Sulfur four isotope NanoSIMS analysis of comet‐81P/Wild 2 dust in impact craters on aluminum foil C2037N from NASA’s Stardust mission

Abstract– We present NanoSIMS four‐isotope S analyses of 24 comet Wild 2 dust impact residues in craters on aluminum foil C2037N returned by NASA’s Stardust mission. Except for one sample, all impact residues have normal S isotopic compositions within 2σ uncertainties of at least two S isotope ratios. This implies that most S‐rich Wild 2 dust impactors formed in the solar system. Instrumental isotope fractionation due to sample topography is the main contribution to our analytical uncertainty. One impact crater residue shows small anomalies of δ³³S = ?57 ± 17‰, and δ³⁴S = ?41 ± 17‰ (1σ uncertainties). Although this could be simply a statistical outlier or the fingerprint of a chemical isotope fractionation it is also possible that the observed anomaly results from the mixture of a cometary FeS particle with a small (150 nm diam.) presolar FeS supernova grain. This would translate into a presolar sulfide abundance of approximately 200 ppm.     

TOF‐SIMS analysis of Allende projectiles shot into silica aerogel

Abstract— Powdered Allende projectiles were fired into silica aerogel at 6.1 km/sec in order to evaluate particle retrieval and analysis techniques for samples from the Stardust mission. Since particles may disintegrate and ablate along the penetration paths in a high‐porosity aerogel, TOF‐SIMS analysis may be a suitable method to determine the distribution of such materials along the tracks as well as potential compositional modifications. Therefore, two ?350 μm‐sized tracks, residing at the surface of a keystone specimen that was flattened between two silicon chips, were analyzed. TOF‐SIMS allows for a detailed study of the chemical composition of particles that survived the impact mostly intact and of fine‐grained material from disintegrated projectiles. In the investigated keystone, material from light gas gun debris dominated. Besides the two tracks, a continuous, 40‐μm‐thick surface layer of implanted material—probably gun residue—was found. One of the two analyzed tracks is compositionally distinct from this surface layer and is likely to contain residual material of an Allende projectile. The analyses clearly demonstrate that tracks, resulting from impactors in the 5–10 μm size range, can be successfully analyzed with TOF‐SIMS.     

Survival of refractory presolar grain analogs during Stardust‐like impact into Al foils: Implications for Wild 2 presolar grain abundances and study of the cometary fine fraction

We present results of FIB–TEM studies of 12 Stardust analog Al foil craters which were created by firing refractory Si and Ti carbide and nitride grains into Al foils at 6.05 km s^?1 with a light‐gas gun to simulate capture of cometary grains by the Stardust mission. These foils were prepared primarily to understand the low presolar grain abundances (both SiC and silicates) measured by SIMS in Stardust Al foil samples. Our results demonstrate the intact survival of submicron SiC, TiC, TiN, and less‐refractory Si₃N₄ grains. In small (<2 μm) craters that are formed by single grain impacts, the entire impacting crystalline grain is often preserved intact with minimal modification. While they also survive in crystalline form, grains at the bottom of larger craters (>5 μm) are typically fragmented and are somewhat flattened in the direction of impact due to partial melting and/or plastic deformation. The low presolar grain abundance estimates derived from SIMS measurements of large craters (mostly >50 μm) likely result from greater modification of these impactors (i.e., melting and isotopic dilution), due to higher peak temperatures/pressures in these crater impacts. The better survivability of grains in smaller craters suggests that more accurate presolar grain estimates may be achievable through measurement of such craters. It also suggests small craters can provide a complementary method of study of the Wild 2 fine fraction, especially for refractory CAI‐like minerals.     

