Almahata Sitta (AhS), an anomalous polymict ureilite, is the first meteorite observed to originate from a spectrally classified asteroid (2008
Chondrule-like objects and Ca-Al-rich inclusions (CAIs) are discovered in the retuned samples from asteroid Ryugu. Here we report results of oxygen isotope, mineralogical, and compositional analysis of the chondrule-like objects and CAIs. Three chondrule-like objects dominated by Mg-rich olivine are16O-rich and -poor with Δ17O (=δ17O – 0.52 × δ18O) values of ~ –23‰ and ~ –3‰, resembling what has been proposed as early generations of chondrules. The16O-rich objects are likely to be melted amoeboid olivine aggregates that escaped from incorporation into16O-poor chondrule precursor dust. Two CAIs composed of refractory minerals are16O-rich with Δ17O of ~ –23‰ and possibly as old as the oldest CAIs. The discovered objects (<30 µm) are as small as those from comets, suggesting radial transport favoring smaller objects from the inner solar nebula to the formation location of the Ryugu original parent body, which is farther from the Sun and scarce in chondrules. The transported objects may have been mostly destroyed during aqueous alteration in the Ryugu parent body.
more » « less- Award ID(s):
- 2004618
- NSF-PAR ID:
- 10397480
- Author(s) / Creator(s):
- ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more »
- Publisher / Repository:
- Nature Publishing Group
- Date Published:
- Journal Name:
- Nature Communications
- Volume:
- 14
- Issue:
- 1
- ISSN:
- 2041-1723
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract TC 3). 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 2008TC 3was 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 2008TC 3. 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 toCI 1 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 3OC and/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 laboratoryVNIR reflectance spectra of AhS stones to fit the F‐type spectrum of the asteroid, suggests that 2008TC 3consisted 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 2008TC 3in 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 inCRE ages 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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Abstract 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 (
CAI s) are underrepresented in the database. Recently found grossite‐bearing inclusions inCO 3 chondrites provide an opportunity to address this matter. We determined the oxygen and magnesium isotopic compositions of individual phases of 10 grossite‐bearingCAI s in the Dominion Range (DOM ) 08006 (CO 3.0) andDOM 08004 (CO 3.1) chondrites. All minerals inDOM 08006CAI s as well as hibonite, spinel, and pyroxene inDOM 08004 are uniformly16O‐rich (Δ17O = −25 to −20‰) but grossite and melilite inDOM 08004CAI s 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‐richCAI s 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‐poorCAI s probably formed before the radionuclide was fully mixed into the solar nebula. All minerals in theDOM 08006CAI s, as well as spinel, hibonite, and Al‐diopside in theDOM 08004CAI s retained their initial oxygen isotopic compositions, indicating homogeneity of oxygen isotopic compositions in the nebular region where theCO grossite‐bearingCAI s originated. Oxygen isotopic heterogeneity inCAI s fromDOM 08004 resulted from exchange between the initially16O‐rich (Δ17O ~−24‰) melilite and grossite and16O‐poor (Δ17O ~0‰) fluid during hydrothermal alteration on theCO chondrite 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 theCO parent asteroid. This and previous studies suggest that O‐isotope exchange during fluid–rock interaction affected mostCAI s in CO ≥3.1 chondrites. -
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Abstract Beckettite (Ca2V6Al6O20; IMA 2015‐001) is a newly discovered refractory mineral, occurring as micrometer‐sized grains intergrown with hibonite and perovskite, and surrounded by secondary grossular, anorthite, coulsonite, hercynite, and corundum. It occurs within highly altered areas in a V‐rich, Type A Ca‐Al‐rich inclusion (CAI), A‐WP1, from the Allende CV3 carbonaceous chondrite. The type beckettite has an empirical formula of (Ca1.99Na0.01)(V3+3.47Al1.40Ti4+0.57Mg0.25Sc0.08Fe2+0.04)(Al5.72Si0.28)O20, with a triclinic structure in space group
