DEPRESSURIZATION OF HYDROTHERMAL VENTS DURING SNOWBALL EARTH: EFFECTS ON OCEAN CHEMISTRY

Depressurization of hydrothermal fluids in contact with basalt at elevated temperatures (350-425^oC) causes substantial increases in Fe²⁺ concentrations, primarily as a result of increased stability of ferrous-chloride aqueous species and decrease in mineral-buffered pH at lower pressures. This observation leads us to speculate that glacioeustatic sea-level fluctuations may have caused variations in iron supply to the oceans, both on Quaternary time scales (~10 bar fluctuations) and at times in the more distant past, e.g., during the Neoproterozoic Snowball Earth episodes, when continental ice sheets may have been considerably larger than those of the Quaternary. The higher fertility of the Quaternary glacial ocean, and the notable association of Neoproterozoic glacial deposits and iron formation, may be at least in part a consequence of this phenomenon.

There are complications, however, including the consequences of phase separation and cooling of the fluid upon ascent from near the magma chamber to the seafloor. The significant variability in Cl^- concentrations of some modern vent fluids indicates that phase separation does occur; the low Fe²⁺ concentration of these fluids indicates that aqueous iron partitions nearly completely into the residual brine. The ultimate fate of this metal-rich brine is unknown, but it may not immediately vent to the overlying ocean. Cooling of vent fluid occurs upon ascent by conduction through wall rocks or mixing with cold seawater. A consequence of this cooling is the precipitation of iron-bearing minerals, particularly pyrite but also epidote and chlorite, and thus the reduction in Fe concentration of the fluid. These complications make unambiguous calculations of variations of iron flux problematic. Although these complications may affect the rate of Fe delivery to seawater, it is still likely that even subtle changes in pressure in subseafloor reaction zones at high temperatures, could have the potential for creating significant reservoirs of Fe-bearing fluids with implications for attendant impact on the paleo-marine environment and its sedimentary record.