Laboratory simulation of impacts on aluminum foils of the Stardust spacecraft: Calibration of dust particle size from comet Wild‐2

Abstract— Metallic aluminum alloy foils exposed on the forward, comet‐facing surface of the aerogel tray on the Stardust spacecraft are likely to have been impacted by the same cometary particle population as the dedicated impact sensors and the aerogel collector. The ability of soft aluminum alloy to record hypervelocity impacts as bowl‐shaped craters offers an opportunistic substrate for recognition of impacts by particles of a potentially wide size range. In contrast to impact surveys conducted on samples from low Earth orbit, the simple encounter geometry for Stardust and Wild‐2, with a known and constant spacecraft‐particle relative velocity and effective surface‐perpendicular impact trajectories, permits closely comparable simulation in laboratory experiments. For a detailed calibration program, we have selected a suite of spherical glass projectiles of uniform density and hardness characteristics, with well‐documented particle size range from 10 μm to nearly 100 μm. Light gas gun buckshot firings of these particles at approximately 6 km s^?1 onto samples of the same foil as employed on Stardust have yielded large numbers of craters. Scanning electron microscopy of both projectiles and impact features has allowed construction of a calibration plot, showing a linear relationship between impacting particle size and impact crater diameter. The close match between our experimental conditions and the Stardust mission encounter parameters should provide another opportunity to measure particle size distributions and fluxes close to the nucleus of Wild‐2, independent of the active impact detector instruments aboard the Stardust spacecraft.     

Analytical scanning and transmission electron microscopy of laboratory impacts on Stardust aluminum foils: Interpreting impact crater morphology and the composition of impact residues

Abstract— The known encounter velocity (6.1 kms^?1) and particle incidence angle (perpendicular) between the Stardust spacecraft and the dust emanating from the nucleus of comet Wild‐2 fall within a range that allows simulation in laboratory light‐gas gun (LGG) experiments designed to validate analytical methods for the interpretation of dust impacts on the aluminum foil components of the Stardust collector. Buckshot of a wide size, shape, and density range of mineral, glass, polymer, and metal grains, have been fired to impact perpendicularly on samples of Stardust Al 1100 foil, tightly wrapped onto aluminum alloy plate as an analogue of foil on the spacecraft collector. We have not yet been able to produce laboratory impacts by projectiles with weak and porous aggregate structure, as may occur in some cometary dust grains. In this report we present information on crater gross morphology and its dependence on particle size and density, the pre‐existing major‐ and trace‐element composition of the foil, geometrical issues for energy dispersive X‐ray analysis of the impact residues in scanning electron microscopes, and the modification of dust chemical composition during creation of impact craters as revealed by analytical transmission electron microscopy. Together, these observations help to underpin the interpretation of size, density, and composition for particles impacted on the Stardust aluminum foils.     

Micron‐scale hypervelocity impact craters: Dependence of crater ellipticity and rim morphology on impact trajectory,projectile size,velocity, and shape

The interstellar collector on NASA's Stardust mission captured many particles from sources other than the interstellar dust stream. Impact trajectory may provide a means of discriminating between these different sources, and thus identifying/eliminating candidate interstellar particles. The collector's aerogel preserved a clear record of particle impact trajectory from the inclination and direction of the resultant tracks. However, the collector also contained aluminum foils and, although impact crater studies to date suggest only the most inclined impacts (>45° from normal) produce crater morphologies that indicate trajectory (i.e., distinctly elliptical), these studies have been restricted to much larger (mm and above) scales than are relevant for Stardust (μm). It is unknown how oblique impact crater morphology varies as a function of length scale, and therefore how well Stardust craters preserve details of impactor trajectory. Here, we present data from a series of impact experiments, together with complementary hydrocode modeling, that examine how crater morphology changes with impact angles for different‐sized projectiles. We find that, for our smallest spherical projectiles (2 μm diameter), the ellipticity and rim morphology provide evidence of their inclined trajectory from as little as 15° from normal incidence. This is most likely a result of strain rate hardening in the target metal. Further experiments and models find that variation in velocity and impactor shape complicate these trends, but that rim morphology remains useful in determining impact direction (where the angle of impact is >20° from normal) and may help identify candidate interstellar particle craters on the Stardust collector.     

