Trace element microanalysis has evolved steadily since the early days of EPMA, yet remains an ext... more Trace element microanalysis has evolved steadily since the early days of EPMA, yet remains an extraordinarily challenging subject. The enhanced capabilities of modern instrumentation, including the use of spectrometers with high X-ray collection efficiencies, high brightness electron sources, and improved stability all contribute to our ability to improve detection limits and analytical spatial resolution. Along with much improved software for data acquisition and analysis, recent progress in EPMA has made the trace realm more accessible than ever. High count precision can be obtained in order to easily bring analytical sensitivity into the single ppm range, but accuracy remains the greatest struggle. With the exception of the calibration, all sources of error encountered in major element analysis are magnified in trace analysis, and other sources become apparent where high spatial resolution is needed. Beam damage and charge effects are difficult problems in high sensitivity, high spatial resolution analysis, particularly in the analysis of insulators. Software can minimize some of the resulting effects on count rates during acquisition in order to improve accuracy, and analysts can empirically evaluate the conditions of analysis (count time, voltage, current, etc.) to try to minimize these effects. Trace analysis is fundamentally an exercise in background characterization, and the acquisition and evaluation of background is a subject of developing methodology. Background curvature and interferences can result in considerable inaccuracy, but can be dealt with via detailed quantitative wavelength scanning or multi-point spectral acquisitions which allow proper regression of the background shape. In the absence of excellent quality trace element secondary standards of similar matrix to unknowns, blank testing and consistency standards can be used to test at least some aspects of the methods employed. Ultimately, the analyst must rely on accuracy evolving from application of the most rigorous protocols.
The public reporting burden for this collection of information is estimated to average 1 hour per... more The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. February 2008 Proceedings Preprint 4. TITLE AND SUBTITLE SPECIMEN HETEROGENEITY ANALYSIS; A PRIMER (PREPRINT) 5a. CONTRACT NUMBER F33615-03-C-5206 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 62102F 6. AUTHOR(S) F. Meisenkothen (UES, Inc.) J.J. Donovan (University of Oregon) 5d. PROJECT NUMBER 4347 5e. TASK NUMBER 13 5f. WORK UNIT NUMBER 43471301 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION REPORT NUMBER
Martian soil composition can illuminate past and ongoing nearsurface processes such as impact gar... more Martian soil composition can illuminate past and ongoing nearsurface processes such as impact gardening [2] and hydrothermal and volcanic activity [3,4]. Though the Mars Exploration Rovers (MER) have analyzed the major-element composition of Martian soils, no soil samples have been returned to Earth for detailed chemical analysis. Rao et al. [1] suggested that Martian meteorite EETA79001 contains melted Martian soil in its impact glass (Lithology C) based on sulfur enrichment of Lithology C relative to the meteorite's basaltic lithologies (A and B) [1,2]. If true, it may be possible to extract detailed soil chemical analyses using this meteoritic sample. We conducted high-resolution (~0.3 µm/pixel) element mapping of Lithology C in thin section EETA79001,18 by energy dispersive spectrometry (EDS). We use these data for principal component analysis (PCA). Fig. 1: Magnesium map of EETA79001 Lithology C. Brighter yellow colors correspond to higher Mg contents of pixel. Yellow, sub-angular relict olivine and elongate quench crystals are set in a glassy matrix. Though visually glassy, EETA79001 Lithology C is heterogeneous. PCA identifies several significant components that may originate either in the basalt or in the soil, but one component with strongly positive eigenvectors for K and Cl likely represents a soluble soil component originating by hydrothermal or evaporitic processes. We are conducting quantitative beam analysis of the glass to determine minor and trace elements, including halogens. We will compare these to compositions of EETA79001 basalt [5,6] and to MER rock and soil compositions [7,8,9] to constrain the origin of components in EETA79001 Lithology C.
Electron Probe Microanalysis (EPMA) requires the acquisition of characteristic X-ray intensities ... more Electron Probe Microanalysis (EPMA) requires the acquisition of characteristic X-ray intensities of elements of interest, which are superimposed upon the background (also called continuum or bremsstrahlung). These background counts must be removed for accurate quantitation, usually by measuring two off-peak channels. An alternative approach, the Mean Atomic Number (MAN) background calculation method, was developed in 1970s. This technique takes advantage of Kramers' law: the generated continuum intensity scales monotonically with the (mean) atomic number of the target. Here, an empirical curve of background intensity versus MAN is generated via measurements at a specific characteristic X-ray peak position for a desired element on a series of standards that do not contain the analyte of interest [1,2]. After a correction for continuum absorption upon the emitted X-rays, we can assume that the MAN curve is a smooth function of the mean atomic number and can easily be used to determine the continuum intensity of a particular material, given its mean atomic number.
