Coupled Ge/Si and Ge isotope ratios as geochemical tracers of seafloor hydrothermal systems: Case studies at Loihi Seamount and East Pacific Rise 9°50′N

Abstract

International audienceGermanium (Ge) and Silicon (Si) exhibit similar geochemical behavior in marine environments but are variably enriched in seafloor hydrothermal fluids relative to seawater. In this study, Ge isotope and Ge/Si ratio systematics were investigated in low temperature hydrothermal vents from Loihi Seamount (Pacific Ocean, 18°54′N, 155°15′W) and results were compared to high-temperature vents from the East Pacific Rise (EPR) at 9°50′N. Loihi offers the opportunity to understand contrasting Ge and Si behavior in low temperature seafloor hydrothermal systems characterized by abundant Fe oxyhydroxide deposition at the seafloor. The results show that both Ge/Si and δ74/70Ge in hydrothermal fluids are fractionated relative to the basaltic host rocks. The enrichment in Ge vs. Si relative to fresh basalts, together with Ge isotope fractionation (Δ74/70Gefluid-basalt up to 1.15‰ at EPR 9°50′N and 1.64‰ at Loihi) are best explained by the precipitation of minerals (e.g. quartz and Fe-sulfides) during higher temperature seawater–rock reactions in the subsurface. The study of Fe-rich hydrothermal deposits at Loihi, largely composed of Fe-oxyhydroxides, shows that Ge isotopes are also fractionated upon mineral precipitation at the seafloor. We obtained an average Ge isotope fractionation factor between Fe-oxyhydroxide (ferrihydrite) and dissolved Ge in the fluid of −2.0 ± 0.6‰ (2sd), and a maximum value of −3.6 ± 0.6‰ (2sd), which is consistent with recent theoretical and experimental studies. The study of a hydrothermal chimney at Bio 9 vent at EPR 9°50′N also demonstrates that Ge isotopes are fractionated by approximately −5.6 ± 0.6‰ (2sd) during precipitation of metal sulfides under hydrothermal conditions. Using combined Ge/Si and estimated Ge isotope signatures of Ge sinks and sources in seawater, we propose a preliminary oceanic budget of Ge which reveals that an important sink, referred as the “missing Ge sink”, may correspond to Ge sequestration into authigenic Fe-oxyhydroxides in marine sediments. This study shows that combining Ge/Si and δ74/70Ge systematics provides a useful tool to trace hydrothermal Ge and Si sources in marine environments and to understand formation processes of seafloor hydrothermal deposits

A Review of the Stable Isotope Bio-geochemistry of the Global Silicon Cycle and Its Associated Trace Elements

“…Recently Baronas et al (2016) have suggested that Ge loss from the ocean appears to be dependent on pore-water oxidation-reduction reactions and the formation authigenic aluminosilicate (see section Sedimentary Processes) minerals within marine sediments. Note, a recent study looking at the links between Ge isotopes and silicon in Hawaiian hydrothermal vents has shown the potential of this new isotopic system to trace the fate of hydrothermal elements released into the ocean (Escoube et al, 2015).…”

Advances and Opportunities in Ore Mineralogy

“…Evidence for modest fractionation during evolution of hydrothermal fluids Ge [134] and references therein, [135] Ge concentrated in sphalerite; potential for tracing processes in seafloor hydrothermal systems.…”

Germanium in Magnetite: A Preliminary Review

“…Experimental study also demonstrates that Ge is likely to be absorpted by or coprecipitated with iron hydroxides and organo-mineral colloids in many superficial aquatic environments at the contact of anoxic groundwaters with surficial oxygenated solutions (Pokrovsky et al, 2006). Some studies also conclude that "missing Ge sink" in ocean is due to Ge sequestration into authigenic Fe-oxyhydroxides in marine sediments (Escoube et al, 2015). However, Ge/Si ratios of iron formations indicate that silica is dominantly derived from weathering of continental landmass, whereas iron has a hydrothermal origin (Hamade et al, 2003).…”

“…Inorganic Ge exists as the same chemical species as Si in natural waters due to their similar atomic radii and Si-O, Ge-O bond lengths and can substitutes for Si in the SiO 2 lattice (Pokrovski and Schott, 1998). Ge/Si ratios have been used to discuss the genesis of banded iron formations because the Ge/Si ratio preserved in iron formation silica should reflect the ratio of the water from which it precipitated and hence reflect its source (e.g., Hamade et al, 2003;Delvigne et al, 2012;Escoube et al, 2015). Previous studies mainly focus on the Ge/Si ratios of Fe-rich layers and Si-rich layers such as chert.…”