Morphology of craters generated by hypervelocity impacts of micron‐sized polypyrrole‐coated olivine particles

To understand the process of cosmic dust particle impacts and translate crater morphology on smoothed metallic surfaces to dust properties, correct calibration of the experimental impact data is needed. This article presents the results of studies of crater morphology generated by impacts using micron‐sized polypyrrole (PPy)‐coated olivine particles. The particles were accelerated by an electrostatic dust accelerator to high speeds before they impacted onto polished aluminum targets. The projectile diameter and velocity ranges were 0.3–1.2 μm and 3–7 km s^?1. After impact, stereopair images of the craters were taken using scanning electron microscope and 3‐D reconstructions made to provide diameter and depth measurements. In this study, not just the dimensions of crater diameters and depths, but also the shape and dimensions of crater lips were analyzed. The craters created by the coated olivine projectiles are shown to have complicated shapes believed to be due to the nonspherical shape of the projectiles.     

Presolar grains in the CM2 chondrite Sutter's Mill

The Sutter's Mill (SM) carbonaceous chondrite is a regolith breccia, composed predominantly of CM2 clasts with varying degrees of aqueous alteration and thermal metamorphism. An investigation of presolar grains in four Sutter's Mill sections, SM43, SM51, SM2‐4, and SM18, was carried out using NanoSIMS ion mapping technique. A total of 37 C‐anomalous grains and one O‐anomalous grain have been identified, indicating an abundance of 63 ppm for presolar C‐anomalous grains and 2 ppm for presolar oxides. Thirty‐one silicon carbide (SiC), five carbonaceous grains, and one Al‐oxide (Al₂O₃) were confirmed based on their elemental compositions determined by C‐N‐Si and O‐Si‐Mg‐Al isotopic measurements. The overall abundance of SiC grains in Sutter's Mill (55 ppm) is consistent with those in other CM chondrites. The absence of presolar silicates in Sutter's Mill suggests that they were destroyed by aqueous alteration on the parent asteroid. Furthermore, SM2‐4 shows heterogeneous distributions of presolar SiC grains (12–54 ppm) in different matrix areas, indicating that the fine‐grained matrix clasts come from different sources, with various thermal histories, in the solar nebula.     

Polymer amide in the Allende and Murchison meteorites

It has been proposed that exothermic gas phase polymerization of amino acids can occur in the conditions of a warm dense molecular cloud to form hydrophobic polymer amide (HPA) (McGeoch and McGeoch 2014). In a search for evidence of this presolar chemistry Allende and Murchison meteorites and a volcano control were diamond burr‐etched and Folch extracted for potential HPA yielding 85 unique peaks in the meteorite samples via matrix‐assisted laser desorption time‐of‐flight mass spectrometry (MALDI TOF/MS). The amino acids after acid hydrolysis in Allende were below the level of detection but many of the Allende peaks via the more sensitive MALDI/TOF analysis could be fitted to a polymer combination of glycine, alanine, and alpha‐hydroxyglycine with high statistical significance. A similar significant fit using these three amino acids could not be applied to the Murchison data indicating more complex polymer chemistry.     

Refractory metal nuggets within presolar graphite: First condensates from a circumstellar environment

