Primitive Solar System Materials Research Articles

Nanoscale Spectroscopic Identification and Characterization of Minerals and Organic Matter in Ryugu Particles

AbstractCarbonaceous asteroids are leftover materials from the early solar system. They did not undergo planetary differentiation processes and thus still retained signatures and clues of the origin and evolution of the solar system. Thus, samples of carbonaceous asteroids provide significant potential for deciphering the origin of the solar system. The Hayabusa2 spacecraft returned the first samples of a carbonaceous asteroid (162173) Ryugu. In this work, we report the nanoscale mineralogy and organic matter content of two Ryugu particles, A0030 and C0034. These were analyzed by highly novel scattering‐type scanning near‐field optical microscopy (s‐SNOM)‐based nanoscale‐Fourier transform infrared (nano‐FTIR) spectroscopy with ∼20 nm spatial resolution. The nano‐FTIR measurements were supported by confocal micro‐Raman imaging and spectroscopy (∼500 nm/pixel spatial sampling). Our investigations show that the two Ryugu particles (a) contain different silicate mineralogies, indicating heterogeneous and incomplete aqueous alteration and (b) are highly rich in organic matter, consisting a variety of molecular functional groups. The spatial distributions of chemical functional groups and their associations based on pseudo‐heterodyne (PsHet) SNOM imaging show that the organic matter is distributed as either diffused or discrete grains. Micro‐Raman spectra show that the Ryugu particles experienced minimal thermal metamorphism, although A0030 was slightly more heated than C0034. The identification of abundant nanoscale organic molecules within the Ryugu grains that could not be identified via micrometer‐scale investigations emphasizes the importance of using nanoscale nondestructive methods for studying primitive solar system materials, such as Ryugu and OSIRIS‐Rex particles and those that will be returned in the future (such as MMX samples).

Chondrite diversity revealed by chromium, calcium and magnesium isotopes

Chondrites are undifferentiated meteorites that can provide information on the compositions of materials in the early solar System, including the building blocks of the terrestrial planets. While most chondrites belong to well-defined groups based on their mineralogy and chemical composition, a minor fraction have unusual characteristics and are classified as ungrouped chondrites. These ungrouped chondrites reflect the diversity of chondritic materials in the early solar system; however, they are not as well studied as grouped meteorites and their origins are poorly understood. In this study, we present high-precision mass-independent Cr, Ca and Mg isotope data for 17 ungrouped chondrites. The ε54Cr and ε48Ca (ε expresses parts per ten thousand mass-independent isotope deviation) data for ungrouped chondrites also provide important constraints for assessing their relationships to the known chondrite groups, and the radiogenic Mg isotope ratios (μ26Mg*) can be used to track the early solar system history. We also present the first high-precision data for a Kakangari (KC) chondrite, an enstatite chondrite, and for four enstatite-rich meteorites. The ε54Cr and ε48Ca values for the KC are −0.44 ± 0.04 and −1.30 ± 0.25, respectively, and ε48Ca value for SAH 97096 (EH3) is −0.19 ± 0.22 that overlaps with that of those of Earth-Moon system and ordinary chondrites. All the carbonaceous chondrite-like (CC) ungrouped chondrites show positive ε54Cr and ε48Ca values, and all the non-carbonaceous chondrite-like (NC) ungrouped chondrites and KCs (also belong to the NC trend) show zero or negative ε54Cr and ε48Ca values. This observation confirms the CC-NC dichotomy for primitive solar system materials. LEW 87232 (KC) also shows the highest 55Mn/52Cr ratio and ε53Cr value amongst all the chondrites. There is a positive trend between 55Mn/52Cr ratios and ε53Cr values among all the chondrites that mostly reflects a mixing between multiple chondritic components. Previously it has been reported that there is a bulk 26Al-26Mg correlation line amongst chondrites. This correlation has been interpreted as being due to mixing of CAIs (high 27Al/24Mg ratios and μ26Mg* values) and other silicate material (e.g., chondrules and matrix). By providing additional 26Al-26Mg chondrite data, we show that there is no 26Al-26Mg correlation line for the chondrites, ruling out the two-endmember (i.e., CAIs and other silicates) mixing model.