P and cell parameters a = 10.367 Å,b = 10.756 Å,c = 8.895 Å, α = 106.0°, β = 96.0°, γ = 124.7°,V = 739.7 Å3, andZ = 2, which leads to a calculated density of 3.67 g cm−3. Beckettite’s general formula is Ca2(V,Al,Ti,Mg)6Al6O20and the endmember formula is Ca2V6Al6O20. Beckettite is slightly16O‐depleted (Δ17O = −16 ± 2‰) compared to the coexisting hibonite and spinel −24 ± 2‰. Beckettite is a primary high‐temperature mineral resulting from igneous crystallization of an16O‐rich V‐rich CAI melt together with V‐bearing hibonite, perovskite, burnettite, spinel, and paqueite. Subsequently, beckettite experienced an incomplete isotope exchange with an16O‐poor aqueous fluid (Δ17O = −3 ± 2‰) on the Allende parent asteroid. -
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Abstract We investigated the metal‐rich chondrite Sierra Gorda (SG) 009, a member of the new G chondrite grouplet (also including NWA 5492, GRO 95551). G chondrites contain 23% metal, very reduced silicates, and rare oxidized mineral phases (Mg‐chromite, FeO‐rich pyroxene). G chondrites are not related to CH‐CB chondrites, based on bulk O, C, and N isotopic compositions, mineralogy, and geochemistry. G chondrites have no fine‐grained matrix or matrix lumps enclosing hydrated material typical for CH‐CB chondrites. G chondrites’ average metal compositions are similar to H chondrites. Siderophile and lithophile geochemistry indicates sulfidization and fractionation of the SG 009 metal and silicates, unlike NWA 5492 and GRO 95551. The G chondrites have average O isotopic compositions Δ17O>0‰ ranging between bulk enstatite (E) and ordinary (O) chondrites. An Al‐rich chondrule from SG 009 has Δ17O>0‰ indicating some heterogeneity in oxygen isotopic composition of G chondrite components. SG 009’s bulk carbon and nitrogen isotopic compositions correspond to E and O chondrites. Neon isotopic composition reflects a mixture of cosmogenic and solar components, and cosmic ray exposure age of SG 009 is typical for O, E, and R chondrites. G chondrites are closely related to O, E, and R chondrites and may represent a unique metal‐rich parent asteroid containing primitive and fractionated material from the inner solar system. Oxidizing and reducing conditions during SG 009 formation may be connected with a chemical microenvironment and possibly could indicate that G chondrites may have formed by a planetesimal collision resulting in the lack of matrix.
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Abstract We studied a thin section of Lewis Cliff (LEW) 87223, an unusual EL3‐related, enstatite chondrite (EC) that has primary and secondary features not observed in other ECs. We studied its metal‐rich nodules, possible shock features, and chondrules, eight of which are Al‐rich chondrules (ARCs). LEW 87223 has petrologic and compositional features similar to EL3s. Enstatite is the dominant mineral; chondrule boundaries are well defined; Si content of metal (0.5–0.6 wt%) is consistent with typical EL3; it has Cr‐bearing troilite, oldhamite, and alabandite; and its O‐isotopic composition is similar to other ECs. However, metal abundance in LEW 87223 (~13 vol%) is slightly higher than in other EL3s and its metal nodules are texturally and mineralogically different from other ECs. Both high and low Ni metals are present, and its alabandite has higher Fe (27.8 wt% Fe) than in other EL3s. Silicates appear darkened in plane polarized light, largely due to reduction of Fe from silicate. A remarkable feature of LEW 87223 is the high abundance of ARCs, which contain Ca‐rich plagioclase and varying amounts of Na‐rich plagioclase along chondrule edges and as veins. This suggests Na metasomatism and the possibility of hydrothermal fluids, potentially related to an impact event. LEW 87223 expands the range of known EC material. It shows that ECs are more diverse and record a wider range of parent body processes than previously known. LEW 87223 is an anomalous EL3, potentially the first member of a new EC group should similar samples be discovered.
