Formation and early transformation of hydrothermal Fe nano-colloids in a black smoker system

Motivated by the goal to determine the chemical form, variability, and potential processes that modulate the flux of ecosystem-limiting metals, like hydrothermal iron (Fe) nano-colloids, and to explore their unique catalytic capabilities, we sampled suspended and dissolved matter in the water column above the Rainbow (36°-33°N) hydrothermal vent field at the Mid-Atlantic Ridge. To investigate the (trans)formation of hydrothermal iron-based nanocolloids, we employed a direct sampling approach that bypasses conventional techniques such as filtration and resuspension. Instead, small amounts of plume fluid were immediately drop-cast onto transmission electron microscopy (TEM) grids and plunge-frozen, preserving dissolved compounds and nanocolloids through vitrification. Using an array of microscopic and spectroscopic techniques, combined with machine learning, allowed detailed characterization of the Fe nanocolloids down to the nano-scale and provided insight into their early (trans)formation and bioavailability.

TEM and synchrotron-based spectroscopy show that the Fe colloids suspended in the hydrothermal plume predominantly consist of poorly ordered ferric Fe-oxyhydroxides most similar to 2-line (2L-Fh) and 6-line ferrihydrites (6L-Fh), which contain local enrichments in P, S, and/or Cu phases. Using the machine learning model SIGMA¹ allowed us to explore the distribution of distinct Fe phases and revealed local P:Fe ratios of 1:2 for 2L-Fhs and 1:6 for 6L-Fhs. Utilizing nano-scale scanning TEM tomography, we showed that some 2L-Fh aggregates contain ferrous chalcopyrite (CuFeS₂) cores. On the outside, the plunge-frozen Fe-nano colloids are covered with the vitrified plume fluid enriched in Mg, Cl, and ± S. Notably, our results do not show associations of Fe with (organic) carbon.

These observations suggest that chalcopyrite forms in the shallow subsurface before venting and acts as a crystallization seed for some fast oxidizing Fe(II) after mixing with seawater. Ferrihydrite (Fh) forms through the formation of Fe₁₃-Keggin clusters², and we argue that part of the clustering process occurred on the surface of the chalcopyrite, resulting in dendritic textures of some 2L-Fh. In contrast, Fh can also nucleate by clustering of Fe without needing a preexisting template, resulting in a more compact morphology. The larger surface area of the dendritic Fh that utilizes metal sulfides for their nucleation results in higher adsorption of PO₄ and, consequently, due to the dehydration of the surface, significantly decreases the dissolution and, therefore, recrystallization, suppressing the transformation into more ordered 6L-Fh. Furthermore, this shows limited interaction between C-rich phases and Fe-bearing precipitates during early (trans)formation in a black smoker system, contrasting previous studies, which suggest that organic compounds play a key role in stabilizing and transporting hydrothermal Fe³.

Our findings shed completely new light on the transport and persistence of vent-derived reduced iron phases, highlighting the role of ferric coatings in protecting nano-scale iron sulfides and challenging the previously proposed importance of complexation with organic matter. Overall, we provide new perspectives on the early (trans)formation processes of vent-derived iron, its interaction with other essential elements, and, eventually, its impact on ocean chemistry.

 

• Tung, P., et al. Geochem., Geophys., Geosystems 24, (2023).
• Weatherill, J. S., et al. Environ. Sci. Technol. 50, 9333–9342 (2016).
• Toner, B. M. et al. Acc. Chem. Res. 49, 128–137 (2016).