Transmission electron microscope (TEM) investigations have revealed Os, Ru, Mo‐rich refractory metal nuggets within four different presolar graphites, from both the high‐density (HD) Murchison (MUR) and low‐density (LD) Orgueil (ORG) fractions. Microstructural and chemical data suggest that these are direct condensates from the gas, rather than forming later by exsolution. The presolar refractory metal nugget (pRMN) compositions are variable (e.g., from 8 < Os atom% < 77), but follow the same chemical fractionation trends as isolated refractory metal nuggets (mRMNs) previously found in meteorites (Berg et al. 2009). From these compositions one can infer a temperature of last equilibration with the gas of 1405–1810 K (e.g., Berg et al. 2009 at approximately 100 dyne cm^?2 pressure), which implies that the host graphites form over roughly the same range (in agreement with predictions) and that the pRMNs are chemically isolated from the gas when captured by graphite. Further, the pRMN compositions give evidence that HD graphites form at a higher T than LD ones. Chemical and phase similarities with the isolated mRMNs suggest that the mRMNs also condense directly from a gas, although from the early solar nebula rather than a presolar environment. Although the pRMNs themselves are too small for detection of isotopic anomalies, NanoSIMS isotopic measurements of their host graphites confirm a presolar origin for the assemblages. The two pRMN‐containing LD graphites show evidence of a supernova (SN) origin, whereas the stellar origins of the pRMNs in HD graphite are unclear, because only less‐diagnostic ¹²C enrichments are detectable (as is commonly true for HD graphites).     

Correlated microanalysis of cometary organic grains returned by Stardust

Abstract– Carbonaceous matter in Stardust samples returned from comet 81P/Wild 2 is observed to contain a wide variety of organic functional chemistry. However, some of this chemical variety may be due to contamination or alteration during particle capture in aerogel. We investigated six carbonaceous Stardust samples that had been previously analyzed and six new samples from Stardust Track 80 using correlated transmission electron microscopy (TEM), X‐ray absorption near‐edge structure spectroscopy (XANES), and secondary ion mass spectroscopy (SIMS). TEM revealed that samples from Track 35 containing abundant aliphatic XANES signatures were predominantly composed of cometary organic matter infilling densified silica aerogel. Aliphatic organic matter from Track 16 was also observed to be soluble in the epoxy embedding medium. The nitrogen‐rich samples in this study (from Track 22 and Track 80) both contained metal oxide nanoparticles, and are likely contaminants. Only two types of cometary organic matter appear to be relatively unaltered during particle capture. These are (1) polyaromatic carbonyl‐containing organic matter, similar to that observed in insoluble organic matter (IOM) from primitive meteorites, interplanetary dust particles (IDPs), and in other carbonaceous Stardust samples, and (2) highly aromatic refractory organic matter, which primarily constitutes nanoglobule‐like features. Anomalous isotopic compositions in some of these samples also confirm their cometary heritage. There also appears to be a significant labile aliphatic component of Wild 2 organic matter, but this material could not be clearly distinguished from carbonaceous contaminants known to be present in the Stardust aerogel collector.     

Microstructural study of micron‐sized craters simulating Stardust impacts in aluminum 1100 targets

Abstract— Various microscopic techniques were used to characterize experimental microcraters in aluminum foils to prepare for the comprehensive analysis of the cometary and interstellar particle impacts in aluminum foils to be returned by the Stardust mission. First, scanning electron microscopy (SEM) and energy dispersive X‐ray spectroscopy (EDS) were used to study the morphology of the impact craters and the bulk composition of the residues left by soda‐lime glass impactors. A more detailed structural and compositional study of impactor remnants was then performed using transmission electron microscopy (TEM), EDS, and electron diffraction methods. The TEM samples were prepared by focused ion beam (FIB) methods. This technique proved to be especially valuable in studying impact crater residues and impact crater morphology. Finally, we also showed that infrared microscopy (IR) can be a quick and reliable tool for such investigations. The combination of all of these tools enables a complete microscopic characterization of the craters.     

