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1. The first samples from Almahata Sitta showing contacts between ureilitic and chondritic lithologies: Implications for the structure and composition of asteroid 2008 TC 3

   https://doi.org/10.1111/maps.13390  

   Goodrich, Cyrena Anne ; Zolensky, Michael E. ; Fioretti, Anna Maria ; Shaddad, Muawia H. ; Downes, Hilary ; Hiroi, Takahiro ; Kohl, Issaku ; Young, Edward D. ; Kita, Noriko T. ; Hamilton, Victoria E. ; et al ( October 2019 , Meteoritics & Planetary Science)

   Almahata Sitta (AhS), an anomalous polymict ureilite, is the first meteorite observed to originate from a spectrally classified asteroid (2008TC₃). However, correlating properties of the meteorite with those of the asteroid is not straightforward because the AhS stones are diverse types. Of those studied prior to this work, 70–80% are ureilites (achondrites) and 20–30% are various types of chondrites. Asteroid 2008TC₃was a heterogeneous breccia that disintegrated in the atmosphere, with its clasts landing on Earth as individual stones and most of its mass lost. We describe AhS 91A and AhS 671, which are the first AhS stones to show contacts between ureilitic and chondritic materials and provide direct information about the structure and composition of asteroid 2008TC₃. AhS 91A and AhS 671 are friable breccias, consisting of a C1 lithology that encloses rounded to angular clasts (<10 μm to 3 mm) of olivine, pyroxenes, plagioclase, graphite, and metal‐sulfide, as well as chondrules (~130–600 μm) and chondrule fragments. The C1 material consists of fine‐grained phyllosilicates (serpentine and saponite) and amorphous material, magnetite, breunnerite, dolomite, fayalitic olivine (Fo 28‐42), an unidentified Ca‐rich silicate phase, Fe,Ni sulfides, and minor Ca‐phosphate and ilmenite. It has similarities toCI1 but shows evidence of heterogeneous thermal metamorphism. Its bulk oxygen isotope composition (δ¹⁸O = 13.53‰, δ¹⁷O = 8.93‰) is unlike that of any known chondrite, but similar to compositions of severalCC‐like clasts in typical polymict ureilites. Its Cr isotope composition is unlike that of any known meteorite. The enclosed clasts and chondrules do not belong to the C1 lithology. The olivine (Fo 75‐88), pyroxenes (pigeonite of Wo ~10 and orthopyroxene of Wo ~4.6), plagioclase, graphite, and some metal‐sulfide are ureilitic, based on mineral compositions, textures, and oxygen isotope compositions, and represent at least six distinct ureilitic lithologies. The chondrules are probably derived from type 3OCand/orCC, based on mineral and oxygen isotope compositions. Some of the metal‐sulfide clasts are derived fromEC. AhS 91A and AhS 671 are plausible representatives of the bulk of the asteroid that was lost. Reflectance spectra of AhS 91A are dark (reflectance ~0.04–0.05) and relatively featureless inVNIR, and have an ~2.7 μm absorption band due toOH⁻in phyllosilicates. Spectral modeling, using mixtures of laboratoryVNIRreflectance spectra of AhS stones to fit the F‐type spectrum of the asteroid, suggests that 2008TC₃consisted mainly of ureilitic and AhS 91A‐like materials, with as much as 40–70% of the latter, and <10% ofOC,EC, and other meteorite types. The bulk density of AhS 91A (2.35 ± 0.05 g cm⁻³) is lower than bulk densities of other AhS stones, and closer to estimates for the asteroid (~1.7–2.2 g cm⁻³). Its porosity (36%) is near the low end of estimates for the asteroid (33–50%), suggesting significant macroporosity. The textures of AhS 91A and AhS 671 (finely comminuted clasts of disparate materials intimately mixed) support formation of 2008TC₃in a regolith environment. AhS 91A and AhS 671 could represent a volume of regolith formed when aCC‐like body impacted into already well‐gardened ureilitic + impactor‐derived debris. AhS 91A bulk samples do not show a solar wind component, so they represent subsurface layers. AhS 91A has a lower cosmic ray exposure (CRE) age (~5–9 Ma) than previously studied AhS stones (11–22 Ma). The spread inCREages argues for irradiation in a regolith environment. AhS 91A and AhS 671 show that ureilitic asteroids could have detectable ~2.7 μm absorption bands.

