Mechanisms to explain the platinum concentration in ferromanganese seamount crusts

Abstract

Ferromanganese seamount crusts cover large areas of hard substrates of sea-floor edifices in the Central Pacific. They can be distinctly enriched in Pt and show Pt/Pd ratios which are several orders of magnitude greater than those orders of magnitude greater than those of seawater. Thirty-four selected samples of the younger and the older crust generation were analyzed for Mn, Fe, Ni, Co and Pt. The bulk Pt concentration varies between 0.14 and 1.02 ppm and averages to 0.51 ± 0.24 ppm. The older generation is generally somewhat richer in Pt (0.63 ± 0.25 ppm) than the younger one (0.39 ± 0.15 ppm). Compared to the Earth's upper continental crust, the ferromanganese material contains about 100 times more Pt. In the studied depth range between 1100 and 3800 m, the Pt concentration is controlled by the water depth. In addition to the bulk data, the distribution of Pt and Pd is recorded through a complete ferromanganese crust profile. The average Pd value amounts to 16 ± 2.8 ppb for the younger and 16 ± 4.8 ppb for the older generation. Two immediate sources may be responsible for the Pt concentration in the ferromanganese crusts: seawater and cosmic spherules. The presented thermodynamic approaches are based on the presumption that the Pt tetrachlorocomplex is the prevailing species under seawater conditions. Precipitation of Pt in seawater may take place if this complex is decomposed by reduction of Pt 2+ to the elemental state. On the other hand, the MnO 2 precipitation is caused by oxidation of dissolved Mn 2+ ions which are supplied from the oxygen-minimum zone. A model reaction by combining the two half-reactions is formulated assuming a coprecipitation of Pt with MnO 2. An alternative mechanism to explain Pt enrichment in ferromanganese crusts might be surface adsorption of anionic Pt tetrachloro-complexes onto positively charged amorphous iron hydroxide particles. Cosmic spherules were discovered in samples of the older ferromanaganese crust generation. The spherules are mostly concentrated in the upper part of this crust layer. Iron meteorites often have Pt concentrations up to 20 ppm. The relative quantity recovered in the grain size class 0.1–0.4 mm only amounts to ∼ 2·10 −4% which is significantly too low to explain the Pt content of the crust samples. However, smaller cosmic particles may exist and also contribute to the Pt enrichment. Suggesting that rough crust surfaces are excellent collectors of denser detrital material, the maximum Pt values observed in the youngest part of the older crust generation may be explained by physical incorporation of cosmic particles.