Stable antimony isotope composition of hydrogenetic ferromanganese crusts

Marine hydrogenetic iron and manganese deposits have been shown to be excellent geochemical archives of seawater chemistry. The behavior of Sb in marine environments and their interactions with marine ferromanganese (oxy)hydroxides, however, are still poorly constrained.

In this study, we report trace element concentrations and the Sb isotope composition in marine Cenozoic hydrogenetic ferromanganese crusts from the southern Central Pacific.

Antimony concentrations are enriched by a factor of approx. 3.4 × 10⁵ compared to ambient seawater concentrations and is mostly of authigenic nature with minimal detrital input. Antimony behaves similarly to particle-reactive Zr (R² = 0.5) and Hf (R² = 0.6) in the hydrogenetic ferromanganese crusts, indicating Sb enrichment by surface precipitation onto ferromanganese colloid particles without significant desorption. The Sb isotope fractionation factor varies between 0.00 and 0.36‰ for the fractionation between Sb in hydrogenetic ferromanganese crusts and Pacific seawater [1]. The range in Sb isotope compositions (δ¹²³Sb = 0.37 - 0.73‰) in the hydrogenetic crusts may be explained by shifts towards heavier values with higher growth rates or different Sb isotope compositions of seawater during the growth of the crusts. Isotopic Sb fractionation in hydrogenetic ferromanganese crusts may be caused by an interplay of factors (e.g. redox transformation, Rayleigh distillation, and biologically controlled mechanisms).

The results of this study build toward a more comprehensive picture of the Sb isotope system in marine system and highlight the potential of Sb as a deep-time tracer of marine redox conditions.

[1] Rouxel et al. (2003) Chem Geol. 200, 25 - 40