• A refractory inclusion returned by Stardust from comet 81P/Wild 2

      Simon, S. B.; Joswiak, D. J.; Ishii, H. A.; Bradley, J. P.; Chi, M.; Grossman, L.; Aléon, J.; Brownlee, D. E.; Fallon, S.; Hutcheon, I. D.; et al. (The Meteoritical Society, 2008-01-01)
      Among the samples returned from comet 81P/Wild 2 by the Stardust spacecraft is a suite of particles from one impact track (Track 25) that are Ca-, Al-rich and FeO-free. We studied three particles from this track that range in size from 5.3 x 3.2 micrometers to 15 x 10 micrometers. Scanning and transmission electron microscopy show that they consist of very fine-grained (typically from ~0.5 to ~2 micrometers) Al-rich, Ti-bearing and Ti-free clinopyroxene, Mg-Al spinel and anorthite, with trace amounts of fine perovskite, FeNi metal and osbornite (TiN) grains. In addition to these phases, the terminal particle, named Inti, also contains melilite. All of these phases, with the exception of osbornite, are common in refractory inclusions and are predicted to condense at high temperature from a gas of solar composition. Osbornite, though very rare, has also been found in meteoritic refractory inclusions, and could have formed in a region of the nebula where carbon became enriched relative to oxygen compared to solar composition. Compositions of Ti-pyroxene in Inti are similar, but not identical, to those of fassaite from Allende inclusions. Electron energy loss spectroscopy shows that Ti-rich pyroxene in Inti has Ti3+/Ti4+ within the range of typical meteoritic fassaite, consistent with formation under reducing conditions comparable to those of a system of solar composition. Inti is 16O-rich, with delta-18O approximately equal to delta-17O approximately equal to -40 per mil, like unaltered phases in refractory inclusions and refractory IDPs. With grain sizes, mineralogy, mineral chemistry, and an oxygen isotopic composition like those of refractory inclusions, we conclude that Inti is a refractory inclusion that formed in the inner solar nebula. Identification of a particle that formed in the inner solar system among the comet samples demonstrates that there was transport of materials from the inner to the outer nebula, probably either in a bipolar outflow or by turbulence.
    • Applied focused ion beam techniques for sample preparation of astromaterials for integrated nanoanalysis

      Graham, G. A.; Teslich, N. E.; Kearsley, A. T.; Stadermann, F. J.; Stroud, R. M.; Dai, Z.; Ishii, H. A.; Hutcheon, I. D.; Bajt, S.; Snead, C. J.; et al. (The Meteoritical Society, 2008-01-01)
      Sample preparation is always a critical step in the study of micrometer-sized astromaterials available for study in the laboratory, whether their subsequent analysis is by electron microscopy or secondary ion mass spectrometry. A focused beam of gallium ions has been used to prepare electron transparent sections from an interplanetary dust particle (IDP), as part of an integrated analysis protocol to maximize the mineralogical, elemental, isotopic, and spectroscopic information extracted from one individual particle. In addition, focused ion beam (FIB) techniques have been employed to extract cometary residue preserved on the rims and walls of microcraters in 1100 series aluminum foils that were wrapped around the sample tray assembly on the Stardust cometary sample collector. Non-ideal surface geometries and inconveniently located regions of interest required creative solutions. These include support pillar construction and relocation of a significant portion of sample to access a region of interest. Serial sectioning, in a manner similar to ultramicrotomy, is a significant development and further demonstrates the unique capabilities of focused ion beam microscopy for sample preparation of astromaterials.
    • Macroscopic subdivision of silica aerogel collectors for sample return missions

      Ishii, H. A.; Bradley, J. P. (The Meteoritical Society, 2006-01-01)
      Silica aerogel collector tiles have been employed for the collection of particles in low Earth orbit and, more recently, for the capture of cometary particles by NASA's Stardust mission. Reliable, reproducible methods for cutting these and future collector tiles from sample return missions are necessary to maximize the science output from the extremely valuable embedded particles. We present a means of macroscopic subdivision of collector tiles by generating large-scale cuts over several centimeters in silica aerogel with almost no material loss. The cut surfaces are smooth and optically clear allowing visual location of particles for analysis and extraction. This capability is complementary to the smaller-scale cutting capabilities previously described (Westphal 2004; Ishii 2005a, 2005b) for removing individual impacts and particulate debris in tiny aerogel extractions. Macroscopic cuts enable division and storage or distribution of portions of aerogel tiles for immediate analysis of samples by certain techniques in situ or further extraction of samples suited for other methods of analysis. The capability has been implemented in the Stardust Laboratory at NASA's Johnson Space Center as one of a suite of aerogel cutting methods to be used in Stardust sample curation.
    • Rapid extraction of dust impact tracks from silica aerogel by ultrasonic microblades