Elucidating evolution processes of Solar System bodies: Approaches by mineralogical study of various kinds of extraterrestrial materials

For the past 30 years, I have performed a mineralogical study of various kinds of extraterrestrial materials focusing on evolution processes of Solar System bodies. The samples analyzed range from primitive chondritic materials returned by spacecrafts to differentiated meteorites including Martian and lunar samples. The first of these analyzed meteorites was an angrite, one of the oldest known achondrites, in collaboration with researchers at NASA's Johnson Space Center, USA. I pointed out the importance of olivine xenocrysts by studying many quenched angrites and showed that bulk compositions of quenched angrites were controlled by resorption degrees of these xenocrysts. Soon I became involved in studying Martian meteorites. In my early works, I found that some shergottite Martian meteorites experienced undercooling of magma and represented parent magma compositions by combining crystallization experimental results. I also revealed that nakhlite Martian meteorites had correlated petrography and mineralogy that could be explained by crystallization at different locations (burial depths) in a common cooling cumulate pile. The presence of remarkable shock features (e.g., darkening of olivine) in Martian meteorites is striking, and it appears that prolonged post-shock heating history largely erased the high-pressure polymorphs in many cases. My research deals with primitive solar system materials including Wild 2 cometary particles and Itokawa asteroidal particles. Studying these samples using electron beam and synchrotron radiation analyses has strengthened the interpretations of their origins. Another important tool employed is electron backscatter diffraction (EBSD) analysis. I applied this technique to identify several new minerals such as dmitryivanovite, andreyivanovite, and kushiroite in early 2000s. EBSD was also used to analyze preferred crystallographic orientation of olivine in brachinites, revealing rigorous magmatic and/or rheological activities in the parent body. At present I am analyzing Ryugu samples returned by the Hayabusa2 spacecraft as a preliminary analysis team member, which broadens our understanding of the formation and evolution processes of solid materials in the early Solar System.

Trace Element Conundrum of Natural Quasicrystals

We report laser ablation inductively coupled plasma mass spectrometry measurements of the trace element contents of the two naturally occurring quasicrystalline minerals, Al63Cu24Fe13 icosahedrite and Al71Ni24Fe5 decagonite, from their type locality in the Khatyrka meteorite. The isolated quasicrystal fragments were mounted separately from any matrix and are larger than the laser beam diameter. When the elements are sorted in order of volatility, a systematic and unique pattern emerges in both bulk natural quasicrystal specimens. They are highly depleted compared to primitive solar system materials (chondritic meteorites) in moderately refractory elements (those with 50% condensation temperatures near 1350–1300 K; V, Co, Mg, Cr) and significantly enriched in moderately volatile elements (those with 50% condensation temperatures between 1250 and 500 K; Sb, B, Ag, Sn, Bi). We compare the chondrite-normalized trace element patterns and ratios of the quasicrystals to those of scoriaceous cosmic spherules and other meteoritic components. The nonmonotonic shapes of the chondrite-normalized trace element patterns in both icosahedrite and decagonite are incompatible with a single condensation process from the gas of the solar nebula. Previous transmission electron microscopy studies show that the natural quasicrystals contain 3–5 vol % of silicate and oxide nanoparticle inclusions, which we consider to be the main host of the measured trace elements. On this basis, we construct a three-stage model for the formation of the quasicrystals and their inclusions: a high-temperature condensation stage and a low-temperature vapor-fractionation stage to make nanoparticles, followed by a third stage that leads to the formation of quasicrystals incorporating the two different types of nanoparticles and their incorporation into the CV chondrite parent body of the Khatyrka meteorite.

Mineralogy and oxygen isotope systematics of magnetite grains and a magnetite‐dolomite assemblage in hydrated fine‐grained Antarctic micrometeorites