Oxygen isotopic compositions and origins of calcium‐aluminum‐rich inclusions and chondrules

Abstract— Primary minerals in calcium‐aluminum‐rich inclusions (CAIs), Al‐rich and ferromagnesian chondrules in each chondrite group have δ¹⁸O values that typically range from ?50 to +5%0. Neglecting effects due to minor mass fractionations, the oxygen isotopic data for each chondrite group and for micrometeorites define lines on the three‐isotope plot with slopes of 1.01 ± 0.06 and intercepts of ?2 ± 1. This suggests that the same kind of nebular process produced the ¹⁶O variations among chondrules and CAIs in all groups. Chemical and isotopic properties of some CAIs and chondrules strongly suggest that they formed from solar nebula condensates. This is incompatible with the existing two‐component model for oxygen isotopes in which chondrules and CAIs were derived from heated and melted ¹⁶O‐rich presolar dust that exchanged oxygen with ¹⁶O‐poor nebular gas. Some FUN CAIs (inclusions with isotope anomalies due to fractionation and unknown nuclear effects) have chemical and isotopic compositions indicating they are evaporative residues of presolar material, which is incompatible with ¹⁶O fractionation during mass‐independent gas phase reactions in the solar nebula. There is only one plausible reason why solar nebula condensates and evaporative residues of presolar materials are both enriched in ¹⁶O. Condensation must have occurred in a nebular region where the oxygen was largely derived from evaporated ¹⁶O‐rich dust. A simple model suggests that dust was enriched (or gas was depleted) relative to cosmic proportions by factors of ～10 to >50 prior to condensation for most CAIs and factors of 1–5 for chondrule precursor material. We infer that dust‐gas fractionation prior to evaporation and condensation was more important in establishing the oxygen isotopic composition of CAIs and chondrules than any subsequent exchange with nebular gases. Dust‐gas fractionation may have occurred near the inner edge of the disk where nebular gases accreted into the protosun and Shu and colleagues suggest that CAIs formed.     

Infrared,ultraviolet, and electron paramagnetic resonance measurements on presolar diamonds: Implications for optical features and origin

Abstract— Infrared (IR) and ultraviolet (UV) absorption spectra were obtained for diamonds from the Allende and Murchison meteorites. In addition, and for the first time, electron paramagnetic resonance spectra were measured. The IR and UV data confirm the suspicion of Russell et al. (1996) that N in presolar diamonds predominantly appears in the form of dispersed N atoms, as is the case for terrestrial type Ib diamonds. In accordance with other observations, our electron paramagnetic resonance measurements suggest a high H content in presolar diamonds. The presolar diamonds most likely originated in a H‐rich region, an environment in which nanometer‐sized diamonds may be more stable than graphite (Badziag et al., 1990). This adds to the evidence—previously based mainly on the twin microstructures of presolar diamonds (Daulton et al., 1996) and the absence of graphite with the same isotopic composition as presolar diamonds (Anders and Zinner, 1993)—for a homogeneous nucleation of presolar diamonds from a gas phase. Based on our results for detection of diamonds in space, we suggest searching for the N‐induced IR and UV absorption features of type Ib diamonds. Other characteristic diamond features that could also be used to detect diamonds in space are the (‐CH_n) IR absorption features due to H‐coated diamonds, as they are described by Allamandola et al. (1993) and the IR multiphonon absorption features of the diamond lattice. The multiphonon features are very weak (Edwards, 1985), but their intensity increases somewhat with increasing temperature (Collins and Fan, 1954), so perhaps a search for them is not totally hopeless.     

Petrography,stable isotope compositions,microRaman spectroscopy,and presolar components of Roberts Massif 04133: A reduced CV3 carbonaceous chondrite

