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Auger Spectroscopy


Figure 7 : Range of Fe contents (at%) in presolar silicates from three different classes of carbonaceous chondrites. The white stars indicate the median Fe content of presolar silicate grains from that meteorite. Figure adapted from [ 3 ].


Models of grain growth indicate that signifi cantly higher amounts of Fe can be incorporated under kinetic or non-equilibrium conditions than during equilibrium conden- sation. T us, together with the fact that the majority of presolar silicate grains have non-stoichiometric compositions ( Figure 6 ), this suggests that many, if not most, presolar silicates condensed under non-equilibrium conditions in the stellar envelopes where they formed. Indeed, as discussed below, this is consistent with the types of stars these grains are thought to come from based on their O isotopic compositions. Stellar sources of the grains . T e majority of O-rich presolar grains belong to one of two groups. Grains that exhibit enrichments in 17 O, with 18 O/ 16 O ratios that are close to the solar system value are thought to originate in low-mass evolved stars, such as red giant or asymptotic giant branch (AGB) stars. Modeling work on the formation of dust in such evolved stars indicates that most silicate dust forms during short high mass-loss episodes that occur aſt er thermal pulses in thermally pulsing AGB stars [ 19 ]. T e stellar environment during these episodes is highly variable, with strong stellar winds and large temperature variations, consistent with the observations from presolar silicate grains. A second group of grains, those that are enriched in 18 O, probably have origins in supernovae. Dust formation in supernovae takes place in the ejecta aſt er the star experiences core collapse and explodes. T is is also an environment that is very unstable and rapidly changing, consistent with what is observed in the grains. New types of presolar grains . Auger spectroscopy has also led to the discovery of new types of presolar grains. In some cases this has confi rmed astronomical observations suggesting their presence in stellar outfl ows. For example, infrared spectra of AGB stars have suggested the presence of silica [ 20 ], despite the fact that this mineral is not expected to condense in stellar environments [ 18 ]. Observations of supernova remnants have also suggested the presence of SiO 2 [ 21 ]. T e identifi cation of actual presolar silica [ 11 – 13 , 15 ] provides ground truth confi r- mation for such observations. Adelaide presolar silicates . Finally, elemental mapping with the Auger Nanoprobe has led to insights into the eff ects of secondary processing on presolar grains. For example, presolar


16


Figure 8 : Distribution of elemental compositions in 42 presolar silicate grains measured by Auger spectroscopy in the carbonaceous chondrite Adelaide. Pyroxene = 1.0 ± 0.1; olivine = 2.0 ± 0.3 [ 4 ].


silicates from the ungrouped carbonaceous chondrite Adelaide exhibit a number of compositional features not seen in the presolar silicates from other primitive meteorites. Adelaide presolar silicates have a median Fe content that, at ~26 at%, is roughly twice as high as that observed in other carbonaceous chondrites ( Figure 7 ). Moreover, the distribution of their compositions is starkly diff erent from that of other presolar- silicate-bearing meteorites ( Figure 8 ). T e vast majority of the grains in Adelaide have (Fe+Mg)/Si ratios greater than the nominal value for olivine (that is, Si-poor compared to olivine). Elemental mapping of the grains in Adelaide shows that the elevated ratios are due to the infi ltration of Fe into the grains ( Figure 9 ). Matrix grains in Adelaide show evidence for recrystallization, indicating that this meteorite appears to have experienced some thermal annealing [ 22 ]. T is process is likely responsible for the elevated Fe contents in most of its presolar silicates, with Fe entering the grains from the surrounding Fe-rich matrix [ 4 ].


Conclusions


Auger spectroscopy has found an important application in planetary science, in the analysis of O-rich presolar grains identifi ed by NanoSIMS. T is technique is ideal for the routine characterization of these grains and has given us a good overview of the compositions of the presolar silicate population. T rough this, we have a gained an understanding of the environments in which these grains formed and the eff ects of the secondary processing they may have experienced on their meteorite parent bodies. Auger spectroscopy has also led to the identifi cation of interesting and unusual grains, which can be investigated further by other techniques.


Acknowledgements T is work was funded by NASA grant NNX14AG25G.


References [1] JF Watts and J Wolstenholme , An Introduction to Surface Analysis by XPS and AES, Wiley & Sons , Chichester , 2003 .


www.microscopy-today.com • 2018 March


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