    

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2. An in situ investigation on the origins and processing of circumstellar oxide and silicate grains in carbonaceous chondrites

   https://doi.org/10.1111/maps.13418  

   Zega, Thomas J. ; Haenecour, Pierre ; Floss, Christine ( November 2019 , Meteoritics & Planetary Science)

   We report on the isotopic, chemical, and structural properties of four O‐rich presolar grains identified in situ in the Adelaide ungrouped C2, LaPaZ Icefield (LAP) 031117CO3.0, and Dominion Range (DOM) 08006CO3.0 chondrites. All four grains have oxygen‐isotopic compositions consistent with origins in the circumstellar envelopes (CSE) of low‐mass O‐rich stars evolved along the red‐giant and asymptotic‐giant branch (RGB,AGB, respectively) of stellar evolution. Transmission electron microscope (TEM) analyses, enabled by focused‐ion‐beam scanning electron microscope extraction, show that the grain from Adelaide is a single‐crystal Mg‐Al spinel, and comparison with equilibrium thermodynamic predictions constrains its condensation to 1500 K assuming a total pressure ≤10⁻³ atm in its hostCSE. In comparison,TEManalysis of two grains identified in theLAP031117 chondrite exhibits different microstructures. GrainLAP‐81 is composed of olivine containing a Ca‐rich and a Ca‐poor domain, both of which show distinct orientations, suggesting changing thermodynamic conditions in the hostCSEthat cannot be precisely constrained.LAP‐104 contains a polycrystalline assemblage of ferromagnesian silicates similar to previous reports of nanocrystalline presolar Fe‐rich silicates that formed under nonequilibrium conditions. Lastly,TEMshows that the grain extracted fromDOM08006 is a polycrystalline assemblage of Cr‐bearing spinel. The grains occur in different orientations, likely reflecting mechanical assembly in their hostCSE. The O‐isotopic and Cr‐rich compositions appear to point toward nonequilibrium condensation. The spinel is surrounded by an isotopically solar pyroxene lacking long‐range atomic order and could have served as a nucleation site for its condensation in the interstellar medium or the inner solar protoplanetary disk.

    

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3. Space weathering of silicates simulated by successive laser irradiation: In situ reflectance measurements of Fo 90 , Fo 99+ , and SiO 2

   https://doi.org/10.1111/maps.12581  

   Loeffler, M. J. ; Dukes, C. A. ; Christoffersen, R. ; Baragiola, R. A. ( January 2016 , Meteoritics & Planetary Science)

   Pulsed‐laser irradiation causes the visible‐near‐infrared spectral slope of olivine (Fo₉₀and Fo₉₉₊) and SiO₂to increase (redden), while the olivine samples darken and the SiO₂samples brighten slightly.XPSanalysis shows that irradiation of Fo₉₀produces metallic Fe. AnalyticalSEMandTEMmeasurements confirm that reddening in the Fo₉₀olivine samples correlates with the production of “nanophase” metallic Fe (npFe⁰) grains, 20–50 nm in size. The reddening observed in the SiO₂sample is consistent with the formation of SiO or other SiO_xspecies that absorb in the visible. The weak spectral brightening induced by laser irradiation of SiO₂is consistent with a change in surface topography of the sample. The darkening observed in the olivine samples is likely caused by the formation of larger npFe⁰particles, such as the 100–400 nm diameter npFe⁰identified during ourTEManalysis of Fo₉₀samples. The Fo₉₀reflectance spectra are qualitatively similar to those in previous experiments suggesting that in all cases formation of npFe⁰is causing the spectral alteration. Finally, we find that the accumulation of successive laser pulses cause continued sample darkening in the Vis‐NIR, which suggests that repeated surface impacts are an efficient way to darken airless body surfaces.

    

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4. Oxygen and Al‐Mg isotopic compositions of grossite‐bearing refractory inclusions from CO 3 chondrites

   https://doi.org/10.1111/maps.13282  

   Simon, Steven B. ; Krot, Alexander N. ; Nagashima, Kazuhide ( April 2019 , Meteoritics & Planetary Science)