Open-source Python3 tools for Thermobarometry: Revealing the good, the bad and the ugly of determining P-T-X conditions in igneous systems
<p>The chemistry of erupted minerals and melts are commonly used to determine the p... more <p>The chemistry of erupted minerals and melts are commonly used to determine the pressures, temperatures and H<sub>2</sub>O contents of magma storage regions beneath volcanic centres. In turn, these estimates are vital for hazard assessment, to understand the formation of critical metal deposits, and to inform models of continental crust formation. In the last few decades, more than 100 empirical and thermodynamic expressions have been calibrated using measurements of phases in experimental studies where these intensive parameters are known. By collating these different models into a computationally-efficient, open-source Python3 package, Thermobar, we can critically assess the performance of thermobarometers in igneous systems, and propagate analytical errors. When we apply published models for different mineral equilibrium to a new experimental dataset not used in model calibration, we find that stated errors vastly underestimate the true uncertainty when these workflows are applied to natural systems.</p><p>Specifically, we find that realistic calculation workflows involving Clinopyroxene (Cpx) equilibrium (e.g., iterating pressure and temperature) have uncertainties spanning the entire crust in most tectonic settings. Using Thermobar functions to propagate analytical error using Monte Carlo simulations, we suggest that these large errors result from imprecise analyses of minor elements such as Na in experimental (and natural) Cpx. Common analytical conditions used for Cpx yield highly correlated pressure-temperature arrays spanning the entire crust, which have been incorrectly interpreted as trancrustal storage in natural systems. Insuffucient analyses of each phase in experimental products means that this analytical error is not sufficiently mediated by averaging, so reported mineral compositions deviate from the true phase composition. This impacts thermobarometer calibration, as well as assessment of these methods using test experimental datasets.</p><p>Overall, we demonstrate that the development of Python3 infrastructure for common quantitative workflows in volcanology is vital to allow rigorous error assessment and model intercomparison; such assessments simply aren’t feasible using traditional tools (e.g., Excel workbooks). Specific changes to analytical, experimental and model calibration workflows (e.g., higher beam currents and count times in Na) will be essential to produce a more robust dataset to calibrate and test the next generation of more precise and accurate Cpx-based barometers. In turn, this will enable more rigorous investigation of magma storage geometries in a variety of tectonic settings (e.g., distinguishing true transcrustal storage vs. storage in discrete reservoirs).</p><p> </p>
We show components with eignenvalues ≥ 1.0. The total variance explained by these components if 5... more We show components with eignenvalues ≥ 1.0. The total variance explained by these components if 53% and 46%, respecFvely. See Figure 3 for experimental regions.
Quantitative WDS Compositional Mapping Using the Electron Microprobe
Abstracts with programs, 2021
Abstract While much progress has been made in electron-probe microanalysis (EPMA) to improve the ... more Abstract While much progress has been made in electron-probe microanalysis (EPMA) to improve the accuracy of point analysis, the same level of attention has not always been applied to the quantification of wavelength-dispersive spectrometry (WDS) X-ray intensity maps at the individual pixel level. We demonstrate that the same level of rigor applied in traditional point analysis can also be applied to the quantification of pixels in X-ray intensity maps, along with additional acquisition and quantitative processing procedures to further improve accuracy, precision, and mapping throughput. Accordingly, X-ray map quantification should include pixel-level corrections for WDS detector deadtime, corrections for changes in beam current (beam drift), changes in standard intensities (standard drift), high-accuracy removal of background intensities, quantitative matrix corrections, quantitative correction of spectral interferences, and, if required, time-dependent corrections (for beam and/or contamination sensitive materials). The purpose of quantification at the pixel level is to eliminate misinterpretation of intensity artifacts, inherent in raw X-ray intensity signals, that distort the apparent abundance of an element. Major and minor element X-ray signals can contain significant artifacts due to absorption and fluorescence effects. Trace element X-ray signals can contain significant artifacts where phases with different average atomic numbers produce different X-ray continuum (bremsstrahlung) intensities, or where a spectral interference, even an apparently minor one, can produce a false-positive intensity signal. The methods we propose for rigorous pixel quantification require calibration of X-ray intensities on the instrument using standard reference materials, as we already do for point analysis that is then used to quantify multiple X-ray maps, and thus the relative time overhead associated with such pixel-by-pixel quantification is small. Moreover, the absolute time overhead associated with this method is usually less than that required for quantification using manual calibration curve methods while resulting in significantly better accuracy. Applications to geological, synthetic, or engineering materials are numerous as quantitative maps not only show compositional 2D variation of fine-grained or finely zoned structures but also provide very accurate quantitative analysis, with precision approaching that of a single point analysis, when multiple-pixel averaging in compositionally homogeneous domains is utilized.
