In situ identification of a CAI candidate in 81P/Wild 2 cometary dust by confocal high resolution synchrotron X-ray fluorescence

ArticleinGeochimica et Cosmochimica Acta 73(18):5483-5492 · September 2009with12 Reads
Impact Factor: 4.33 · DOI: 10.1016/j.gca.2009.06.008

We detected additional CAI-like material in STARDUST mission samples of comet 81P/Wild 2. Two highly refractory cometary dust fragments were identified in the impact track 110 [C2012, 0, 110, 0, 0] by applying high resolution synchrotron induced confocal and conventional XRF analysis (HR SR-XRF). The use of a polycapillary lens in front of the detector for confocal spectroscopy dramatically improves the fidelity of particle measurements by removing contribution from the surrounding aerogel. The high spatial resolution (300 × 300 nm2; 300 × 1000 nm2) obtained allowed the detailed non-destructive in situ (trapped in aerogel) study of impacted grains at the sub-μm level.For the two largest particles of the track, the terminal particle and a second particle along the impact track, Ca concentration is up to 30 times higher than CI and Ti is enriched by a factor of 2 compared to CI. High resolution (HR) SR-XRF mapping also reveals that the highest concentrations of Ca, Ti, Fe (and Ni) measured within each grain belongs to different areas of the respective maps which indicate that the particles are composed of several chemically diverse mineral phases. This is in agreement with the finding of a complex phase assemblage of highly refractory minerals in the first ever detected Stardust mission CAI grain “Inti” of Track 25.Principle component analysis (PCA) is a powerful tool for extracting the dominant mineral components and was applied to the two grains indicating that regions in the terminal particle and the second particle are consistent with anorthite or grossite and gehlenite, monticellite or Dmitryivanovite (CaAl2O4), respectively.Our new findings demonstrate that the HR SR-XRF with confocal geometry and PCA analysis is capable of identifying CAI-like fragments without the need to extract particles from the aerogel matrix which is a time-consuming, complex and destructive process.Furthermore, the detection of new CAI-like fragments in the coma dust of comet 81P/Wild 2 strengthens the observation that strong mixing effects and, therefore, mass transport before or during comet formation must have occurred at least up to the region where Kuiper Belt comets formed (∼30 AU).

