Dataset Information

Electron Microprobe/SIMS Determinations of Al in Olivine: Applications to Solar Wind, Pallasites and Trace Element Measurements.

ABSTRACT: Electron probe microanalyzer measurements of trace elements with high accuracy are challenging. Accurate Al measurements in olivine are required to calibrate SIMS implant reference materials for measurement of Al in the solar wind. We adopt a combined EPMA/SIMS approach that is useful for producing SIMS reference materials as well as for EPMA at the ~100 μg g^-1 level. Even for mounts not polished with alumina photoelectron spectroscopy shows high levels of Al surface contamination. In order to minimize electron beam current density, a rastered 50 × 100 μm electron beam was adequate and minimized sensitivity to small Al-rich contaminants. Reproducible analyses of eleven SIMS-cleaned spots on San Carlos olivine agreed at 69.3 ± 1.0 μg g^-1• The known Al mass fraction was used to calibrate an Al implant into San Carlos. Accurate measurements of Al were made for olivines in the pallasites: lmilac, Eagle Station and Springwater. Our focus was on Al in olivine, but our technique could be refined to give accurate electron probe measurements for other contamination-sensitive trace elements. For solar wind, it is projected that the Al/Mg abundance ratio can be determined to 6%, a factor of 2 more precise than the solar spectroscopic ratio.

SUBMITTER: Paque JM

PROVIDER: S-EPMC8350971 | biostudies-literature | 2020 Sep

REPOSITORIES: biostudies-literature

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Electron Microprobe/SIMS Determinations of Al in Olivine: Applications to Solar Wind, Pallasites and Trace Element Measurements.

Paque J M JM Hofmann A E AE Burnett D S DS Guan Y Y Jurewicz A J G AJG Woolum D S DS Ma C C Rossman G R GR

Geostandards and geoanalytical research 20200608 3

Electron probe microanalyzer measurements of trace elements with high accuracy are challenging. Accurate Al measurements in olivine are required to calibrate SIMS implant reference materials for measurement of Al in the solar wind. We adopt a combined EPMA/SIMS approach that is useful for producing SIMS reference materials as well as for EPMA at the ~100 μg g<sup>-1</sup> level. Even for mounts not polished with alumina photoelectron spectroscopy shows high levels of Al surface contamination. In ...[more]

PMID: 34381324

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Project description:The performance of multi-collector secondary ion mass spectrometry (MC-SIMS) for Mg isotope ratio analysis was evaluated using 17 olivine and 5 pyroxene reference materials (RMs). The Mg isotope composition of these RMs was accurately and precisely determined by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS), and these measured isotope ratios were used to evaluate SIMS instrumental mass bias as a function of the forsterite (Fo) content of olivine. The magnitude of the Mg isotope matrix effects were ~3‰ in δ25Mg, and are a complex function of olivine Fo content, that ranged from Fo59.3 to Fo100. In addition to these Mg isotope matrix effects, Si+ ion yields and Mg+/Si+ ion ratios varied as a complex function of the Fo content of the olivine RMs. For example, Si+ ion yields varied by ~33%. Based on the observations, we propose instrumental bias correction procedures for SIMS Mg isotope analysis of olivine using a combination of Mg+/Si+ ratios and Fo content of olivine. Using this correction method, the accuracy of δ25Mg analyses is 0.3‰, except for analysis of olivine with Fo86-88 where instrumental biases and Mg+/Si+ ratios change dramatically with Fo content, making it more difficult to assess the accuracy of Mg isotope ratio measurements by SIMS over this narrow range of Fo content. Five pyroxene RMs (3 orthopyroxenes and 2 clinopyroxenes) show smaller ranges of instrumental bias (~1.4‰ in δ25Mg) as compared to the olivine RMs. The instrumental bias for the 3 orthopyroxene RMs do not define a linear relationship with respect to enstatite (En) content, that ranged from En85.5 -96.3. The clinopyroxene RMs have similar En and wollastonite (Wo) contents but have δ25Mg values that differ by 0.5‰ relative to their δ25Mg values determined by MC-ICP-MS. These results indicate that additional factors (e.g., minor element abundances) likely contribute to SIMS instrumental mass fractionation. In order to better correct for these SIMS matrix effects, additional pyroxene RMs with various chemical compositions and known Mg isotope ratios are needed.

Project description:The solar wind is a continual outflow of plasma and magnetic field from the Sun's upper atmosphere-the corona-that expands to fills the solar system. Variability in the near-Earth solar-wind conditions can produce adverse space weather that impacts ground- and space-based technologies. Consequently, numerical fluid models of the solar wind are used to forecast conditions a few days ahead. The solar-wind inner-boundary conditions are supplied by models of the corona that are, in turn, constrained by observations of the photospheric magnetic field. While solar eruptions-coronal mass ejections (CMEs)-are treated as time-dependent structures, a single coronal "snapshot" is typically used to determine the ambient solar-wind for a complete model run. Thus, all available time-history information from previous coronal-model solutions is discarded and the solar wind is treated as a steady-state flow, unchanging in the rotating frame of the Sun. In this study, we use 1 year of daily-updated coronal-model solutions to comprehensively compare steady-state solar-wind modelling with a time-dependent method. We demonstrate, for the first time, how the SS approach can fundamentally misrepresent the accuracy of coronal models. We also attribute three key problems with current space-weather forecasting directly to the steady-state approach: (1) the seemingly paradoxical result that forecasts based on observations from 3-days previous are more accurate than forecasts based on the most recent observations; (2) high inconsistency, with forecasts for a given day jumping significantly as new observations become available, changing CME propagation times by up to 17 h; and (3) insufficient variability in the heliospheric magnetic field, which controls solar energetic particle propagation to Earth. The time-dependent approach is shown to alleviate all three issues. It provides a consistent, physical solution which more accurately represents the information present in the coronal models. By incorporating the time history in the solar wind along the Sun-Earth line, the time-dependent approach will provide improvements to forecasting CME propagation to Earth.

Dataset Information

Electron Microprobe/SIMS Determinations of Al in Olivine: Applications to Solar Wind, Pallasites and Trace Element Measurements.

Publications

Electron Microprobe/SIMS Determinations of Al in Olivine: Applications to Solar Wind, Pallasites and Trace Element Measurements.

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