Gartman, Amy , Findlay, Alyssa J. , Hannington, Mark D., Garbe-Schönberg, Dieter , Jamieson, John W. and Kwasnitschka, Tom (2019) The role of nanoparticles in mediating element deposition and transport at hydrothermal vents. Geochimica et Cosmochimica Acta, 261 . pp. 113-131. DOI 10.1016/j.gca.2019.06.045.

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Abstract

Precipitation processes in hydrothermal fluids exert a primary control on the eventual distribution of elements, whether that sink is in the subseafloor, hydrothermal chimneys, near-field metalliferous sediments, or more distal in the ocean basin. Recent studies demonstrating abundant nanoparticles in hydrothermal fluids raise questions as to the importance of these nanoparticles relative to macro minerals, as well as the fate of such particles in hydrothermal systems. Here we evaluate the particle geochemistry of black smoker fluids from Niua South vent field, including nanoparticles and macro minerals, in order to consider how the processes of mineral precipitation affect mineral size and morphology, and how this mineral precipitation may dictate element sinks as hydrothermal fluids begin to mix with seawater. We find that the Niua vent fluids are dominated by sulfide and sulfate minerals, with the mineralogy of major and minor minerals changing with temperature, degree of mixing with seawater and rate of precipitation. The majority of particles are submicron in size, and sulfide minerals become larger and exhibit more crystalline morphology with increasing seawater content in the fluids. Minor minerals include gold and bismuth tellurides, and nanoparticulate chalcopyrite and nano-zinc sulfide occur. These findings are consistent with major mineral classes and precipitation processes observed in other systems, while providing further insight into the details of mineral precipitation at Niua including the separate and combined influences of boiling, mixing and cooling during hydrothermal fluid transport and initial interactions with seawater. This work demonstrates that boiling and rapid mixing encourages the formation of nanoparticles, whereas conductive cooling encourages particle growth. Further, these data demonstrate that the possible influence of nanoparticles in hydrothermal systems are not restricted to enhancing element transport, but may also include restricting mineral growth and affecting physicochemical properties of hydrothermal chimneys.

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