AbstractWe report the mineralogy and texture of magnetite grains, a magnetite‐dolomite assemblage, and the adjacent mineral phases in five hydrated fine‐grained Antarctic micrometeorites (H‐FgMMs). Additionally, we measured the oxygen isotopic composition of magnetite grains and a magnetite‐dolomite assemblage in these samples. Our mineralogical study shows that the secondary phases identified in H‐FgMMs have similar textures and chemical compositions to those described previously in other primitive solar system materials, such as carbonaceous chondrites. However, the oxygen isotopic compositions of magnetite in H‐FgMMs span a range of ∆17O values from +1.3‰ to +4.2‰, which is intermediate between magnetites measured in carbonaceous and ordinary chondrites (CCs and OCs). The δ18O values of magnetites in one H‐FgMM have a ~27‰ mass‐dependent spread in a single 100 × 200 μm particle, indicating that there was a localized control of the fluid composition, probably due to a low water‐to‐rock mass ratio. The ∆17O values of magnetite indicate that H‐FgMMs sampled a different aqueous fluid than ordinary and carbonaceous chondrites, implying that the source of H‐FgMMs is probably distinct from the asteroidal source of CCs and OCs. Additionally, we analyzed the oxygen isotopic composition of a magnetite‐dolomite assemblage in one of the H‐FgMMs (sample 03‐36‐46) to investigate the temperature at which these minerals coprecipitated. We have used the oxygen isotope fractionation between the coexisting magnetite and dolomite to infer a precipitation temperature between 160 and 280 °C for this sample. This alteration temperature is ~100–200 °C warmer than that determined from a calcite‐magnetite assemblage from the CR2 chondrite Al Rais, but similar to the estimated temperature of aqueous alteration for unequilibrated OCs, CIs, and CMs. This suggests that the sample 03‐36‐46 could come from a parent body that was large enough to attain temperatures as high as the OCs, CIs, and CMs, which implies an asteroidal origin for this particular H‐FgMM.

Sulfur isotope study of sulfides in CI, CM, C2ung chondrites and volatile-rich clasts – Evidence for different generations and reservoirs of sulfide formation

Abstract Deciphering aspects of the solar system’s formation process and the origin of planetary bodies can be achieved by examining primitive solar system materials, as these materials reflect the early solar system composition and may represent the building blocks of planetary bodies. Along these lines, knowing the original composition of carbonaceous chondrite meteorites is a valuable asset for determining the conditions in the parent bodies where they formed. Therefore, to determine the key characteristics of the parent bodies from which the carbonaceous chondrites and primitive materials are derived, we examined chemical and sulfur isotope compositions of sulfides in CM, CI and C2ung carbonaceous chondrites as well as from CM- and CI-like volatile-rich clasts; such an investigation allows us to explore the origin of these sulfides and to determine the primordial S composition of their parent body source region. In this study, sulfides from 7 CM, CI, and C2ung carbonaceous chondrites and 16 chondritic and achondritic breccias containing volatile-rich clasts were analyzed by electron microprobe and SIMS. Different sulfides were found, which shows evidence of different formation origins. Based on compositions and exsolution textures, we suggest that one fraction of the sulfides in both clasts and chondrites formed at high temperatures prior to incorporation into the parent body. The other sulfides most likely have a secondary origin and precipitated during fluid–rock interaction. Furthermore, differences in the S isotopic signature of the sulfides in chondrites correlate with the degree of aqueous alteration of the carbonaceous host rocks (CM or CI). Studying the sulfides of the volatile-rich clasts in brecciated chondrites and achondrites, a similar fractionation cannot be seen. Even though the mineralogy of CI chondrites and CI-like clasts is similar, the sulfides in CI chondrites appear to be enriched in heavy isotopes compared to those in the clasts (δ34S + 1‰ (CI) vs −2‰ (CI-like clast). This could have been caused by different alteration conditions, or it represents a different sampling reservoir. In this study a large S isotopic fractionation between pentlandite and pyrrhotite was found in large primarily formed sulfides showing exsolution textures, indicating that pentlandite prefers to incorporate light S isotopes. Considering the S isotope composition of the exsolved phase which can be found in CM- and CI-like clasts, the pristine δ34S value of the original monosulfide solid solution (mss) is estimated to be ∼−2‰. This value possibly resembles the sampling reservoir from which the sulfides formed, indicating that both CM- and CI-like clasts derived from a similar reservoir, and this reservoir is different from the formation reservoir of the CI chondrites.