Here, we report the mineralogy, petrography, C‐N‐O‐stable isotope compositions, degree of disorder of organic matter, and abundances of presolar components of the chondrite Roberts Massif (RBT) 04133 using a coordinated, multitechnique approach. The results of this study are inconsistent with its initial classification as a Renazzo‐like carbonaceous chondrite, and strongly support RBT 04133 being a brecciated, reduced petrologic type >3.3 Vigarano‐like carbonaceous (CV) chondrite. RBT 04133 shows no evidence for aqueous alteration. However, it is mildly thermally altered (up to approximately 440 °C); which is apparent in its whole‐rock C and N isotopic compositions, the degree of disorder of C in insoluble organic matter, low presolar grain abundances, minor element compositions of Fe,Ni metal, chromite compositions and morphologies, and the presence of unequilibrated silicates. Sulfides within type I chondrules from RBT 04133 appear to be pre‐accretionary (i.e., did not form via aqueous alteration), providing further evidence that some sulfide minerals formed prior to accretion of the CV chondrite parent body. The thin section studied contains two reduced CV3 lithologies, one of which appears to be more thermally metamorphosed, indicating that RBT 04133, like several other CV chondrites, is a breccia and thus experienced impact processing. Linear foliation of chondrules was not observed implying that RBT 04133 did not experience high velocity impacts that could lead to extensive thermal metamorphism. Presolar silicates are still present in RBT 04133, although presolar SiC grain abundances are very low, indicating that the progressive destruction or modification of presolar SiC grains begins before presolar silicate grains are completely unidentifiable.     

Rare earth element measurements and mapping of minerals in the Allende CAI, 7R19‐1, by NanoSIMS ion microprobe

We have established analytical procedures for quantitative rare earth element (REE) measurements by NanoSIMS 50L ion microprobe with 2–10 μm spatial resolution. Measurements are performed by multidetection using energy filtering under several static magnetic field settings. Relative sensitivity factors and REE oxide/REE element secondary ion ratios that we determined for the NanoSIMS match values previously determined for other ion microprobes. REE measurements of 100 ppm REE glass standards yielded reproducibility and accuracy of 0.5–2.5% and 5–15%, respectively. REE measurements of minerals of an Allende type‐A CAI, 7R19‐1, were performed using three different methods: spot analysis, line profile, and imaging. These data are in excellent agreement with previous REE measurements of this inclusion by IMS‐3f ion microprobe. The higher spatial resolution NanoSIMS measurements provide additional insight into the formation process of this CAI and offer a promising new tool for analysis of fine‐grained and complexly zoned materials.     

Comet 81P/Wild 2: The size distribution of finer (sub‐10 μm) dust collected by the Stardust spacecraft

Abstract– The fluence of dust particles <10 μm in diameter was recorded by impacts on aluminum foil of the NASA Stardust spacecraft during a close flyby of comet 81P/Wild 2 in 2004. Initial interpretation of craters for impactor particle dimensions and mass was based upon laboratory experimental simulations using projectiles less than >10 μm in diameter and the resulting linear relationship of projectile to crater diameter was extrapolated to smaller sizes. We now describe a new experimental calibration program firing very small monodisperse silica projectiles (470 nm–10 μm) at approximately 6 km s^?1. The results show an unexpected departure from linear relationship between 1 and 10 μm. We collated crater measurement data and, where applicable, impactor residue data for 596 craters gathered during the postmission preliminary examination phase. Using the new calibration, we recalculate the size of the particle responsible for each crater and hence reinterpret the cometary dust size distribution. We find a greater flux of small particles than previously reported. From crater morphology and residue composition of a subset of craters, the internal structure and dimensions of the fine dust particles are inferred and a “maximum‐size” distribution for the subgrains composing aggregate particles is obtained. The size distribution of the small particles derived directly from the measured craters peaks at approximately 175 nm, but if this is corrected to allow for aggregate grains, the peak in subgrain sizes is at <100 nm.     

The use of Auger spectroscopy for the in situ elemental characterization of sub‐micrometer presolar grains

Abstract— Presolar grains are small samples of stardust that can be found at low abundances in some of the most unaltered types of extraterrestrial materials. While earlier laboratory studies of stardust mainly focused on grain types that can be extracted from bulk meteorites by acid dissolution techniques, such as silicon carbide and graphite, recent analyses of presolar silicates rely on isotope imaging searches for locating these grains in situ. Since presolar silicates are generally less than a micrometer in diameter and represent at best only a few hundred ppm of their host materials (e.g., primitive meteorites or interplanetary dust particles), locating and studying these particles can be analytically challenging. Recently, we began using scanning Auger spectroscopy for the in situ elemental characterization of presolar silicate grains as a complement to NanoSIMS isotopic studies for obtaining spatially matched compositional data. Auger spectroscopy is a well‐established analytical technique for elemental characterizations in the material sciences, but has not been widely used in geological applications. We discuss the application of this technique to sub‐micrometer sized silicate grains and address practical issues such as sample preparation, measurement settings, spatial resolution, data processing, and elemental quantification.     