This is the peer-reviewed, final accepted version for American Mineralogist, published by the Min... more This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America. The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.
PRSUPR, an integrated program to acqr ple fixed or scanning spectrometer electror le automation i... more PRSUPR, an integrated program to acqr ple fixed or scanning spectrometer electror le automation includes automated standard scribed. X-ray count and coordinate data are ng predigitized standard samples referenced to random-access disk files for on-line or of I marks or user-defined locations from a preusing the clrzuFsample matrix correctior on' Automation for unknown sample speciset ofunknown and standard sample autom lom selected points' line traverses' all.d x-y integrated into the program. Extensive u from user-defined cartesian grids with rectsimple installation program are provided polygon boundaries using mouse' trackball' mer. The program operates on a standard II In addition' the program can operate unatand supports automation, acquisition, and ing any combination of the above standard for a wide variety ofmicroprobe hardware nation procedures' omation includes multiple element analysis
Faced with time constraints and with beam sensitive materials, electron probe micro-analyser (EPM... more Faced with time constraints and with beam sensitive materials, electron probe micro-analyser (EPMA) operators often desire faster analyses to save time and/or to minimize sample damage from exposure to the electron beam. One of the most common approaches to decreasing analytical time is to reduce the amount of time spent counting x-rays both on peak and on background. This results in fewer x-ray counts, however, which translates into reduced analytical precision and higher detection limits. Increasing the beam current can compensate for this reduced count time, but may also result in increased sample damage and an attendant decrease in accuracy.
We present methods combining backscattered-electron (BSE) mosaic imaging, quantitative spot-mode ... more We present methods combining backscattered-electron (BSE) mosaic imaging, quantitative spot-mode electron-probe microanalysis (EPMA), and quantitative compositional mapping by EPMA and micro-xray fluorescence (µXRF) to provide a framework for detailed analysis of terrestrial and lunar samples. BSE imaging provides a base map for the characterization of samples by EPMA. Recent developments in image stitching provide a convenient method of processing BSE mosaic image sets. Characterizing cm-sized samples by EPMA and µXRF provides complementary information about sample chemistry. We acquire BSE mosaic and x-ray stage maps on a JEOL JXA-8200 electron microprobe equipped with five WDS spectrometers and a silicon drift EDS (SDD) that are used to acquire major and minor element maps. Quantitative EPMA maps are acquired using Probe Image, Probe for EPMA, and CalcImage software using conventional EPMA standardization, mean atomic number (MAN) background correction, and a full Φ(pz) correction at each pixel in the map. Maps are acquired with 400-1024 pixels at 0.X-15 µm and 10-100 msec per pixel at analytical conditions of 15kV and 50-100 nA. Large areas are covered by selection of appropriate pixel resolution and step size. An EDAX Orbis µXRF equipped with an SDD is used for spectrum image stage mapping using a 30 µm polycapillary optic, similar pixel resolution, and dwell times of 50-500 msec.
Martian meteorite Elephant Moraine A79001 (EET 79001) has received considerable attention for the... more Martian meteorite Elephant Moraine A79001 (EET 79001) has received considerable attention for the unusual composition of its shock melt glass, particularly its enrichment in sulfur relative to the host shergottite. It has been hypothesized that Martian regolith was incorporated into the melt or, conversely, that the S-enrichment stems from preferential melting of sulfide minerals in the host rock during shock. We present results from an electron microprobe study of EET 79001 including robust measurements of major and trace elements in the shock melt glass (S, Cl, Ni, Co, V, and Sc) and minerals in the host rock (Ni, Co, and V). We find that both S and major element abundances can be reconciled with previous hypotheses of regolith incorporation and/or excess sulfide melt. However, trace element characteristics of the shock melt glass, particularly Ni and Cl abundances relative to S, cannot be explained either by the incorporation of regolith or sulfide minerals. We therefore propose an alternative hypothesis whereby, prior to shock melting, portions of EET 79001 experienced acid-sulfate leaching of the mesostasis, possibly groundmass feldspar, and olivine, producing Al-sulfates that were later incorporated into the shock melt, which then quenched to glass. Such activity in the Martian near-surface is supported by observations from the Mars Exploration Rovers and laboratory experiments. Our preimpact alteration model, accompanied by the preferential survival of olivine and excess melting of feldspar during impact, explains the measured trace element abundances better than either the regolith incorporation or excess sulfide melting hypothesis does.
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