   The distribution of the short‐lived radionuclide²⁶Al in the early solar system remains a major topic of investigation in planetary science. Thousands of analyses are now available but grossite‐bearing Ca‐, Al‐rich inclusions (CAIs) are underrepresented in the database. Recently found grossite‐bearing inclusions inCO3 chondrites provide an opportunity to address this matter. We determined the oxygen and magnesium isotopic compositions of individual phases of 10 grossite‐bearingCAIs in the Dominion Range (DOM) 08006 (CO3.0) andDOM08004 (CO3.1) chondrites. All minerals inDOM08006CAIs as well as hibonite, spinel, and pyroxene inDOM08004 are uniformly¹⁶O‐rich (Δ¹⁷O = −25 to −20‰) but grossite and melilite inDOM08004CAIs are not; Δ¹⁷O of grossite and melilite range from ~ −11 to ~0‰ and from ~ −23 up to ~0‰, respectively. Even within this small suite, in the two chondrites a bimodal distribution of the inferred initial²⁶Al/²⁷Al ratios (²⁶Al/²⁷Al)₀is seen, with four having (²⁶Al/²⁷Al)₀≤1.1 × 10⁻⁵and six having (²⁶Al/²⁷Al)₀≥3.7 × 10⁻⁵. Five of the²⁶Al‐richCAIs have (²⁶Al/²⁷Al)₀within error of 4.5 × 10⁻⁵; these values can probably be considered indistinguishable from the “canonical” value of 5.2 × 10⁻⁵given the uncertainty in the relative sensitivity factor for grossite measured by secondary ion mass spectrometry. We infer that the²⁶Al‐poorCAIs probably formed before the radionuclide was fully mixed into the solar nebula. All minerals in theDOM08006CAIs, as well as spinel, hibonite, and Al‐diopside in theDOM08004CAIs retained their initial oxygen isotopic compositions, indicating homogeneity of oxygen isotopic compositions in the nebular region where theCOgrossite‐bearingCAIs originated. Oxygen isotopic heterogeneity inCAIs fromDOM08004 resulted from exchange between the initially¹⁶O‐rich (Δ¹⁷O ~−24‰) melilite and grossite and¹⁶O‐poor (Δ¹⁷O ~0‰) fluid during hydrothermal alteration on theCOchondrite parent body; hibonite, spinel, and Al‐diopside avoided oxygen isotopic exchange during the alteration. Grossite and melilite that underwent oxygen isotopic exchange avoided redistribution of radiogenic²⁶Mg and preserved undisturbed internal Al‐Mg isochrons. The Δ¹⁷O of the fluid can be inferred from O‐isotopic compositions of aqueously formed fayalite and magnetite that precipitated from the fluid on theCOparent asteroid. This and previous studies suggest that O‐isotope exchange during fluid–rock interaction affected mostCAIs in CO ≥3.1 chondrites.

    

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5. Investigating the response of biotite to impact metamorphism: Examples from the Steen River impact structure, Canada

   https://doi.org/10.1111/maps.13011  

   Walton, E. L. ; Sharp, T. G. ; Hu, J. ; Tschauner, O. ( November 2017 , Meteoritics & Planetary Science)

   Impact metamorphic effects from quartz and feldspar and to a lesser extent olivine and pyroxene have been studied in detail. Comparatively, studies documenting shock effects in other minerals, such as double chain inosilicates, phyllosilicates, carbonates, and sulfates, are lacking. In this study, we investigate impact metamorphism recorded in crystalline basement rocks from the Steen River impact structure (SRIS), a 25 km diameter complex crater inNWAlberta, Canada. An array of advanced analytical techniques was used to characterize the breakdown of biotite in two distinct settings: along the margins of localized regions of shock melting and within granitic target rocks entrained as clasts in a breccia. In response to elevated temperature gradients along shock vein margins, biotite transformed at high pressure to an almandine‐Ca/Fe majorite‐rich garnet with a density of 4.2 g cm⁻³. The shock‐produced garnets are poikilitic, with oxide and silicate glass inclusions. Areas interstitial to garnets are vesiculated, in support of models for the formation of shock veins via oscillatory slip, with deformation continuing during pressure release. Biotite within granitic clasts entrained within the hot breccia matrix thermally decomposed at ambient pressure to produce a fine‐grained mineral assemblage of orthopyroxene + sanidine + titanomagnetite. These minerals are aligned to the (001) cleavage plane of the original crystal. In this and previous work, the transformation of an inosilicate (pargasite) and a phyllosilicate (biotite) to form garnet, an easily identifiable, robust mineral, has been documented. We contend that in deeply eroded astroblemes, high‐pressure minerals that form within or in the environs of shock veins may serve as one of the possibly few surviving indicators of impact metamorphism.

    

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