Formation of Fe-Mn coatings on foraminifera from the ultraslow-spreading southwest Indian Ridge: Implications for hydrothermal and diagenetic overprints in sediments

Abstract

Detailed fine-scale morphological, mineralogical, and geochemical analyses were conducted on the Si-(Fe) hydroxide formations, planktonic foraminifera (PF) and the Fe-Mn coatings within a typical sediment sample collected from near the newly-discovered active hydrothermal vent field at the ultraslow-spreading Southwest Indian Ridge (SWIR). The analyses were conducted with a combination of different techniques, including Laser Scanning Confocal Microscope (LSCM), Laser Raman, high-resolution XRD, SEM-EDS, C-O isotope measurement, EMPA, and in-situ LA-ICP-MS. The mixed-type hydrothermal-hydrogenetic Si-(Fe) hydroxide formations (commonly occur as cavity-fillings inside the PF chambers) contain a considerable amount of organic matter, and are mainly composed of nontronite, goethite, tridymite, and opal-A with the bulk compositions of 8.13–40.91 wt% SiO2, 6.85–54.50 wt% FeO, and 0.43–23.99 wt% Al2O3. Such high Al content can be mainly attributed to the precipitation and accumulation of hydrothermal-derived Al-rich materials, and the Si-Fe-Al assemblage reveals that it is mainly sourced from plagioclase dissolution/alteration during subseafloor fluid-rock interaction. Meanwhile, the relative enrichments of high field strength elements (HFSEs), heavy rare earth elements (HREE), and Cr-Ni-Sc also imply that these Si-(Fe) hydroxide formations have another source from pyroxene alteration. In addition, their distinct microtextures (filamentous, orbicular, or beaded-like) are indicative of bacterial-related precipitation. The hydrogenetic Fe-Mn coatings have a layered texture and exhibit no apparent fluorescence, and are mainly composed of vernadite and goethite with the bulk compositions of 23.98–30.06 wt% FeO and 8.55– 21.52 wt% MnO. Their average Fe/Mn ratios, growth rates, and Zn-As contents are slightly higher than those of the open-ocean SWIR Fe-Mn deposits, which suggests an additional Fe input from submarine hydrothermal activity. Moreover, the altered PF shell is mainly composed of calcite and have three color types (white/yellowish/yellow). The contents of Si, Al, Fe, Cr, Ni, Cu, Zn, Ba, HFSEs, and REE increase as the yellowish color deepens, which indicates the diagenetic contamination from basalt alteration. Consequently, a new diagenetic Fe-carbonate phase (ankerite) was formed in the inner layer of the altered PF shells, and close correlations were also found between their C-O isotope ratios. Both the δ13C and δ18O values tend to drop from white- to yellow-type PF shells, accompanied with the decreasing contents of Ca, Na, Mg, and Sr. All these parameters and signatures would help to unravel the hydrothermal influence and diagenetic overprinting, which could cause confusion when using foraminifera shell geochemistry (e.g., Sr/Ca and Mg/Ca ratios) as a marine tracer in paleoclimate reconstruction.