Christian Vollmer

Si Isotopic Compositions of Presolar Silicate Grains from Red Giant Stars and Supernovae

We have measured the silicon isotopic compositions of 38 presolar silicate grains from the carbonaceous chondrite Acfer 094, which have been previously studied for oxygen isotopes. The goals of this study are to further disentangle stellar sources of the 18O-enriched (Group IV) and the most 17O-enriched (Group I) silicate grains and to put further constraints on the Galactic chemical evolution (GCE) of the Si isotopes. Our results show that the majority (6 out of 8) of the 18O enriched silicates have enhanced 28Si, in qualitative agreement with the signature of presolar silicon carbide (SiC) X grains from supernovae. Three highly 17O-enriched grains ( -->17O/16O > 3 × 10−3) have close to solar 29Si/28Si but enhanced 30Si/28Si, possibly indicating an origin in binary systems. Alternative stellar sources are 3.5-4 M☉ asymptotic giant branch (AGB) stars. Si isotopic compositions of the majority of presolar silicates fall along the SiC mainstream line, although most grains plot to the 30Si-poor side of this line. Most presolar silicate grains therefore formed in red giant branch (RGB)/AGB stars of roughly solar metallicity, and incorporated less (or no) Si processed by slow neutron capture reactions (s-process) than presolar SiC grains, because silicates form before large amounts of 12C and s-processed material has been dredged up to the surface. The inferred shift of the Si isotopes due to the different dredge-up of matter from the He intershell in C-rich AGB stars is Δ29Si = 3‰-21‰ and Δ30Si = 17‰-30‰, which is compatible with predictions for 1.5-3 M☉ AGB stars of solar metallicity when C/O > 1.

Stellar MGSIO3 perovskite : A shock-transformed stardust silicate found in a meteorite

We have discovered an isotopically highly anomalous MgSiO3 grain in the ungrouped carbonaceous chondrite Acfer 094 with a perovskite-like crystal structure resembling the dominant high-pressure mineral of the Earths lower mantle. Oxygen isotopic ratios of the silicate grain are 17O/ 16O = (4.91 ± 0.36) × 10-3 (12 times the solar value) and 18O/16O = (1.36 ± 0.19) × 10-3 (0.4 times the solar value). This signature points to condensation in the ejecta of a ~2 M☉, close-to-solar metallicity red giant branch (RGB) or asymptotic giant branch (AGB) star. Alternatively, the grain could have formed in the ejecta of a nova, in which 17O is highly overabundant. TEM analysis of the grain revealed a high pressure perovskite-like crystal structure not predicted by equilibrium condensation in low-pressure stellar environments. A possible formation scenario is transformation of a silicate precursor triggered by a shock wave, either in the interstellar medium (ISM) or originating from the grains parent star. Shock waves must thus be considered as a potential mechanism to recrystallize silicates, or even convert them into high-pressure structures. Alternatively, nonequilibrium condensation or crystallization by a chemical vapor deposition (CVD)-like process, also invoked for the formation of nanodiamonds, is a distinct possibility, although more speculative.

Trachyandesitic volcanism in the early Solar System

Significance Volcanism is a fundamental geological process on planets and was substantial during crustal growth on planetary bodies in the early Solar System, as witnessed by ubiquitous rocks of basaltic composition, e.g., on Earth, Moon, Mars, and asteroids. Besides basaltic volcanism, trachyandesite lavas are generated on Earth. The first occurrence of a trachyandesite lava in the meteorite collections demonstrates that trachyandesitic, alkali-, and silica-rich volcanism takes place not only on Earth today but already occurred on a small planetesimal ∼4.56 billion years ago. It sets new constraints on mechanisms and styles of early Solar System volcanism. Volcanism is a substantial process during crustal growth on planetary bodies and well documented to have occurred in the early Solar System from the recognition of numerous basaltic meteorites. Considering the ureilite parent body (UPB), the compositions of magmas that formed a potential UPB crust and were complementary to the ultramafic ureilite mantle rocks are poorly constrained. Among the Almahata Sitta meteorites, a unique trachyandesite lava (with an oxygen isotope composition identical to that of common ureilites) documents the presence of volatile- and SiO2-rich magmas on the UPB. The magma was extracted at low degrees of disequilibrium partial melting of the UPB mantle. This trachyandesite extends the range of known ancient volcanic, crust-forming rocks and documents that volcanic rocks, similar in composition to trachyandesites on Earth, also formed on small planetary bodies ∼4.56 billion years ago. It also extends the volcanic activity on the UPB by ∼1 million years (Ma) and thus constrains the time of disruption of the body to later than 6.5 Ma after the formation of Ca–Al-rich inclusions.

