Geochemistry and iron isotope systematics of hydrothermal plume fall-out at East Pacific Rise 9°50′N

Abstract

While gross hydrothermal fluxes entering the ocean are known to be significant, much remains unknown about the fate of this material as it disperses through the oceans, and its impact upon ocean biogeochemistry. Mineral precipitation within hydrothermal plumes removes hydrothermally-sourced metals from solution and also acts to scavenge trace elements from the surrounding water column. Here, we investigate the fate of particulate Fe released from high-temperature hydrothermal venting at EPR 9°50'N and its potential impact on local deep-ocean Fe-isotopic and geochemical budgets. We measured the geochemical composition, mineralogy and Fe isotope systematics of hydrothermal plume products in order to determine whether mineral precipitation imposes characteristic Fe-isotope "fingerprints" for hydrothermally sourced Fe in the deep ocean. Our sampling includes sediment trap deployments after the eruptive event of Jan. 2006, allowing the examination of temporal changes of hydrothermal fluxes over a 160 day period. Results show that Fe isotope composition in the high-temperature vent fluids is rather constant over the sampling period 2004-2008, and that secular variations of δ 56 Fe values of plume particles from-0.03 to-0.91‰ (relative to IRMM-14 standard) could be explained by local processes leading to variable mixing extents of hydrothermal, biogenic and lithogenic particles. Through geochemical modeling, we have calculated the relative abundances of hydrothermal plume components such as sulfides, Fe oxyhydroxides, organic matter, biogenic and lithogenic phases. We demonstrate that Fe isotope fractionation in the hydrothermal plume occurs during the formation and rapid settling of Fe-sulfides that are characterized by δ 56 Fe values ranging from-0.73 ± 0.13‰ to-0.86± 0.13‰, which is systematically lower than the endmember hydrothermal fluids (δ 56 Fe =-0.4‰). This study suggests that both the initial Fe isotope composition of the high-temperature vent fluids and its initial Fe/H 2 S ratio (i.e. Fesulfide precipitation versus Fe-oxyhydroxide precipitation) should impose characteristic Fe isotope "fingerprints" for hydrothermally derived Fe in the deep ocean.

Dissolved iron and iron isotopes in the southeastern Pacific Ocean

“…While no measurements of the native hydrothermal vent fluid δ 56 Fe in the southern EPR vents of this study have been measured, all vent fluids measured to date along both the northern EPR and the Mid‐Atlantic Ridge have δ 56 Fe <0‰, ranging from −0.21 to −0.69‰ [ Bennett et al , ; Rouxel et al , ; Rouxel et al , ; Severmann et al , ]. Together with the fact that altered basalts in the high‐temperature hydrothermal reaction zone have been recorded to have heavier δ 56 Fe of up to +1.3‰ [ Rouxel et al , ], it has been hypothesized that light Fe isotopes are preferentially leached during hydrothermal vent fluid generation.…”

“…Thus, the heavy δ 56 Fe values of +0.54 ± 0.14‰ (2 SD) recorded in this study in the distal hydrothermal plume provide us with new information about chemical mechanisms of hydrothermal Fe transport from the EPR. First, the dFe is likely not in a nanoparticulate pyrite form, which would be expected to have δ 56 Fe values lighter than vent fluid [Bennett et al, 2009], which we suggest is likely <0‰ [Rouxel et al, 2008b;Rouxel et al, 2016]. Colloidal pyrite (nanopyrite) formation [Gartman et al, 2014;Yucel et al, 2011] is one of two potential mechanisms for how dFe might escape precipitation and become stabilized in hydrothermal plumes, an observation that has come to be known as the leaky vent hypothesis [Toner et al, 2012].…”

Distribution and isotopic signature of ligand-leachable particulate iron along the GEOTRACES GP16 East Pacific Zonal Transect

“…This is supported by only a minor contribution from Fe(II) minerals to pFe in plume samples at Stations 18 and 20 during GP16 (Fitzsimmons et al, 2017). There is therefore no evidence in our  56 Fe labile data of the variability associated with early (buoyant) plume dynamics, attributed to Fe-sulfide formation, and Fe-oxyhydroxide precipitation from partial Fe(II) oxidation (Klar et al, 2017;Rouxel et al, 2016;Severmann et al, 2004).…”

“…Numerous studies have examined iron isotope fractionation associated with processes that take place within the ocean, including oxidation, reduction, adsorption, dissolution and biological uptake (e.g. Dauphas and Rouxel, 2006;Dideriksen et al, 2008;Staubwasser et al, 2013Staubwasser et al, , 2006, and during mixing of hydrothermal vent fluids with seawater (Bennett et al, 2009;Klar et al, 2017;Rouxel et al, 2016;Severmann et al, 2004).…”

“…Analysis of plume particulate material revealed that up to 60% of total mass in the small size fraction consisted of newly precipitated Fe oxyhydroxides (Fitzsimmons et al, 2017;Lam et al, this issue), which are known to form near hydrothermal vents as dFe in hot, anoxic hydrothermal fluid mixes with cold, oxygenated sea water (Rudnicki and Elderfield, 1993 (Beard et al, 2003;Bennett et al, 2009;Klar et al, 2017;Rouxel et al, 2016;Severmann et al, 2004). We therefore speculate that the near-field  56 Fe labile closely resembles the iron isotope signature of the vent fluid end-member at the SEPR vent from which the plume originated.…”