Anette von der Handt

Ancient, highly heterogeneous mantle beneath Gakkel ridge, Arctic Ocean

The Earth’s mantle beneath ocean ridges is widely thought to be depleted by previous melt extraction, but well homogenized by convective stirring. This inference of homogeneity has been complicated by the occurrence of portions enriched in incompatible elements. Here we show that some refractory abyssal peridotites from the ultraslow-spreading Gakkel ridge (Arctic Ocean) have very depleted 187Os/188Os ratios with model ages up to 2 billion years, implying the long-term preservation of refractory domains in the asthenospheric mantle rather than their erasure by mantle convection. The refractory domains would not be sampled by mid-ocean-ridge basalts because they contribute little to the genesis of magmas. We thus suggest that the upwelling mantle beneath mid-ocean ridges is highly heterogeneous, which makes it difficult to constrain its composition by mid-ocean-ridge basalts alone. Furthermore, the existence of ancient domains in oceanic mantle suggests that using osmium model ages to constrain the evolution of continental lithosphere should be approached with caution.

Accurate determination of Fe3+/∑Fe of andesitic glass by Mössbauer spectroscopy

Abstract To evaluate the accuracy of Fe3+ and Fe2+ ratios in silicate glasses determined by Mössbauer spectroscopy, we examine in detail the temperature (47-293 K) of Mössbauer spectra for two andesitic glasses, one quenched at 1 atm, 1400 °C (VF3) and the other at 3.5 GPa, 1600 °C (M544). Variabletemperature Mössbauer spectra of these two glasses are used to characterize the recoilless fraction, f, by two different methods-a relative method (RM) based on the temperature dependence of the ratios of Fe3+ and Fe2+ Mössbauer doublets and the second based on the temperature dependence of the center shift (CS) of the doublets. The ratio of the recoilless fractions for Fe3+ and Fe2+, CT, can then be used to adjust the observed area of the Mössbauer doublets into the Fe3+/∑Fe ratio in the sample. We also evaluated the contributions of non-paramagnetic components to the Fe in the glasses by determining the influence of applied magnetic field on sample magnetization. Finally, for the VF3 glass, we determined the Fe3+/∑Fe independently by wet chemical determination of the FeO content combined with careful electron microprobe analyses of total Fe. Recoilless fractions determined with the CS method (CSM) are significantly smaller than those determined with the relative method and suggest larger corrections to room-temperature Fe3+/∑Fe ratios. However, the RM determinations are believed to be more accurate because they depend less on the assumption of the Debye harmonic model and because they produce more nearly temperature-independent estimates of Fe3+/∑Fe ratios. Non-linear responses of sample magnetizations to applied magnetic fields indicate that the glasses contain a small (0.4-1.1% for VF3) superparamagnetic component that is most likely to be nanophase precipitates of (Fe,Mg)Fe2O4 oxide, but corrections for this component have negligible influence on the total Fe3+/∑Fe determined for the glass. For the VF3 glass, the Fe3+/∑Fe produced by uncorrected room-temperature Mössbauer spectroscopy [0.685 ± 0.014 in two standard deviation (2σ)] agrees within 3% of that determined by wet chemistry (0.666 ± 0.030 in 2σ). The Fe3+/∑Fe corrected for recoilless fraction contributions is 0.634 ± 0.078(2σ), which is 7.5% lower than the uncorrected room-temperature ratio, but also agrees within 5% of wet chemical ratio. At least for this andesitic glass, the room-temperature determination of Fe3+/∑Fe is accurate within analytical uncertainty, but room-temperature Mössbauer determinations of Fe3+/∑Fe are always systematically higher compared to recoilless-fraction corrected ratios.

Quantitative EPMA of Nitrogen in Silicate Glasses

Nitrogen is the dominant gas in the Earth’s atmosphere and the key to planetary habitability. However, exchange processes of nitrogen between deep Earth reservoirs (crust, mantle and core) and the surface (atmo-, hydroand biosphere) are still not well understood. Accurate determination of nitrogen at low concentrations is important to place constraints on N partitioning during planetary differentiation and to address its role in the evolution of the early atmosphere.

Focused Interest Group on Microanalytical Standards (FIGMAS): Assessing the Quality, Availability and Need for Standards in the Microanalytical Community

It has been recognized over the past years that different electron microprobe and scanning electron microprobe laboratories use different sets of standards or reference materials for quantitative analysis. Unfortunately, some of these standards either have become unavailable (e.g., some natural minerals from the Smithsonian Institution collection) or are only available to a restricted group of people (e.g., internal reference materials). Other synthetic materials are also available commercially or provided by other institutions and research centers. However, they sometimes lack either broad availability or acceptable characterization (e.g., NIST glasses, Corning glasses, Drake & Weill REE-glasses... [1,2]). Another important problem for the community is a clear assessment of standard quality (the “Good”, “Bad” and “Ugly” of Carpenter [3]): “good” homogeneous standards with accurate compositional information and without impurities or inclusions are rare, whereas “bad” standards, which lack good characterization, are more common. Individual lab managers do commonly examine their own standard collections to re-evaluate compositional homogeneity and test the accuracy of published compositions. Standards are also frequently re-analyzed at individual labs using various techniques, and therefore multiple accepted compositions for individual standards may exist.

Focused Interest Group on Microanalytical Standards (FIGMAS): An Update

Reference materials and standards (RMS) play a crucial role in quantitative microbeam analysis, as they are the basis for instrumental calibration, data quality assurance (e.g., secondary standards) and interlaboratory comparison. Good standards that have been evaluated and certified for their homogeneity and reference composition are rare and only available from a handful of recognized providers. Whereas some crucial RMS have or will soon become unavailable, some researchers continue to develop and test new RMS [1-3], at times becoming more widely disseminated. Unfortunately, practical constraints make RMS development often difficult [4], and leave analysts with a set of “second choice” reference materials instead of certified standards. Such reference materials may suffer from various shortcomings such as questionable provenance, homogeneity or impurity issues (natural samples), contamination (synthetic materials), or incomplete or inaccurate reference compositions. Furthermore, variations between individual batches and/or updated reference compositions [1, 5]) can lead to confusion and the potential risk for mixing old and new values. Efforts have to be made to keep a record, evaluate, and guarantee the quantity of RMS for the next century.

Microanalytical Standards, Reference and Research Materials: Continuing the Effort toward Breaking the Accuracy Barrier

Reference materials and standards play a critical role in quantitative microbeam methods, as they are necessary for instrumental calibration, quality assurance (e.g. secondary standards) and inter-laboratory comparison. Certified materials (“good” standards) are available from a handful of providers, having been tested for micro-homogeneity and reference composition through independent methods and round robins. However, this process is time consuming and costly [1]. Hence, most reference materials in use are not formally certified, and many analysts use compilation values or their own preferred values.