path models investigated phosphorus behavior in systems approximating those likely to be encountered as reaction zone fluids ascend through sheeted dyke complexes and higher permeability lavas that characterize the oceanic crust. We repeated calculations performed by Tosca and Tutolo (13), which involved separation of the hydrothermal fluid from the rock buffer and fluid upflow through mafic rocks along a prescribed cooling and decompression pathway from 400°C/400 bar to 250°C/250 bar, approximately consistent with conditions associated with venting of hydrothermal fluids at the modern seafloor.…”

“…This system also included the effects of reaction between the hydrothermal fluid and hypothetical wall rocks within the oceanic crust (13,44), which involved reaction with varying masses of fresh crystalline basalt chosen to represent average modern mid-oceanic ridge-type basalt (MORB). This allowed us to evaluate the effect of water/rock ratio (from 1 to 5000), which, for modern subseafloor systems and ancient hydrothermally altered equivalents, is inferred to have ranged between approximately 250 to 1000 based on minor, trace element, and isotope geochemistry (45)(46)(47)(48)(49).…”

“…Most models for BIF and jaspilite deposition invoke upwelling of dissolved Fe(II)-rich deeper water followed by biological oxidation in the surface ocean (7,8), either indirectly involving molecular oxygen produced by Cyanobacteria or directly by microaerophilic Fe-oxidizing bacteria or anoxygenic phototrophs. As with modern hydrothermal sediments, the oxidation of Fe(II) and its settling through the water column is considered to be a major pathway for the deposition of seawater P. These models rely on the widely held assumption that the primary Fe phases were Fe(III)oxyhydroxides; however, this is now being debated following recent studies of BIFs that suggest that the earliest phases were Fe(II)silicates (9)(10)(11)(12)(13).…”