TEM foil preparation of sub‐micrometre sized individual grains by focused ion beam technique

Analysis of presolar silicate grains provides new knowledge on interstellar and circumstellar environments and can be used to test models of the Galactic chemical evolution. However, structural information of these grains is rare because sample preparation for transmission electron microscopy is very difficult due to the small dimensions of these grains (<0.5 μm). With the use of the focused ion beam technique thin foils from these grains for transmission electron microscopy analysis can be prepared. Nevertheless, reaching the required precision of some tens of nanometres for the preparation of the transmission electron microscopy foil in the place of interest is not trivial. Furthermore, in the current samples, the grain of interest can only be identified by its different isotopic composition; i.e. there is no contrast difference in scanning electron microscopy or transmission electron microscopy images which allow the identification of the grain. Therefore, the grain has to be marked in some way before preparing the transmission electron microscopy foil. In the present paper, a method for transmission electron microscopy foil preparation of grains about 200 to 400 nm in diameter is presented. The method utilizes marking of the grain by Pt deposition and milling of holes to aid in the exact orientation of the transmission electron microscopy foil with respect to the grain. The proposed method will be explained in detail by using an example grain.

Fluid-induced organic synthesis in the solar nebula recorded in extraterrestrial dust from meteorites

Significance Organic matter from the parent molecular cloud of our solar system can be located in primitive extraterrestrial samples like meteorites and cometary grains. This pristine matter contains among the most primitive organic molecules that were delivered to the early Earth 4.5 billion years ago. We have analyzed these organics by a high-resolution electron microscope that is exceptionally suited to study these beam-sensitive materials. Different carbon and nitrogen functional groups were identified on a submicron scale and can be attributed to early cometary and meteoritic organic reservoirs. Our results demonstrate for the first time to our knowledge that certain highly aromatic and nitrogen-containing ubiquitous organics were transformed from an oxygen-rich organic reservoir by parent body fluid synthesis in the early solar system. Isotopically anomalous carbonaceous grains in extraterrestrial samples represent the most pristine organics that were delivered to the early Earth. Here we report on gentle aberration-corrected scanning transmission electron microscopy investigations of eight 15N-rich or D-rich organic grains within two carbonaceous Renazzo-type (CR) chondrites and two interplanetary dust particles (IDPs) originating from comets. Organic matter in the IDP samples is less aromatic than that in the CR chondrites, and its functional group chemistry is mainly characterized by C–O bonding and aliphatic C. Organic grains in CR chondrites are associated with carbonates and elemental Ca, which originate either from aqueous fluids or possibly an indigenous organic source. One distinct grain from the CR chondrite NWA 852 exhibits a rim structure only visible in chemical maps. The outer part is nanoglobular in shape, highly aromatic, and enriched in anomalous nitrogen. Functional group chemistry of the inner part is similar to spectra from IDP organic grains and less aromatic with nitrogen below the detection limit. The boundary between these two areas is very sharp. The direct association of both IDP-like organic matter with dominant C–O bonding environments and nanoglobular organics with dominant aromatic and C–N functionality within one unique grain provides for the first time to our knowledge strong evidence for organic synthesis in the early solar system activated by an anomalous nitrogen-containing parent body fluid.

Transmission Electron Microscopy of Al-rich Silicate Stardust from Asymptotic Giant Branch Stars

We report on transmission electron microscopy (TEM) investigations of two mineralogically unusual stardust silicates to constrain their circumstellar condensation conditions. Both grains were identified by high spatial resolution nano secondary ion mass spectrometry (NanoSIMS) in the Acfer 094 meteorite, one of the most pristine carbonaceous chondrites available for study. One grain is a highly crystalline, highly refractory (Fe content < 0.5 at%), structurally undisturbed orthopyroxene (MgSiO3) with an unusually high Al content (1.8 ± 0.5 at%). This is the first TEM documentation of a single crystal pyroxene within the complete stardust silicate data set. We interpret the microstructure and chemistry of this grain as being a direct condensate from a gas of locally non-solar composition (i.e., with a higher-than-solar Al content and most likely also a lower-than-solar Mg/Si ratio) at (near)-equilibrium conditions. From the overabundance of crystalline olivine (six reported grains to date) compared to crystalline pyroxene (only documented as a single crystal in this work) we infer that formation of olivine over pyroxene is favored in circumstellar environments, in agreement with expectations from condensation theory and experiments. The second stardust silicate consists of an amorphous Ca-Si rich material which lacks any crystallinity based on TEM observations in which tiny (<20 nm) hibonite nanocrystallites are embedded. This complex assemblage therefore attests to the fast cooling and rapidly changing chemical environments under which dust grains in circumstellar shells form.

