Molybdenum and nickel partitioning during the oxidative precipitation of iron: implications for paleo-environmental proxies during BIF formation; Mo K-edge XANES data

Description

The deposition of banded iron formations (BIFs) from Precambrian oceans likely affected the abundance and bioavailability of trace metal nutrients. Utilizing BIFs as proxies for paleo-ocean composition requires careful constraints on the mechanisms and partition coefficients of trace element incorporation into BIF precursor phases. These precursors likely varied in their structure and Fe oxidation state over time and with local and global fluctuations in ocean chemistry. We studied the partitioning of two bio-relevant trace metals, nickel(II) and molybdenum(VI), during the oxidative precipitation of iron oxyhydroxides from solution under a range of oxygen fluxes and fluid chemistries. We characterized the mechanisms of Ni and Mo incorporation by measuring elemental yields after sequential phase dissolution and by Mo K-edge X-ray absorption near-edge structure (XANES) spectral analysis. This dataset accompanies the publication of the same name and comprises Mo K-edge XANES spectra for products from the above experiments and for a series of standards and synthesized iron phases with sorbed Mo. The pre-edge peak varies in intensity with Mo oxidation state and coordination. Using the prominence of this pre-edge peak as a proxy, we show that Mo is reduced over the course of Fe(II) oxidation, and the extent of Mo oxidation depends on solution chemistry and the mineralogy of newly formed precipitates. Mo undergoes late re-oxidation in some experiments as the last of Fe(II) is oxidized.

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Mo K-edge (E0 = 20000 eV) X-ray absorption fine structure (XAFS) measurements were performed at the National Synchrotron Light Source II (NSLS-II) Beamline for Material measurement (6-BM) in Brookhaven, NY, using a paraboloid collimating mirror, Si (111) monochromator and a flat harmonic rejection mirror. Monochromator energy was monitored using reference metal foils. Powdered precipitates were smeared over folded tape mounts and vacuum sealed in plastic bags under anoxic conditions at Temple University before being transported to the beamline. Experimental samples and synthesized single phase iron-trace metal precipitates were measured in fluorescence mode using a 4-element vortex silicon-drift detector. Two scans consisting of the following energy grid were collected and merged to reduce signal to noise ratio: 10 eV increments from -200 eV to -30 eV (relative to E0), 2 eV increments from -30 eV to -10 eV, 0.25 eV increments from -10 eV to 25 eV, and 0.5k increments from 25 eV to 12k with respective time steps of 1.5s, 1.5s, 1.5s, and 0.5k. Fe Kα fluorescence was attenuated with aluminum metal foils. Single element trace metal standards were measured in transmission mode using a vortex silicon-drift detector with an energy grid that employed a 10 eV increment from -200 eV to -130 eV, 2.0 eV increments from -130 eV to -110 eV, 0.25eV increments from -110 eV to +35 eV, and 0.05k increment from +35 eV to 15k with 0.5s time steps. Data reduction was carried out in Athena.

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