Origins of meteoritic nanodiamonds investigated by coordinated atom‐probe tomography and transmission electron microscopy studies

To constrain the origins of meteoritic nanodiamonds, the abundance ratios of stable C isotopes in acid residues from the carbonaceous chondritic meteorite Allende CV3 were measured using coordinated atom‐probe tomography (APT) and transmission electron microscopy (TEM). We combined our data with previously published APT data. A statistical analysis of this combined data set suggests an upper bound of 1 in 10² on the subpopulation that could have a large isotopic enrichment in ¹³C relative to ¹²C, consistent with the possible detection by secondary ion mass spectrometry of a similar enrichment in a 1 in 10⁵ fraction, abundant enough to account for the Xe‐HL anomalous isotopic component carried by the acid residues. Supernovae are believed to be the source of Xe‐HL, leading to the mystery of why all other supernova minerals do not carry Xe‐HL. The lack of Xe‐HL in low‐density disordered supernova graphite suggests that the isotopically anomalous component is the nanodiamonds, but the disordered C in the residue is not ruled out. We discuss possible origins of the disordered C and implications of our results for proposed formation scenarios for nanodiamonds. At least 99% of the meteoritic acid residue exhibits no unambiguous evidence of presolar formation, although production with solar isotope ratios in asymptotic giant branch stars is not ruled out. Comparison of TEM and APT results indicates that a minority of the APT reconstructions may preferentially sample disordered C rather than nanodiamonds. If this is the case, a presolar origin for a larger fraction of the nanodiamonds remains possible.     

A cornucopia of presolar and early solar system materials at the micrometer size range in primitive chondrite matrix

Abstract— We have used a variety of complementary microanalytical techniques to constrain the mineralogy, trace‐element distributions, and oxygen‐isotopic compositions in a 50 × 50 μm area of Acfer 094 matrix. The results reveal the exceptional mineralogical and compositional heterogeneity of this material at the sub‐μm level. We observe μm‐scale and sub‐μm grains with elemental associations suggesting feldspar, metal with widely varying Ni contents, and a Cr‐Fe alloy (in addition to forsterite, pyroxene, sulfide, ferrihydrite, and amorphous groundmass previously described). A new class of μm‐scale CAI (μCAI) is also observed, which show sub‐μm compositional zoning, and a range of oxygen isotopic compositions. Unlike the larger CAIs in Acfer 094, which are uniformly ¹⁶O‐enriched, two of the three μCAIs we analyzed are isotopically normal. We also observed a Li‐rich hotspot that detailed analysis by ToF‐SIMS suggests may be a LiCr‐oxide grain. Within the resolution of the NanoSIMS, this grain has isotopically normal Li. Finally, in our 50 × 50 μm area, we positively identified a presolar grain that is the most ¹⁸O‐rich silicate found so far in meteorites. The grain may originate from an asymptotic giant branch (AGB) star, or more likely, a supernova. In line with previous TEM studies (Greshake 1997), we find no evidence for clastic material (e.g., fragmental chondrules) in the matrix of Acfer 094: although the matrix is volatile‐depleted, this depletion does not appear to result from dilution of a primordial starting material with (depleted) chondrule fragments. Assuming that matrix experienced the depletion event, our data on the detailed mineralogy of Acfer 094 are currently equivocal in constraining the nature of that event. We observe carrier phases for several elements consistent with conditions approaching equilibrium condensation; however, the presence of an amorphous groundmass is suggestive of more rapid cooling.