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X-ray microspectroscopy is a powerful analytical method in geoscientific and environmental research as it provides a unique combination of nanoscale imaging with high spectroscopic sensitivity at relatively low beam-related sample damage. In this study, "classical"scanning transmission soft X-ray microscopy (STXM) with X-ray absorption spectroscopy and the recently established soft X-ray ptychography are applied to the analysis of selected rock varnish samples from urban and arid desert environments. X-ray ptychography enhances the spatial resolution relative to STXM by up to 1 order of magnitude. With its high chemical sensitivity, it can resolve nanoscale differences in valence states of the key varnish elements manganese (Mn) and iron. Our results emphasize the complex nanoarchitecture of rock varnish as well as the diverse mineralogy of the Mn oxy-hydroxide matrix and its embedded dust grains. In contrast to the fast-growing urban varnish, the slow-growing arid desert varnish revealed a remarkable nanoscale stratification of alternating Mn valence states, providing hints on the layer-wise and still enigmatic growth process.

Precise and accurate determination of major components in alkali silicate glasses provides critical information concerning element distribution, petrogenesis and magmatic evolution. However, as one of the most important in situ microanalytical technologies, the application of electron probe microanalysis (EPMA) was limited by the time-dependent loss of X-ray intensity of alkali ions (especially Na) during electron beam bombardment on alkali silicate glasses. In this study, an optimized analytical method was developed using a JEOL JXA 8100 EPMA, aiming to get a long enough incubation duration suitable for accurate analysis of major element mass fractions. The optimal analytical conditions were determined as follows: 15 kV accelerating voltage, 2 nA beam current, 20 μm beam diameter, 40 s peak counting time, Na and K analyzed with top priority, time-saving mode with no additional detector movement and no peak search functioned. The availability of this low beam current density condition (6.37 × 10−3 nA μm−2) to 24 anhydrous reference glasses, with their compositions covering the entire spectrum from the ultramafic to highly siliceous range, was verified by the good agreement of the EPMA measured data with their GeoReM preferred values. Similarly, the methodology presented here is also suitable for analysis of hydrous glasses with H2O contents up to approximately 9 wt%, accompanied by no obvious Na loss found and no complicated mathematical corrections needed, and therefore can be used for the estimation of H2O contents in H2O-rich (>1 wt%) geological glasses by the difference method. Despite its analytical accuracy and precision (>0.5 wt%) not better than those of SIMS and FTIR, EPMA can be used as a substitute technique for H2O determination with its main advantages of excellent spatial resolution (down to 5–10 μm) and analytical convenience, especially for small melt inclusions or tiny experimental quench products, and the case with no appropriate calibration standards or no reported extinction coefficients available for those compositions of interest. In daily application, our optimized method is low cost and easy to implement and can achieve comparable precision and detection limits with acceptable efficiency (∼4–5 min/point for ten major elements) relative to routine analysis conditions. [ABSTRACT FROM AUTHOR]

Phytoplankton dynamics in coastal ecosystems is increasingly altered by land‐based human activities. Yet, this global vision conceals major disparities, among sites and through time. As conventional monitoring time series are quite sparse and relatively short, biological records of environmental variability appear as relevant tools to gain insights into phytoplankton dynamics over larger temporal and spatial scales. Here, we present results of an interdisciplinary project dealing with chemical information archived in shells of Pecten maximus (Bivalvia; Pectinidae), known to form daily growth striae on its shell surface. Several individuals were collected in the Bay of Brest (France) in 2011 and 2012, and analyzed for the molybdenum and lithium concentrations in their soft tissues and the element‐to‐calcium ratios (Mo : Ca and Li : Ca) in their calcitic striae. All shells revealed high synchrony and reproducibility in their Mo : Ca and Li : Ca profiles, characterized with a major peak at the end of May and in mid‐June 2011, respectively. Detailed analysis of physical, chemical, and biological variables measured in seawater during an extensive 9‐month environmental survey enabled a meticulous description of phytoplankton dynamics in 2011 and its impact on shell geochemistry. Main findings strongly suggest that (1) the timing of Mo : Ca peaks reflects the occurrence of silicon limitation and diatom aggregation periods, (2) the height of these peaks relates to the amplitude of the first spring diatom bloom, and (3) Li : Ca serves as a proxy for the temporal dynamics of diatom biovolume and of biogenic silica recycling at the sediment–water interface. [ABSTRACT FROM AUTHOR]

To expand the newly developed ARM glasses as reference materials for in situ microanalysis of isotope ratios and iron oxidation state by a variety of techniques such as SIMS, LA‐MC‐ICP‐MS and EPMA, we report Li‐B‐Si‐O‐Mg‐Sr‐Nd‐Hf‐Pb isotope data and Fe2+/ΣFe ratios for these glasses. The data were mainly obtained by TIMS, MC‐ICP‐MS, IR‐MS and wet‐chemistry colorimetric techniques. The quality of these data was cross‐checked by comparing different techniques or by comparing the results from different laboratories using the same technique. All three glasses appear to be homogeneous with respect to the investigated isotope ratios (except for B in ARM‐3) and Fe2+/ΣFe ratios at the scale of sampling volume and level of the analytical precision of each technique. The homogeneity of Li‐B‐O‐Nd‐Pb isotope ratios at the microscale (30–120 μm) was estimated using LA‐MC‐ICP‐MS and SIMS techniques. We also present new EPMA major element data obtained using three different instruments for the glasses. The determination of reference values for the major elements and their uncertainties at the 95% confidence level closely followed ISO guidelines and the Certification Protocol of the International Association of Geoanalysts. The ARM glasses may be particularly useful as reference materials for in situ isotope ratio analysis. [ABSTRACT FROM AUTHOR]

Hadean zircons, from the Jack Hills (Western Australia) and other localities, are currently the only window into the earliest terrestrial felsic crust, the formation of which remains enigmatic. Based upon new experimental results, generation of such early crust has been hypothesized to involve the partial melting of hydrated peridotite interacting with basaltic melt at low pressure (<10 km), but it has yet to be demonstrated that such liquids can indeed crystallize zircons comparable to Jack Hills zircon. We used thermodynamic and geochemical modeling to test this hypothesis. The predicted zircon saturation temperatures of <750 °C, together with the model zircon Th, U, Nb, Hf, Y, and rare earth element (REE) contents at 700 °C, δ18OVSMOW (Vienna standard mean ocean water) signatures, and co-crystallizing mineral assemblage were compared to those of the Jack Hills zircon. This comparison was favorable with respect to crystallization temperature, most trace-element contents, and mineral inclusions in zircon. The discrepancy in δ18OVSMOW signatures may be explained by hotter conditions of Hadean protocrust hydration. Our work supports the idea that felsic magma generation at shallow depths involving a primordial weathered ultramafic protocrust and local basaltic intrusions is indeed a viable mechanism for the formation of felsic crust on early Earth. [ABSTRACT FROM AUTHOR]