Platinum stable isotope fractionation and adsorption on marine ferromanganese oxide substrates

In the modern oxic ocean, ferromanganese sediments in the form of crusts and nodules precipitate directly from seawater over geological timescales. These slow growing ferromanganese sediments incorporate many trace elements, recording changes in ocean chemistry such as variations in the sources of metals to the marine environment and redox changes linked to ocean circulation and atmosphere conditions at the time of precipitation. Ferromanganese sediments are the primary sink for platinum (Pt) in the modern marine environment, with Pt concentrations two orders of magnitude higher than in average upper continental crust. With five stable isotopes and several oxidation states in nature, the Pt stable isotope system has potential to add to the existing suite of stable isotope tracers of ocean environment and processes. This is the first study to evaluate the Pt stable isotope system in a low temperature environment. To investigate Pt in the marine environment and its potential as (paleo)environmental tracer, chemical separation and a double spike multi-collector inductively coupled plasma mass spectrometry method was developed to measure the platinum isotopic composition of ferromanganese sediments. Measured ¹⁹⁸Pt/¹⁹⁴Pt are reported as δ¹⁹⁸Pt values, representing permil variations relative to the IRMM-010 Pt standard. The external reproducibility in natural, bulk, ferromanganese nodule samples is ca. ± 0.089 ‰ (2 sd), likely limited by sample homogeneity. The incorporation of Pt into the ferromanganese sediments is expected to impart a stable isotopic fractionation, such that the Pt isotopic composition recorded in the sediments will be offset from seawater. To constrain the incorporation mechanism of Pt onto ferromanganese sediments, a series of adsorption experiments were conducted to evaluate the oxidation state, coordination chemistry and speciation of the platinum adsorbed onto the surface of iron oxyhydroxide and manganese oxide substrates using X-ray absorption spectrometry. The experiments show Pt adsorbs to goethite, ferrihydrite, and δ-MnO₂ as Pt⁴⁺ at variable pH (pH = 2, 4, 6, and 8), with δ-MnO₂ significantly more effective than the Fe hydroxyoxides at adsorbing Pt, by a factor of > 19 at pH = 8. The presence of Pt⁴⁺ suggests a chemisorptive redox mechanism in which Pt²⁺ from seawater is adsorbed and oxidised to Pt⁴⁺ on the mineral surface. The lighter isotopes of Pt are preferentially adsorbed during this process resulting in an isotopically heavy solution, with an average offset of ca. 0.4 ‰ between the solid and the solution (α¹⁹⁸Ptli-so ~ 1.0006, for δ-MnO₂ at pH = 8). By analogy to the ocean, this suggests that the platinum in seawater will be isotopically heavier than the platinum sequestered into marine ferromanganese sediments. A globally-distributed suite of modern ferromanganese crusts and nodules was measured to investigate the Pt isotopic composition of natural ferromanganese sediments. Both hydrogenetic and diagenetic crusts and nodules are isotopically heavier than any other natural materials measured to date; up to ca 0.8 ‰ heavier than IRMM-010 and up to ca. 1.0 ‰ than terrestrial mantle and crustal samples. Suspected hydrothermal inputs to ferromanganese deposits result in wide range of isotopic compositions, suggesting hydrothermal processes may introduce both heavier and lighter Pt on a local scale. Given that ferromanganese sediments adsorb lighter Pt, this indicates that seawater will have a heavy Pt isotopic composition. The Pt concentration in seawater is too low for direct Pt isotopic measurement, as is the concentration in the primary source of Pt to the marine environment, river waters. However, the Pt isotopic character of riverine input was evaluated through a series of leaching experiments as a simplified analogy for oxidative weathering of terrestrial sources. Leaching of three Pt-bearing crustal samples (standard reference materials SARM-76, WPR-1, and PTA-1) indicate that the most mobile platinum, most likely dominated by sulphide minerals, is isotopically heavy relative to the bulk platinum isotopic signature of the samples, with an isotopic offset of up to 5.3 ‰ with respect to the residue, which is consistent with an isotopically heavy seawater composition. A first order model of the marine Pt isotopic system has been developed, with a predicted seawater composition of δ¹⁹⁸Pt = 0.6 – 1.5 ‰. The measureable isotopic differences among the components of the marine and crustal cycle indicate that Pt shows promise as a new non-traditional stable isotopic tracer of low temperature processes.