Coupled Ge/Si and Ge isotope ratios as new geochemical tracers of seafloor hydrothermal systems: A case study at Loihi Seamount

Germanium isotope and Ge/Si ratio systematics were investigated in low temperature hydrothermal vents from Loihi Seamount (Hawaii, USA) and results were compared to high-temperature vents from East Pacific Rise at 9-10°N. Loihi offers the opportunity to understand a low temperature basalt leach system with surface deposits mainely composed of iron oxyhydroxides. The results show that both Ge/Si and ?74/70Ge in hydrothermal fluids are fractionated relative to the host basaltic rock with Ge/Si ~ 30 and 5.7 µmol/mol and ?74Ge ~ 1.9 and 1.55‰ for Loihi and EPR (Bio9) respectively vs. Ge/Si ~ 2.2 µmol/mol; ?74Ge ~ 0.56‰ for basalt. The relative enrichment in Ge vs. Si together with Ge isotope fractionation can be explained by quartz precipitation in the reaction zone at depth. Using Ge mass balance between hydrothermal fluids and fresh basalts, we calculated a ?74Gequartz-fluid of about -5.0‰. Although the fractionation of Ge isotopes in quartz is presently unknown, such apparent large Ge isotope fractionation at elevated temperature suggests concomitant loss of Ge and Si during fluid upflow resulting in Rayleigh-type effect and/or a fractionation during the basalt dissolution. The study of microbial mats at Loihi Seamount, composed essentially of Fe-oxyhydroxide with minor phases of amorphous silica and volcanic materials, also suggest that Ge isotopes are fractionated upon precipitation at the seafloor during seawater - hydrothermal fluids mixing. We obtained a maximum Ge isotope fractionation between Fe-oxyhydroxide (ferrihydrite) and dissolved Ge in the fluid of about -2.74‰. Isotopic variations observed in the different mats have being interpreted as reflecting the percentage of Ge sequestration in microbial mats; the lower values corresponding with the highest Ge trapping. This result is consistent with recent experimental estimation. This study shows that combining Ge/Si and ?74Ge systematics provide a useful tool to trace hydrothermal Ge and Si sources in marine environments and to understand formation processes of seafloor hydrothermal deposits. Preliminary mass balance of germanium isotope in seawater reveals that the missing Ge sink may correspond to Ge sequestration into Fe-oxyhydroxides within marine sediments.