Geochemical, mineralogical, and Sr–Nd isotopic compositions of ferromanganese encrustations from Central Indian Ridge at 06°38.5′S

Abstract

Mineralogical, geochemical, and isotopic (Sr and Nd) studies of 14 ferromanganese encrustations on basaltic substrates from Central Indian Ridge at 06°38.5′S indicate that except for two (NC3 and NC4), the samples are hydrogenous type. NC3 and NC4 have slightly higher growth rates, lower bulk trace metal contents, and low ∑REE. They have a lower content of high field strength (e.g., Hf, Ta, Zr, and Nb), incompatible (e.g., Pb, Th, and U), and lithophilic elements (e.g., Sr and Ba) than others. In addition, NC3 has a lower 87Sr/86Sr ratio (0.708825) than the remaining samples (range 0.709169–0.709253) but higher 143Nd/144Nd ratio (0.512296) than others (range 0.512258–0.512279). Both NC3 and NC4 have a higher radiogenic component indicated by their low εNd value (−6.67) than the remaining samples (range −7.00 to −7.41). These evidences together indicate a possible contribution of trace metals by a local mixed source of hydrothermal and hydrogenous origin during their growth history.

Equilibrium Fractionation of Stable Strontium Isotopes During Adsorption to Mn Oxides

“…The adsorption isotherm of Sr in synthetic seawater experiments (Figure 2d) suggests that under current seawater conditions ([Sr] = 8 μg g 1 ) (Bernat et al, 1972), the Sr concentration in δ-MnO 2 is approximately 3,000 μg g 1 , corresponding to a Sr/Mn ratio of about 7.5 mg g 1 . This estimate is in line with observed ratios in natural ferromanganese nodule/crust samples, such as NOD-A-1 (2.3 mg g 1 ) and NOD-P-1 (9.5 mg g 1 ) (Axelsson et al, 2002), and other samples from global oceans (from 3 to 10 mg g 1 , except several samples with extremely low Mn content) (Banerjee et al, 2018;Ingram et al, 1990;Javier Gonzalez et al, 2016;Sano et al, 1985), reinforcing the reliability of the experimental findings. The study's results, along with previous research, indicate that Mn oxides are likely the primary phase for Sr adsorption in ferromanganese deposits.…”

“…Ferromanganese nodules/crusts, formed in low‐energy, low‐detrital waters, have the potential to document ocean chemistry evolution, thereby recording the Sr isotopic composition of seawater (Banerjee et al., 2018; Koschinsky & Heine, 2017; Wasylenki et al., 2014). The suitability of ferromanganese nodules/crusts as proxies for reconstructing seawater 88 Sr/ 86 Sr needs further assessment.…”

“…To address this, we conducted adsorption experiments on Fe/Mn oxides at room temperature to quantify Sr isotope fractionation between solutions and minerals. Key minerals in ferromanganese deposits, such as ferrihydrite and vernadite (δ‐MnO 2 ), were used (Banerjee et al., 2018). We investigated the impact of time, pH, initial Sr concentration, and ionic strength (IS) on Sr isotope fractionation.…”

Geochemical and Sr‐Nd‐Pb‐Fe Isotopic Constraints on the Formation of Fe‐Si Oxyhydroxide Deposits at the Ultraslow‐Spreading Southwest Indian Ridge

“…The numbers on the mixing curves correspond to the fraction of seawater. The data for Indian Ocean basalts (Meyzen et al., 2005) and Fe‐Mn crusts (Banerjee et al., 2018) are shown for comparison.…”