Rare earth element and yttrium (REY) geochemistry of 3.46-2.45 Ga greenalite-bearing banded iron formations: New insights into iron deposition and ancient ocean chemistry

Description

Finely laminated chert enclosing nanoparticles of greenalite and apatite is ubiquitous in Archean-Paleoproterozoic ferruginous cherts, jaspilites and Banded Iron Formations (BIFs) and are considered to be primary deposits. The cherts and BIFs are chemical sedimentary rocks interpreted to have been precipitated in marine settings prior to the first permanent rise in atmospheric oxygen at the Great Oxidation Event (GOE) ca. 2.45-2.32 M.y. ago. As chemical sediments, they are potential archives of the solutions from which they precipitated, incorporating signals from hydrothermal fluids and ambient seawater. Previous studies of rare earth elements and Y (REY) in pre-GOE BIFs have mostly found an “Archean seawater signature” with positive Eu anomalies, attributed to the influence of high-temperature hydrothermal processes, and positive anomalies for La, Gd and Y, ascribed to seawater. REY abundances determined by in situ LA-ICP-MS are presented for well-preserved, laminated greenalite-bearing cherts from ten formations of pre-GOE ferruginous cherts and BIFs from Western Australia. The samples come from a wide range of depositional environments, e.g., submarine proximal volcanic environments, basin floor, slope and deep marine shelf, and are between 3.46 Ga to 2.45 Ga in age. Five groups with different REY profiles are identified, namely (i) mafic-volcanic-influenced vent-proximal chert; (ii) felsic volcanic- and sediment-associated chert, (iii) ferruginous cherts in shelf sediments, (iv) the Nammuldi Member of the Marra Mamba Iron Formation and Joffre Member of the Brockman Iron Formation, and (v) the Dales Gorge Member of the Brockman Iron Formation. Of these, only the Dales Gorge Member BIF has a typical Archean seawater signature while the others have REY patterns likely reflecting differing source fluids and environments of deposition but not necessarily global ocean chemistry. Analyses of chert containing sub-micron-sized particles of greenalite and apatite indicate that the likely hosts of the REEs are apatite, siderite and possibly greenalite. The REY profiles of the greenalite-bearing cherts may differ from those of bulk samples of the same formations, perhaps reflecting a diagenetic overprint in the bulk samples, whereas the greenalite-bearing cherts likely preserve their depositional compositions, locked in by early silicification. Data stored here include: Table S1: Individual point analyses (ppm) of Rare Earth Elements + Y in greenalite‐bearing chert samples from Western Australia. Table S2: Individual point analyses (ppm) of selected elements in greenalite‐bearing chert samples from Western Australia.

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The cherts were studied with petrographic microscopes and BSE imaging using TESCAN VEGA 3 and FEI Verios SEMs. Each SEM was fitted with an energy dispersive X-ray spectrometer (EDS), which was used for quantitative and qualitative chemical analysis of mineral grains. For mineral grains too small for quantitative analysis by EDS (<1-2 microns), optical properties and EDS spectra, combined with grain shape, were used for tentative identification of mineral species. To confirm the identity of submicron inclusions, lamellae (ca. 10 x 5 m) for TEM studies were cut from chert in polished thin sections. Focused ion beam techniques were used to prepare ca. 100 nm thick TEM lamellae using an FEI Helios NanoLab G3 CX DualBeam instrument. TEM data were collected at 200 kV using an FEI Titan G2 80–200 TEM/STEM with ChemiSTEM technology. High resolution TEM (HRTEM) images and electron diffraction techniques, high-angle annular dark-field (HAADF) scanning TEM (STEM) images and EDS element distribution maps and spectra were used to identify the nanoparticles. TEM images were processed using TIA (TEM Imaging and Analysis) software from FEI, ImageJ (Fiji) software and Digital Micrograph software from Gatan Incorporated. EDS spectra and maps were collected with an FEI Super-X EDS detector and were processed using Esprit software from Bruker Corporation. Trace elements in the cherts were analyzed at Adelaide Microscopy, University of Adelaide, using a RESOlution-LR 193 nm ArF excimer laser ablation system with a S155 large format sample chamber. The laser was coupled to an Agilent 7900x ICP-MS. Samples were ablated with a laser spot size of 51 µm, a fluence at the sample of ~10 J/cm2 on the chert and 3.5 J/cm2 on the glass reference materials, and a repetition rate of 5 Hz. Samples were ablated in a He atmosphere (flow rate 0.35 L/min) and the aerosol mixed with Ar carrier gas (flow rate 1.01 L/min) immediately after ablation for transport to the ICP-MS. Each analysis consisted of 30 s of gas background and 30 s of ablation signal with the laser firing. The NIST SRM612 glass was used as the primary reference material for calibration and drift correction, and the USGS GSD-1G glass was analyzed as a secondary reference material for quality control. The following isotopes were measured: 23Na, 24Mg 27Al, 29Si, 31P, 39K, 43Ca, 47Ti, 52Cr, 55Mn, 57Fe, 59Co, 60Ni, 65Cu, 66Zn, 85Rb, 88Sr, 89Y, 90Zr, 93Nb, 133Cs, 137Ba, 139La, 140Ce, 141Pr, 146Nd, 147Sm, 153Eu, 157Gd, 159Tb, 163Dy, 165Ho, 166Er, 169Tm, 172Yb, 175Lu, 178Hf, 181Ta, 232Th and 238U, with counting times of 5-20 ms for each mass and a total sweep time of 0.59 seconds. Data was processed using LADR software (https://norsci.com/?p=ladr) using standard reference values from the GeoREM database (http://georem.mpch-mainz.gwdg.de/). Data was quantified using 29Si as the internal standard element and normalizing the sum of the element oxides to 100 wt%.

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