A new population of dust from stellar explosions among meteoritic stardust

Primitive Solar System materials host small amounts of refractory dust grains predating the formation of the Sun and its planetary system. These ‘presolar’ grains condensed in the ejecta of evolved stars, novae and supernovae1. Their highly anomalous isotopic compositions cannot be explained by chemical or physical processes within the Solar System; instead, they represent the nucleosynthetic signatures of their stellar parents. Among this ‘true stardust’, silicates are the most abundant type of dust available for single-grain analyses2, with typical sizes of approximately 150 nm (ref. 3). Unlike presolar silicon carbides, aluminium oxides or graphites, which can be separated chemically from meteorites, presolar silicates have to be identified in situ, as they would be destroyed by extraction agents. Instrumental restrictions have constrained almost all previous magnesium isotopic measurements to presolar aluminium oxides, and the contribution of radiogenic 26Mg from 26Al decay has precluded unambiguous conclusions about their initial magnesium isotopes. Recent technical advances have enabled the undisturbed in situ investigation of magnesium isotopes in presolar silicates with unprecedented spatial resolution ( 200-nm-sized presolar silicates significantly higher than previously inferred1. A minor but important fraction of silicate stardust believed to come from red giant stars is shown to have a supernova origin instead, making the supernova dust fraction among >200-nm-sized presolar silicates significantly higher than previously inferred.

A Centimeter-wave Study of Methanol and Ammonia Isotopologues in Sgr B2(N): Physical and Chemical Differentiation between Two Hot Cores

Abstract We present new radio-frequency interferometric maps of emission from the 14NH3, 15NH3, and NH2D isotopologues of ammonia and the 12CH3OH and 13CH3OH isotopologues of methanol toward Sgr B2(N). With a resolution of ∼3″ (0.1 pc), we are able to spatially resolve emission from two hot cores in this source and separate it from absorption against the compact H ii regions in this area. The first (N1) is the well-known v = 64 km s−1 core, and the second (N2) is a core 6″ to the north at v = 73 km s−1. Using emission from 15NH3 and hyperfine satellites of 14NH3 metastable transitions, we estimate the 14NH3 column densities of these sources and compare them to those of NH2D. We find that the ammonia deuteration fraction of N2 is roughly 10–20 times higher than that in N1. We also measure an [15NH3/14NH3] abundance ratio that is apparently 2–3 times higher in N2 than in N1, which could indicate a correspondingly higher degree of nitrogen fractionation in N2. In addition, we find that N2 has a factor of 7 higher methanol abundance than N1. Together, these abundance signatures suggest that N2 is a younger source, for which species characteristic of grain chemistry at low temperatures are currently being actively liberated from ice mantles and have not yet reached chemical equilibrium in the warm gas phase. The high D abundance and possible high 15N abundance in NH3 found in N2 are interesting for studying the potential interstellar origin of abundances in primitive solar system material.

The stardust abundance in the local interstellar cloud at the birth of the Solar System

Primitive Solar System materials, such as certain types of meteorites, interplanetary dust particles and cometary matter, contain small quantities of refractory dust grains that are older than our Solar System. These ‘presolar grains’ condensed in the winds of evolved stars and in the ejecta of stellar explosions, and they were part of the interstellar gas and dust cloud from which our Solar System formed 4.57 billion years ago 1 . Interstellar dust is not only stardust but forms in the interstellar medium as well, predominantly as silicates, and, to a lesser extent, as carbonaceous dust and iron particles 2 . Presolar grains represent a sample of stardust, and their abundances in primitive Solar System materials can be used to constrain the fraction of stardust among interstellar dust. Here we show that the size distribution of presolar silicates follows that observationally derived for interstellar dust, at least in the diameter range 100–500 nm, that current estimates of presolar grain abundances (mass fractions) are at least a factor of 2 too low, and that several per cent of the interstellar dust in the interstellar cloud pre-dating our Solar System was stardust, making it a minor but still important ingredient of the starting material from which our Solar System formed.

