Auger Spectroscopy in Planetary Science: Elemental Analysis of Presolar Silicate Grains
Christine Floss
Laboratory for Space Sciences and Department of Physics , Washington University , One Brookings Drive , St. Louis , MO 63130 fl
oss@wustl.edu
Abstract: Auger spectroscopy has not found widespread application in the geological sciences, largely because specimens are often non- conductors that charge under electron beams. However, it has proven to be useful for the characterization of sub-micrometer mineral grains in certain meteorites, where sample charging issues can be mitigated. We describe the development of analytical protocols for the measurement of presolar silicate grains using Auger spectroscopy and review how this technique has helped our understanding of the stellar origin and subsequent evolution of these grains.
Keywords: Auger spectroscopy, presolar grains, NanoSIMS, elemental analysis, meteorites
Introduction
Auger spectroscopy is a well-established surface analytical technique in materials science [ 1 ], where it is referred to by a variety of acronyms, principally AES (Auger electron spectroscopy) and SAM (scanning Auger microscopy). However, AES has not found widespread application in the geological sciences. T is is largely due to problems with sample charging: since most geological samples are insulators, they need to be coated prior to analysis, which cannot be done in Auger spectroscopy because of the surface sensitivity of this technique. However, Auger spectroscopy has found an important niche application in planetary science, where it can be used for the characterization of sub-micrometer mineral grains. Presolar grains, as their name implies, are grains that are older than the solar system. T ey formed in the outfl ows of evolved stars and in the ejecta of stellar explosions (supernovae) and were a signifi cant component of the molecular cloud from which our solar system formed. A fraction of them survived solar system formation and can be found in small amounts (< 0.1%) in primitive meteorites that have remained essentially unaltered since their formation 4.6 billion years ago. Presolar grains are recognized by their isotopic signatures, which diff er substantially from those observed in other solar system materials. T ese grains can be studied in the laboratory to gain a better understanding of stellar evolution and nucleosynthesis of the elements. T ey also provide information about conditions in the stellar sources in which they formed and the environments they have traversed subsequent to formation, including the interstellar medium,
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the early solar nebula, and the parent bodies of the meteorites in which they are found. Auger spectroscopy fi rst began to be applied to the elemental analysis of sub-micrometer mineral grains about ten years ago and, since that time, has advanced to the point where it is used routinely to characterize presolar silicate grains identifi ed in primitive meteorites [ 2 , 3 ]. T is article summarizes the development of this technique for planetary science applica- tions and provides examples of how Auger spectroscopy has provided constraints on the stellar origin(s) of presolar silicate grains, as well as how they have been aff ected by secondary processing in the parent bodies of the meteorites in which they are found.
Materials and Methods NanoSIMS . Presolar silicate grains are identifi ed in situ in standard meteorite thin sections using nano-scale secondary ion mass spectrometry (Cameca NanoSIMS). In SIMS instru- ments (also known as ion microprobes), a high-energy primary ion beam is directed at a sample, from which secondary particles (neutrals, as well as positively and negatively charged species) are then ejected. Charged particles of one polarity (“secondary ions”) can then be extracted into a mass spectrometer and are counted by an ion detector (which can be an electron multiplier, a Faraday cup, or a channel plate). T e NanoSIMS, which is well suited for presolar grain research, has a lateral resolution
Figure 1 : Schematics illustrating the coaxial beam optics of (a) the Cameca NanoSIMS 50 and (b) the Physical Electronics PHI 700 Auger Nanoprobe. In both diagrams the primary beams are shown in blue, and the secondary beams are shown in red.
doi: 10.1017/S1551929518000202
www.microscopy-today.com • 2018 March
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