Abstract
Understanding the role of hydrothermal iron (Fe) within the marine biogeochemical Fe cycle has undergone a paradigm shift with the discovery of mechanisms that stabilize hydrothermal Fe and enhance its dispersal in the ocean. This shift has spurred interest in provenance studies using Fe stable isotopes. However, the dispersal and fate of hydrothermal Fe, along with its impact on Fe isotope distribution, remain incompletely understood. In this study, we present Fe isotope data from a young post-eruptive back-arc hosted hydrothermal vent system. Here we investigate the Fe isotopic compositions of host rock, hydrothermal fluids and precipitates from the newly discovered Nifonea vent field in Vanuatu. Located in a young back-arc basin, Nifonea provides a unique opportunity to (1) study processes affecting Fe isotope fractionation, represented by the 56Fe/54Fe signature (δ56Fe), in environments characterized by short-lived heat pulses and relatively low water depths, and (2) better constrain Fe isotope effects resulting from subsurface sulfide precipitation and phase separation. The results indicate the complex interplay of sulfide formation and phase separation producing large spatial variability of fluid Fe isotopic compositions with generally low δ56Fe. High temperatures from recent volcanic events are interpreted to facilitate slow precipitation of chalcopyrite with systematically higher δ56Fe compared to hydrothermal fluids causing considerable Fe isotope effects. Variations in Fe/H2S ratios lead to different transformation processes and Fe isotope systematics at individual Nifonea vent sites. In addition, phase separation at relatively low-pressure conditions produces low-Cl vapour phases and appears to strongly partition the Fe isotopes into vapour and liquid phases. These results further support predictions from previous research (Bennett et al., 2009) however, they also reveal the very complex interplay of various processes affecting the Fe isotope systematics in hydrothermal plumes and preclude a generalized isotopic signature for hydrothermally derived Fe for input into mass balance models. This combination of mechanisms underscores the diversity of such vent fields and highlights post-eruptive venting in back-arc spreading centers as a crucial source of dissolved Fe to the deep ocean in back-arc spreading centers as a crucial source of dissolved Fe to the deep ocean.