      Ishii, H. A.; Graham, G. A.; Kearsley, A. T.; Grant, P. G.; Snead, C. J.; Bradley, J. P. (The Meteoritical Society, 2005-01-01)
      In January 2006, NASA's Stardust mission will return with its valuable cargo of the first cometary dust particles captured at hypervelocity speeds in silica aerogel collectors and brought back to Earth. Aerogel, a proven capture medium, is also a candidate for future sample return missions and low-Earth orbit (LEO) deployments. Critical to the science return of Stardust as well as future missions that will use aerogel is the ability to efficiently extract impacted particles from collector tiles. Researchers will be eager to obtain Stardust samples as quickly as possible; tools for the rapid extraction of particle impact tracks that require little construction, training, or investment would be an attractive asset. To this end, we have experimented with diamond and steel microblades. Applying ultrasonic frequency oscillations to these microblades via a piezo-driven holder produces rapid, clean cuts in the aerogel with minimal damage to the surrounding collector tile. With this approach, intact impact tracks and associated particles in aerogel fragments with low-roughness cut surfaces have been extracted from aerogel tiles flown on NASA's Orbital Debris Collector (ODC) experiment. The smooth surfaces produced during cutting reduce imaging artifacts during analysis by scanning electron microscopy (SEM). Some tracks have been dissected to expose the main cavity for eventual isolation of individual impact debris particles and further analysis using techniques such as transmission electron microscopy (TEM) and nano-secondary ion mass spectrometry (nanoSIMS).
    • Recovering the elemental composition of comet Wild 2 dust in five Stardust impact tracks and terminal particles in aerogel

      Ishii, H. A.; Brennan, S.; Bradley, J. P.; Luening, K.; Ignatyev, K.; Pianetta, P. (The Meteoritical Society, 2008-01-01)
      The elemental (non-volatile) composition of five Stardust impact tracks and terminal particles left from capture of comet 81P/Wild 2 dust were mapped in a synchrotron X-ray scanning microprobe with full fluorescence spectra at each pixel. Because aerogel includes background levels of several elements of interest, we employ a novel "dual threshold" approach to discriminate against background contaminants: an upper threshold, above which a spectrum contains cometary material plus aerogel and a lower threshold below which it contains only aerogel. The difference between normalized cometary-plus-background and background-only spectra is attributable to cometary material. The few spectra in-between are discarded since misallocation is detrimental: cometary material incorrectly placed in the background spectrum is later subtracted from the cometary spectrum, doubling the loss of reportable cometary material. This approach improves accuracy of composition quantification. We present the refined whole impact track and terminal particle elemental abundances for the five impact tracks. One track shows mass increases in Cr and Mn (1.4x), Cu, As and K (2x), Zn (4x), and total mass (13%) by dual thresholds compared to a single threshold. Major elements Fe and Ni are not significantly affected. The additional Cr arises from cometary material containing little Fe. We exclude Au intermixed with cometary material because it is found to be a localized surface contaminant carried by comet dust into an impact track. The dual threshold technique can be used in other situations where elements of interest in a small sample embedded in a matrix are also present in the matrix itself.
    • The formation of boundary clinopyroxenes and associated glass veins in type B1 CAIs

      Paque, J. M.; Beckett, J. R.; Ishii, H. A.; Aléon-Toppani, A.; Burnett, D. S.; Teslich, N.; Dai, Z. R.; Bradley, J. P. (The Meteoritical Society, 2009-01-01)
      We used focused ion beam thin section preparation and scanning transmission electron microscopy (FIB/STEM) to examine the interfacial region between spinel and host melilite for spinel grains in type B1 inclusions from the Allende and Leoville carbonaceous chondrites. Boundary clinopyroxenes decorating spinel surfaces have compositions similar to those of coarser clinopyroxenes from the same region of the inclusion, suggesting little movement after formation. Host melilite displays no anomalous compositions near the interface and late-stage minerals are not observed, suggesting that boundary pyroxenes did not form by crystallization of residual liquid. Allende spinels display either direct spinel-melilite contact or an intervening boundary clinopyroxene between the two phases. Spinel-melilite interfacial regions in a Leoville B1 are more complex, with boundary clinopyroxene, as observed in Allende, but also variable amounts of glass, secondary calcite, perovskite, and an Mg-, Al-, OH-rich and Ca-, Si-poor crystalline phase that may be a layered double hydrate. One possible scenario of formation for the glass veins is that open system alteration of melilite produced a porous, hydrated aggregate of Mg-carpholite or sudoite + aluminous diopside that was shock melted and quenched to a glass. The hydrated crystalline phase we observed may have been a shocked remnant of the precursor phase assemblage, but is more likely to have formed later by alteration of the glass. In the mantle, boundary clinopyroxenes may have been crystallized from Ti-rich liquids formed by the direct dissolution of perovskite and an associated Sc-Zr-rich phase or as a reaction product between dissolving perovskite and liquid. In the core, any perovskite and associated Ti-enriched liquids that may have originally been present disappeared before the growth of boundary clinopyroxene, and the observed boundary clinopyroxene may have nucleated and grown from the liquid, along with the larger core clinopyroxene.