Stardust Interstellar Preliminary Examination <scp>VI</scp>: Quantitative elemental analysis by synchrotron X‐ray fluorescence nanoimaging of eight impact features in aerogel

<jats:title>Abstract</jats:title><jats:p>Hard X‐ray, quantitative, fluorescence elemental imaging was performed on the <jats:styled-content style="fixed-case">ID</jats:styled-content>22<jats:styled-content style="fixed-case">NI</jats:styled-content> nanoprobe and <jats:styled-content style="fixed-case">ID</jats:styled-content>22 microprobe beam lines of the European Synchrotron Research facility (<jats:styled-content style="fixed-case">ESRF</jats:styled-content>) in Grenoble, France, on eight interstellar candidate impact features in the framework of the <jats:styled-content style="fixed-case">NASA</jats:styled-content> Stardust Interstellar Preliminary Examination (<jats:styled-content style="fixed-case">ISPE</jats:styled-content>). Three features were unambiguous tracks, and the other five were identified as possible, but not definite, impact features. Overall, we produced an absolute quantification of elemental abundances in the 15 ≤ Z ≤ 30 range by means of corrections of the beam parameters, reference materials, and fundamental atomic parameters. Seven features were ruled out as interstellar dust candidates (<jats:styled-content style="fixed-case">ISDC</jats:styled-content>) based on compositional arguments. One of the three tracks, I1043,1,30,0,0, contained, at the time of our analysis, two physically separated, micrometer‐sized terminal particles, the most promising <jats:styled-content style="fixed-case">ISDC</jats:styled-content>s, Orion and Sirius. We found that the Sirius particle was a fairly homogenous Ni‐bearing particle and contained about 33 fg of distributed high‐Z elements (Z > 12). Orion was a highly heterogeneous Fe‐bearing particle and contained about 59 fg of heavy elements located in hundred nanometer phases, forming an irregular mantle that surrounded a low‐Z core. X‐ray diffraction (<jats:styled-content style="fixed-case">XRD</jats:styled-content>) measurements revealed Sirius to be amorphous, whereas Orion contained partially crystalline material (Gainsforth et al. 2014). Within the mantle, one grain was relatively Fe‐Ni‐Mn‐rich; other zones were relatively Mn‐Cr‐Ti‐rich and may correspond to different spinel populations. For absolute quantification purposes, Orion was assigned to a mineralogical assemblage of forsterite, spinel, and an unknown Fe‐bearing phase, while Sirius was most likely composed of an amorphous Mg‐bearing material with minor Ni and Fe. Owing to its nearly chondritic abundances of the nonvolatile elements Ca, Ti, Co, and Ni with respect to Fe, in combination with the presence of olivine and spinel as inferred from <jats:styled-content style="fixed-case">XRD</jats:styled-content> measurements, Orion had a high probability of being extraterrestrial in origin.</jats:p>