Petrography and classification of NWA 7402: A new sulfide-rich unequilibrated ordinary chondrite

We classify a new chondritic find Northwest Africa (NWA) 7402. This meteorite is highly unequilibrated, and is therefore potentially significant for the study of primitive Solar System materials. Mineralogy, mineral chemistry, and modal abundances of minerals indicate that NWA 7402 is most likely an L chondrite. However, the specimen contains a higher abundance of sulfide than commonly seen in ordinary chondrites. The structural order of organic matter in the matrix and the chromium content of Fe-rich olivine grains indicate a petrologic type of 3.1. NWA 7402 largely escaped thermal metamorphism, and secondary phases formed by aqueous alteration are rare to absent. Minor planar fractures and undulatory extinction of olivine grains suggest that NWA 7402 experienced shock up to stage 2 or 3. Terrestrial weathering is heterogeneous in the specimen; much of the stone's exterior shows substantial Fe oxidation (weathering grade 2), while some parts of the interior remain relatively fresh (weathering grade 1). NWA 7402 has some unusual features that should be investigated further. The sulfide abundance is higher than reported sulfide contents for other L chondrites, and the chromium content of the olivines does not fall on the trend established for unequilibrated ordinary chondrites by Grossman and Brearley (2005).

NanoSIMS and more: New tools in nuclear astrophysics

Primitive Solar System materials contain nm- to μm-sized presolar grains that formed in the winds of evolved stars and in the ejecta of stellar explosions. These samples of stardust can be analysed in terrestrial laboratories with sophisticated analytical instrumentation in great detail. Of particular importance are coordinated studies of individual grains by Secondary Ion Mass Spectrometry (SIMS), Resonance Ionization Mass Spectrometry (RIMS) and Focused Ion Beam/Transmission Electron Microscopy (FIB/TEM) from which detailed information on isotopic compositions and mineralogies can be obtained. A key tool is the NanoSIMS 50 ion probe which permits to do isotope measurements of light and many intermediate-mass elements with spatial resolutions of <100 nm. A new type of RIMS instrument, “CHILI”, is currently under construction and is aimed to provide <100 nm resolution for isotope studies of intermediate-mass and heavy elements. Another promising analysis technique for future studies is Atom Probe Tomography (APT) which might be useful to create 3D-elemental and isotopic maps of presolar grains at the nanometer scale.

Interstellar and Solar System organic matter preserved in interplanetary dust

AbstractInterplanetary dust particles (IDPs) collected in the Earths stratosphere derive from collisions among asteroids and by the disruption and outgassing of short-period comets. Chondritic porous (CP) IDPs are among the most primitive Solar System materials. CP-IDPs have been linked to cometary parent bodies by their mineralogy, textures, C-content, and dynamical histories. CP-IDPs are fragile, fine-grained (&lt; um) assemblages of anhydrous amorphous and crystalline silicates, oxides and sulfides bound together by abundant carbonaceous material. Ancient silicate, oxide, and SiC stardust grains exhibiting highly anomalous isotopic compositions are abundant in CP-IDPs, constituting 0.01-1% of the mass of the particles. The organic matter in CP-IDPs is isotopically anomalous, with enrichments in D/H reaching 50x the terrestrial SMOW value and 15N/14N ratios up to 3x terrestrial standard compositions. These anomalies are indicative of low T (10-100 K) mass fractionation in cold molecular cloud or the outermost reaches of the protosolar disk. The organic matter shows distinct morphologies, including sub-um globules, bubbly textures, featureless, and with mineral inclusions. Infrared spectroscopy and mass spectrometry studies of organic matter in IDPs reveals diverse species including aliphatic and aromatic compounds. The organic matter with the highest isotopic anomalies appears to be richer in aliphatic compounds. These materials also bear similarities and differences with primitive, isotopically anomalous organic matter in carbonaceous chondrite meteorites. The diversity of the organic chemistry, morphology, and isotopic properties in IDPs and meteorites reflects variable preservation of interstellar/primordial components and Solar System processing. One unifying feature is the presence of sub-um isotopically anomalous organic globules among all primitive materials, including IDPs, meteorites, and comet Wild-2 samples returned by the Stardust mission. We will present an overview of the current state of understanding of the properties and origins of organic matter in primitive IDPs.

