Behavior of iron isotopes in hydrothermal systems: Beebe and Von Damm vent fields on the Mid-Cayman ultraslow-spreading ridge

It is now clear that, in some parts of the ocean, inputs of hydrothermal iron (Fe) can make a more significant contribution to the Fe inventory than previously thought. While the Fe isotopic signature of seawater has proved useful for distinguishing between inputs of Fe from atmospheric deposition and seafloor sediments, the Fe isotope signature of hydrothermal vent fluids may change during mixing of vent fluids and seawater. To better constrain the processes leading to these changes, the Fe isotopic compositions (δ56Fe) of dissolved and total dissolvable Fe have been determined in high temperature vent fluids and the buoyant hydrothermal plumes at the Beebe and the Von Damm vent fields, which are located along the ultraslow Mid-Cayman spreading center in the Caribbean Sea. Our results show that the δ56Fe value of dissolved Fe in the earliest stages of buoyant plume formation was lower (as low as −4.08‰) than measured in a high temperature, low-Mg, vent fluid sample (−0.28‰). This indicates that the iron isotopic signature of dissolved Fe is principally controlled by oxidation of Fe(II) and precipitation of Fe-(oxyhydr)oxides that preferentially incorporate heavy Fe isotopes. In support of this, the δ56Fe value of labile particulate Fe was higher than the δ56Fe value of dissolved Fe. Nevertheless, at Beebe, the δ56Fe value of total dissolvable Fe increased as the proportion of Fe predicted to have been lost from the plume increased, consistent with preferential fall-out of Fe-sulfides that are enriched in light Fe isotopes. The very low δ56Fe values of dissolved Fe in the Beebe buoyant plume are consistent with (i) the high Fe/H2S ratio of the vent fluids, and (ii) high Fe(II) oxidation rates, relative to other vent sites.