The Inventory of Presolar Grains in Primitive Meteorites: A NanoSIMS Study of C-, N-, and O-isotopes in NWA 852

Meteorites as well as interplanetary and cometary dust contain small amounts of mineral grains that formed in the winds of evolved stars or in the ejecta of stellar explosions and survived incorporation into solid bodies of our own solar system. Investigation of the abundance and distribution of these “presolar grains” in primitive solar system matter can shed light on parent body processes as well as possible heterogeneities in the early solar nebula. We investigated presolar silicate and oxide grains in the CR chondrite NWA 852 with a NanoSIMS 50. Abundances of 77 ppm for silicates, 39 ppm for oxides, and 160 ppm for silicon carbide, as well as evidence for N-enriched organic molecular cloud material were observed. Although NWA 852 has the lowest presolar silicate/oxide-ratio observed so far for presolar-grain-rich material, indicating extensive aqueous alteration, a significant fraction of O-anomalous grains (silicates and oxides) remained intact. Thus, this meteorite may be a link between presolargrain-rich, pristine CR chondrites and CRs with lower presolar grain abundances.

Heavy Element Abundances in Presolar Silicon Carbide Grains from Low-Metallicity AGB Stars

Primitive meteorites contain small amounts of presolar minerals that formed in the winds of evolved stars or in the ejecta of stellar explosions. Silicon carbide is the best studied presolar mineral. Based on its isotopic compositions it was divided into distinct populations that have different origins: Most abundant are the mainstream grains which are believed to come from 1.5–3 M⊙ AGB stars of roughly solar metallicity. The rare Y and Z grains are likely to come from 1.5–3 M⊙ AGB stars as well, but with subsolar metallicities (0.3–0.5 times solar). Here we report on C and Si isotope and trace element (Zr, Ba) studies of individual, submicrometer-sized SiC grains. The most striking results are: (1) Zr and Ba concentrations are higher in Y and Z grains than in mainstream grains, with enrichments relative to Si and solar of up to 70 times (Zr) and 170 times (Ba), respectively; (2) For the Y and Z grains there is a positive correlation between Ba concentrations and amount of s-process Si. This correlation is well explained by predictions for 2–3 M⊙ AGB stars with metallicities of 0.3–0.5 times solar. This confirms low-metallicity stars as most likely stellar sources for the Y and Z grains.

Presolar Silicates In Meteorites And Interplanetary Dust Particles

Presolar silicates remained unrecognized in primitive solar system materials for a long time and only the recent development of advanced ion probe imaging techniques led to their discovery. Silicates turned out to be the most abundant presolar mineral. To date more than 200 presolar silicates have been found. Most grains (>70%) exhibit enrichments in 17O with 17O/16O ratios of up to 25x the solar ratio, close‐to‐solar or slightly lower than solar 18O/16O ratios, and Si‐isotopic ratios close to the SiC mainstream line, although slightly shifted to the 30Si‐poor side. These grains most likely formed in the winds of 1–2.5 M⊙ RGB/AGB stars. 17O/16O ratios of >3×l0−3 may be the result of mass transfer in binary systems as was recently proposed for presolar oxide grains. This scenario is also supported by the Si‐isotopic ratios. Some (a few %) silicate grains have enrichments in 17O and strong depletions in l8O, probably the result of cool bottom processing during the RGB or AGB phase. Presolar silicate grains with moderate to strong excesses in l8O (∼10% of all silicates) most likely have a supernova origin.Presolar silicates remained unrecognized in primitive solar system materials for a long time and only the recent development of advanced ion probe imaging techniques led to their discovery. Silicates turned out to be the most abundant presolar mineral. To date more than 200 presolar silicates have been found. Most grains (>70%) exhibit enrichments in 17O with 17O/16O ratios of up to 25x the solar ratio, close‐to‐solar or slightly lower than solar 18O/16O ratios, and Si‐isotopic ratios close to the SiC mainstream line, although slightly shifted to the 30Si‐poor side. These grains most likely formed in the winds of 1–2.5 M⊙ RGB/AGB stars. 17O/16O ratios of >3×l0−3 may be the result of mass transfer in binary systems as was recently proposed for presolar oxide grains. This scenario is also supported by the Si‐isotopic ratios. Some (a few %) silicate grains have enrichments in 17O and strong depletions in l8O, probably the result of cool bottom processing during the RGB or AGB phase. Presolar silicate grains ...

Lessons Learned from Electron Microscopy of Deformed Opalinus Clay

Using a combined approach of ion-beam milling and electron microscopy, we observe, describe and quantify the microstructure of naturally and synthetically deformed Opalinus Clay (OPA) and deduce its microstructural evolution and underlying deformation mechanisms. The investigated samples derive from the so-called Main Fault, a 10 m offset fold-bend thrust fault crossing the Mont Terri Rock Laboratory in the Swiss Jura Mountains. The samples are slightly overconsolidated, experienced a burial depth of 1350 m and a maximum temperature of 55 °C. Most impact on strain is attributed to frictional sliding and rigid body rotation. However, trans-granular fracturing, dissolution-precipitation of calcite, clay particle neoformation and grain deformation by intracrystalline plasticity have a significant contribution to the fabric evolution. The long-term in-situ deformation behavior of OPA is inferred to be more viscous than measured at laboratory conditions.