NEW CONSTRAINTS ON THE ABUNDANCES OF SILICATE AND OXIDE STARDUST FROM SUPERNOVAE IN THE ACFER 094 METEORITE

We studied about 5000 μm{sup 2} of fine-grained matrix material in the Acfer 094 meteorite by high-resolution (nominal 50 nm) NanoSIMS ion imaging for the presence of O-rich presolar (stardust) grains. This approach permits identifying presolar grains down to 150 nm in lower-resolution (nominal 100 nm) ion imaging surveys. The number density of identified presolar grains is a about a factor of two to three higher than what was found by lower-resolution ion imaging studies. The abundances of grains of O isotope Group 3 and 4 are higher than previously found. None of the presolar grains shows the strong enrichments in {sup 16}O expected from model predictions for the majority of supernova (SN) grains. Other potential O-rich SN grains, the Group 4 and some of the Group 3 grains, make up 33% by number and 19% by mass. This is clearly higher than the ∼10% (by number) inferred before and the 5% (by mass) estimated by a model for stellar dust in the interstellar medium. Our work shows that O-rich SN grains might be more abundant among the population of presolar grains in primitive solar system materials than currently thought, even without the {supmore » 16}O-rich grains as predominantly expected from SN models.« less

Deuterium/hydrogen microscopy in astrogeological material using an elastic recoil approach

A new experimental setup for quantitative hydrogen isotopic-ratio microscopy in thin samples (up to 12μm) is under development at the Lund Ion Beam Analysis Facility. This technique is derived from the proton–proton scattering technique and has been proven to have the same beneficial features, namely a detection limit below 1wt-ppm and a depth resolution better than 1μm. The method gives absolute quantitative information about H or D content in atoms per cm2 and does not depend on the structure, chemical environment or other so called “matrix effects”. This work presents an evaluation of the developed technique through measurements on unique material from samples from the Tagish Lake meteorite, which has been suggested to be one of the most primitive solar system materials yet studied. We discuss the capabilities of the technique through the results from measurements on a geological standard.

High-Resolution and Analytical TEM Investigation of Space Radiation Processing Effects in Primitive Solar System Materials and Airless Planetary Surface Environments

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Carbonates in CM chondrites: Complex formational histories and comparison to carbonates in CI chondrites

CM chondrites are primitive solar-system materials that have undergone high degrees of aqueous alteration, resulting in the formation of secondary minerals including carbonates. Two different carbonate minerals (calcite ⁄ aragonite and dolomite) together constitute 1.4-2.8 vol% of CM chondrites. In contrast, CI chondrites contain four different carbonate minerals: calcite ⁄ aragonite, dolomite, breunnerite, and siderite. CI chondrites have abundant dolomite, a mineral that seems to be absent in the most aqueously altered CM chondrites. In this study, carbonates in seven CM chondrites (Y-791198, LaPaz Icefield 04796, Cold Bokkeveld, Nogoya, Queen Alexandra Range 93005, Allan Hills 83100, and Meteorite Hills 01070) were studied petrographically and by electron microprobe. The results indicate that carbonate formation in CM chondrites differs from that in CI chondrites and is more complex than previously recognized. Our studies of CM chondrites indicate that (1) carbonates formed on the parent asteroid in an aqueous environment that gradually changed in composition, (2) at some stage, Ca and Mg activities in the environment were high enough to form metastable dolomite, and (3) dolomites disappeared in the most aqueously altered CM chondrites.

Detection of N$^\mathsf{{15}}$NH+in L1544

Excess levels of 15N isotopes which have been detected in primitive solar system materials are explained as a remnant of interstellar chemistry which took place in regions of the protosolar nebula. Chemical models of nitrogen fractionation in cold clouds predict an enhancement in the gas-phase abundance of 15N-bearing molecules, thus we have searched for 15N variants of the N2H+ ion in L1544, which is one of the best candidate sources for detection owing to its low central core temperature and high CO depletion. With the IRAM 30m telescope we have obtained deep integrations of the N2H+(1-0) line at 91.2 GHz. The N2H+(1-0) line has been detected toward the dust emission peak of L1544. The 14N/15N abundance ratio in N2H+ resulted 446+/-71, very close to the protosolar value of ~450, higher than the terrestrial ratio of ~270, and significantly lower than the lower limit in L1544 found by Gerin et al. (2009, ApJ, 570, L101) in the same object using ammonia isotopologues.

