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Auger Spectroscopy


sections that we analyze, in fact, do need to be carbon-coated for the NanoSIMS analyses. T is carbon coat is removed during the NanoSIMS measurements, leaving clean raster areas surrounded by material retaining the carbon-coating ( Figure 2 ). T is combination of factors ensures that sample charging generally is not a problem for the Auger analysis of these grains.


Results T e isotopic composition of a presolar grain provides information about its stellar origin [ 6 ], but additional elemental information from Auger spectroscopy is useful in understanding the conditions under which a grain condensed and is necessary for comparison with astronomical data [ 7 ]. T is is particularly true for silicates because of the large number of compositionally and structurally distinct entities in this class of minerals [ 8 ]. T e elemental information obtained from Auger spectroscopy provides an overview of the diversity of the presolar silicate grain population. Auger analyses of the elements in over 40 presolar grains found in the carbonaceous chondrite meteorite Adelaide are given in [ 4 ]. In addition to single-phase grains, compound grains that consist of more than one distinct phase have also been found. In some cases these grains may consist of aggregates containing both silicate and oxide grains [ 4 ]. Figure 5 shows an example of such a grain, which consists of a Mg- and Si-rich upper part and an Al- and Ca-rich lower part. Compound grains consisting of compositionally diverse silicate grains have also been identifi ed [ 9 ].


Discussion


Figure 5 : Secondary electron image and false color Auger elemental maps of a compound presolar grain from the carbonaceous chondrite Adelaide. The grain is outlined in red, based on the isotopic anomaly observed in the NanoSIMS raster ion images (not shown), and consists of a Mg- and Si-rich upper part (1) and an Al- and Ca-rich lower part (2). Figure adapted from [ 4 ].


Figure 6 : Distribution of elemental compositions in >400 presolar silicate grains, from a number of primitive meteorites, measured by Auger spectroscopy. Pyroxene = 1.0 ± 0.1; olivine = 2.0 ± 0.3. Data sources: [ 9 , 12 – 17 ].


2018 March • www.microscopy-today.com


Compositions of presolar grains . Over the last ten years more than 400 O-rich presolar grains have been measured by Auger spectroscopy. Some of these grains (~50) have been oxides, phases such as wüstite [ 10 ], magnetite [ 11 ], and silica (SiO 2 ) [ 9 , 12 – 13 ]. However, the majority of the grains are ferromag- nesian silicates. Figure 6 shows the distribution of compositions based on the ratios of the cations Fe, Mg, and Ca, relative to Si. T is ratio is 1 for pyroxene and 2 for olivine. Roughly 40% of the grains have stoichiometries consistent with either olivine or pyroxene. Ca-, Na-, and Al-bearing silicates such as melilite and feldspar do not appear to be present in the presolar silicate population. A large fraction of the grains, moreover, have non-stoichiometric compositions that do not correspond to any known minerals: 10% of the grains are more Si-rich than pyroxene, 20% are more Si-poor than olivine, and 28% have compositions that are intermediate between olivine and pyroxene. High Fe contents . Another observation arising from our Auger measurements of presolar silicates is that most of these grains have high (and highly variable) abundances of Fe ( Figure 7 ). Iron contents range from 0 at% up to almost 50 at%, with median Fe contents of 12–14 at%. T e high Fe abundances observed in many of the grains were unanticipated because equilibrium condensation calculations predict the formation of Mg-rich silicates, such as enstatite and forsterite [ 18 ]. Although secondary alteration can enhance Fe contents, and appears to have played a role in some meteorites (see below), most of the meteorites with high presolar silicate abundances show little evidence of alteration, suggesting that much of the Fe is primary.


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