    • "Thousands of grains were collected by the Stardust spacecraft during its encounter with comet 81P/Wild 2 in February 2004 and were subsequently delivered to Earth in January 2006. Numerous high-temperature materials were recovered from their host aerogel tracks including Ca, Al-rich inclusions (CAIs) (Zolensky et al., 2006; Simon et al., 2008; Schmitz et al., 2009; Matzel et al., 2010; Joswiak et al., 2013 ), ferromagnesian (FMG) chondrule fragments (Nakamura et al., 2008; Gainsforth et al., 2010; Bridges et al., 2012; Joswiak et al., 2012 Nakashima et al., 2012; Ogliore et al., 2012 ), possible amoeboid olivine aggregate (AOA) fragments (NakamuraMessenger et al., 2011; Nakashima et al., 2012) and anhydrous mineral and rock grains (Zolensky et al., 2006; Joswiak et al., 2012). Presolar grains (Messenger et al., 2009; Leitner et al., 2010), organic materials (Sandford et al., 2006; Matrajt et al., 2008) and exotic sub-100 nm Ti, V nitride inclusions (Simon et al., 2008) were also found. "
    [Show abstract] [Hide abstract] ABSTRACT: A 4 × 6 μm terminal particle from Stardust track 130, named Bidi, is composed of a refractory assemblage of Fo97 olivine, Al- and Ti-bearing clinopyroxene and anorthite feldspar (An97). Mineralogically, Bidi resembles a number of components found in primitive chondritic meteorites including Al-rich chondrules, plagioclase-bearing type I ferromagnesian chondrules and amoeboid olivine aggregates (AOAs). Measured widths of augite/pigeonite lamellae in the clinopyroxene indicate fast cooling rates suggesting that Bidi is more likely to be a chondrule fragment than an AOA. Bulk element concentrations, including an Al2O3 content of 10.2 wt%, further suggests that Bidi is more akin to Al-rich rather than ferromagnesian chondrules. This is supported by high anorthite content of the plagioclase feldspar, overall bulk composition and petrogenetic analysis using a cosmochemical Al2O3–Ca2SiO4–Mg2SiO4 phase diagram. Measured minor element abundances of individual minerals in Bidi generally support an Al-rich chondrule origin but are not definitive between any of the object types. Oxygen isotope ratios obtained from olivine (+minor high-Ca pyroxene)fall between the TF and CCAM lines and overlap similar minerals from chondrules in primitive chondrites but are generally distinct from pristine AOA minerals. Oxygen isotope ratios are similar to some minerals from both Al-rich and type I ferromagnesian chondrules in unequilibrated carbonaceous, enstatite and ordinary chondrites. Although no single piece of evidence uniquely identifies Bidi as a particular object type, the preponderance of data, including mineral assemblage, bulk composition, mineral chemistry, inferred cooling rates and oxygen isotope ratios, suggest that Bidi is more closely matched to Al-rich chondrules than AOAs or plagioclase-bearing type I ferromagnesian chondrules and likely originated in a chondrule-forming region in the inner solar system.
    Full-text · Article · Nov 2014 · Geochimica et Cosmochimica Acta
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    • "Unfortunately, no information is given about the size, density or composition of the standard solution drops or how the different densities of standard foils and investigated particles are taken into account. In [21] the authors reconstruct different mineral phases of Ca and Al-rich inclusions in cometary matter. Two heterogeneous particles embedded in aerogel are analysed which are considerably larger than the probing volume (case C) and, therefore, the confocal setup is mainly used to reduce information from the surrounding material. "
    [Show abstract] [Hide abstract] ABSTRACT: 3D micro X-ray fluorescence analysis (3D Micro-XRF) is a non-destructive method for the investigation of elemental compositions of specimens with which three-dimensionally resolved information can be obtained. This is rendered possible through the formation of a probing volume resulting from the overlap of a condensed X-ray beam and the acceptance of a polycapillary lens in front of an energy-dispersive detector. Various setup schemes have been developed in the last years, which can be divided into synchrotron instrumentation and X-ray tube based spectrometers. Established in 2003/2004 numerous applications have been published up until now. Quantification of data though is still a topic of considerable interest and has been reported only for limited number of publications. This review aims to give an overview of work on quantitative 3D Micro-XRF. As the method can be applied with adapted setups to a variety of analytical problems, quantification also has to be flexible and different schemes have been developed.
    Full-text · Article · Nov 2012 · Spectrochimica Acta Part B Atomic Spectroscopy
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  • [Show abstract] [Hide abstract] ABSTRACT: Compared to other microscopic analytical tools X-ray microscopy techniques have the advantage that the large penetration depth of X-rays in matter allows one to investigate the interior of an object without destructive sample preparation. In combination with X-ray fluorescence tomography, analytical information from inside of a specimen can be obtained. Different X-ray analytical techniques can be used to produce contrast, X-ray absorption, fluorescence, and diffraction, to yield chemical, elemental, and structural information about the sample. Scanning microscopy on the basis of various lens systems in synchrotron radiation sources provides a routine spatial resolution of now about 100 nanometer but in the foreseeable future a 10–20 nanometer spatial resolution can be expected. X-ray absorption spectrometry can also provide chemical (speciation) information on the sample. All this makes X-ray microscopy attractive to many fields of science. In this paper the techniques are briefly reviewed and a number of applications in the earth, planetary and cosmos sciences are illustrated with state-of-the art examples, while applications in the environmental sciences and biology are also briefly discussed.
    Full-text · Article · Dec 2010 · The European Physical Journal Conferences
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