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Redox sensitive trace elements in pyrite, including nodules, are increasingly used to infer the chemical conditions of ancient oceans—but considerable uncertainty remains regarding the mechanism and timing of nodule formation. Resolving these uncertainties is important because pyrite nodules must form in connection with the overlying water column, rather than during late diagenesis, to reflect the composition of the global ocean. Existing models for pyrite nodule formation have been specific to pyrite textures from individual sites, and we lack a unified model that can explain the compositional and textural diversity observed in nodules from different localities. In this study we examine ten pyrite nodules from several geological periods (Neoarchean to Carboniferous) using in situ LA-ICP-MS and SHRIMP-SI analyses. We present transects of spot analyses of trace elements (As, Ag, Cu, Co, Ni, Sb and Se) and S isotope ratios for each nodule. The pyrite nodules can be classified according to three main categories: those with (1) little to no trace element or isotopic zonation of the nodule from core to margin, (2) strong zonation from core to margin, and (3) minor zonation near the core but more significant zonation near the margin of the nodule. We further illustrate this zonation with a NanoSIMS element map from an additional pyrite nodule. These results are interpreted to indicate nodule formation along a spectrum between two end-member mechanisms. We suggest that the absence of trace element or isotopic zonation reflects nodule growth by a pathway that is analogous to the pervasive growth mechanism for carbonate nodules. This model involves the production of many nucleation sites that are evenly distributed within the volume that the nodule eventually occupies. Consequently, this mechanism results in a chemically homogenous nodule. Pyrites formed this way are suitable for paleoceanographic reconstruction. The other end-member mechanism is analogous to the concentric growth model for carbonate concretions. In this scenario, the core of the nodule forms first and is followed by the addition of concentric layers—each with a progressively different trace element content and δ34S signature as diagenesis progresses. Despite having limited utility for reconstructing ancient seawater, these late forming nodules may track the evolving availability of bioessential trace elements for the subsurface biosphere with important implications for global biogeochemical cycles. Spatial trends in trace elements and S isotopes thus speak to the mechanisms of pyrite nodule formation and provide a framework for evaluating nodule suitability for a range of paleoenvironmental studies. [ABSTRACT FROM AUTHOR]

The Vindhyan Basin in central India preserves a thick (~5 km) sequence of sedimentary and lesser volcanic rocks that provide a valuable archive of a part of the Proterozoic (~1800–900 Ma) in India. Here, we present an analysis of key sedimentary pyrite textures and their trace element and sulfur isotope compositions in the Bijaigarh Shale (1210 ± 52 Ma) in the Vindhyan Supergroup, using reflected light microscopy, LA-ICP-MS and SHRIMP-SI, respectively. A variety of sedimentary pyrite textures (fine-grained disseminated to aggregates, framboids, lags, and possibly microbial pyrite textures) are observed reflecting quiet and strongly anoxic water column conditions punctuated by occasional high-energy events (storm incursions). Key redox sensitive or sensitive to oxidative weathering trace elements (Co, Ni, Zn, Mo, Se) and ratios of (Se/Co, Mo/Co, Zn/Co) measured in sedimentary pyrites from the Bijaigarh Shale are used to infer atmospheric redox conditions during its deposition. Most trace elements are depleted relative to Proterozoic mean values. Sulfur isotope compositions of pyrite, measured using SHRIMP-SI, show an increase in δ34S as we move up stratigraphy with positive δ34S values ranging from 5.9‰ (lower) to 26.08‰ (upper). We propose limited sulphate supply caused the pyrites to incorporate the heavier isotope. Overall, we interpret these low trace element signatures and heavy sulfur isotope compositions to indicate relatively suppressed oxidative weathering on land during the deposition of the Bijaigarh Shale. [ABSTRACT FROM AUTHOR]

We present protocols for high-spatial resolution measurement of oxygen isotope ratios of goethite (α-FeOOH) with the Sensitive High Mass Resolution Ion Microprobe – Stable Isotopes (SHRIMP-SI) and propose a natural sample as a potential goethite reference material (RM) for ion microprobe analysis. We assess the effects of goethite chemical composition, crystallographic orientation, and texture on the accuracy and repeatability of SHRIMP-SI δ18O (δ18O SIMS) results. Synthetic goethites evaluated as potential δ18O SIMS RM are powdery, porous, and finely crystalline; they do not yield repeatable results. A dense colloform goethite from the Capão topaz mine, Minas Gerais, Brazil, fulfills major prerequisites: it is stoichiometrically relatively pure, yields repeatable oxygen isotope results, and occurs in abundance to produce a RM for long-term use. We use an average laser fluorination δ18O LF-VSMOW value of −17.3 ± 0.3‰ (1SD) obtained for five aliquots of this RM to normalize all δ18O SIMS measurements. Multiple δ18O SIMS analyses of a large fragment of the Capão L4 (CL4) RM analyzed in three different runs yield an overall repeatability of −17.3 ± 0.5‰ (2SD, n = 294) for all three runs combined. Natural variability and crystal orientation effects are the main reasons for the excess spread of the δ18O SIMS results compared to the spot internal precision (ca. 0.2‰). All δ18O SIMS analyses (n = 1027) in various aliquots of CL4, randomly oriented and analyzed in 26 sessions during eight distinct runs, yield an overall repeatability of −17.3 ± 0.7‰ (2SD), confirming that CL4 is a suitable SIMS RM. After ascertaining its suitability as a RM, we used CL4 to standardize analyses of other natural goethite samples with the SHRIMP-SI and compared δ18O SIMS and laser fluorination results to test the relationship between natural properties (e.g., porosity, minor elements substituting for Fe), preparation procedures (e.g., polish and relief), instrument conditions, and the overall precision and accuracy of the SIMS analyses. Samples containing minor elements substituting for Fe (e.g., Al, Mn, Cu, Zn, etc.) or as contaminants (e.g., Si, P) require significant matrix corrections. Because we could not find homogenous natural goethite samples showing a large range in metal concentrations, we extrapolate our calibration curves beyond the composition of our calibration goethite samples. δ18O SIMS results corrected for instrument mass fractionation (using CL4) and compositionally dependent matrix effects (using several calibration goethites of known elemental composition) are less precise but statistically indistinguishable from their laser fluorination results. However, porous samples are unsuitable for SHRIMP-SI δ18O analysis. Dense colloform samples yield repeatable results for individual growth bands, showing that the high spatial resolution, moderate precision, and speed of analysis of the SHRIMP-SI can resolve variations in oxygen isotope composition acquired during sample growth. (U Th)/He geochronology of equivalent aliquots from the same goethite samples reveal that the combination of the two methods permits the extraction of temporal variation in the isotopic compositions of meteoric solutions in the geological past. [ABSTRACT FROM AUTHOR]