Characterization of Antarctic micrometeorites by thermoluminescence

Abstract— In order to explore the nature and history of micrometeorites, we have measured the thermoluminescence (TL) properties of four micrometeorites, three cosmic spherules, and one irregular scoriaceous particle, that we found in a survey of 17 micrometeorites. These micrometeorites have TL sensitivities ranging from 0.017 ± 0.002 to 0.087 ± 0.009 (on a scale normalized to 4 mg of the H3.9 chondrite Dhajala). The four micrometeorites have very similar TL peak temperatures and TL peak widths, and these distinguish them from CI, most CM, CV, CO, and ordinary chondrites. However, the TL properties of these micrometeorites closely resemble those of the unusual CM chondrite MacAlpine Hills (MAC) 87300 and terrestrial forsterites. Heating experiments on submillimeter chips of a CM chondrite and a H5 chondrite suggest that these TL properties are have not been significantly affected by atmospheric passage. Thus we suggest that there is no simple linkage between these micrometeorites and the established meteorite classes, and that forsterite is an important component of these micrometeorites, as it is in many primitive solar system materials.

Mass-independent fractionation of oxygen isotopes in the mesostasis of a chondrule from the Semarkona LL3.0 ordinary chondrite

A large chondrule from Semarkona, the most primitive ordinary chondrite known, has been discovered to contain a record of mass transport during its formation. In most respects, it is a normal Type I, group A1, low-FeO chondrule that was produced by reduction and mass-loss during the unidentified flash-heating event that produced the chondrules, the most abundant structural component in primitive meteorites. We have previously measured elemental abundances and abundance profiles in this chondrule. We here report oxygen isotope ratio abundances and ratio abundance profiles. We have found that the mesostasis is zoned in oxygen isotope ratio, with the center of the chondrule containing isotopically heavier oxygen than the outer regions, the outer regions being volatile rich from the diffusion of volatiles into the chondrule during cooling. The δ 17O values range from −2.0‰ to 9.9‰, while δ 18O range from −1.9‰ to 9.6‰. More importantly, a plot of δ 17O against δ 18O has a slope of 1.1 ± 0.2 (1 σ) and 0.88 ± 0.10 (1 σ) when measured by two independent methods. Co-variation of δ 17O with δ 18O that does not follow mass-dependent fractionation has often been seen in primitive solar system materials and is usually ascribed to the mixing of different oxygen reservoirs. We argue that petrographic and compositional data indicate that this chondrule was completely melted at the time of its formation so that relic grains could not have survived. Furthermore, there is petrographic and compositional evidence that there was no aqueous alteration of this chondrule subsequent to its formation. Although it is possible to formulate a series of exchanges between the chondrule and external 16O-rich and 16O-poor reservoirs that may explain the detailed oxygen isotope systematics of this chondrule, such a sequence of events looks very contrived. We therefore hypothesize that reduction, devolatilization, and crystallization of the chondrule melt may have produced 16O-rich olivines and 16O-poor mesostasis plotting on a slope-one line as part of the chondrule-forming process in an analogous fashion to known chemical mass-independent isotopic fractionation mechanisms. During cooling, volatiles and oxygen near the terrestrial line in oxygen isotope composition produced the outer zone of volatile rich and 16O-rich mesostasis. The chondrule therefore not only retains a record of considerable mass transport accompanying formation, but also may indicate that the isotopes of oxygen underwent mass-independent fractionation during the process.

NanoSIMS: A new tool in cosmochemistry

The Cameca NanoSIMS 50 is a new generation ion microprobe, characterized by high spatial resolution (down to 50 nm), high transmission, and simultaneous detection of up to six isotopes. After 4 years of operation the NanoSIMS has now become a well established tool in cosmochemistry which has significantly contributed to a better understanding of the presence of presolar matter in primitive solar system materials and on the early solar system history. Examples of important breakthroughs are: (i) discovery of silicate stardust in interplanetary dust particles and primitive meteorites; (ii) extension of isotopic studies to submicrometer-sized presolar SiC and spinel grains separated from primitive meteorites by chemical and physical treatments; (iii) identification of isotopic heteorogeneities within micrometer-sized presolar grains; (iv) coordinated NanoSIMS-TEM studies made it possible to obtain simultaneous information on the mineralogy, structure, and isotopic composition of presolar materials; (v) detection of radiogenic 60Ni from the decay of radioactive 60Fe (half life 1.5 Ma) and of radiogenic 53Cr from the decay of radioactive 53Mn (half life 3.7 Ma) in meteoritic minerals.