Transmission electron microscopy of a refractory inclusion from the Allende meteorite: Anatomy of a pyroxene

Abstract— A crystal of clinopyroxene from the coarse-grained refractory inclusion Egg 6 of the Allende meteorite has been studied in detail by transmission electron microscopy. The pyroxene crystal contains euhedral, dislocation-free inclusions of pure spinel MgAl₂O₄, without any topotactic relation to the host. Extensive dislocation walls at equilibrium, characteristic of high-temperature anneal, are present in the crystal. Alteration products are occasionally observed at the spinel-pyroxene interface close to regions where dislocation walls decorated with bubbles (or voids) are present. The bubbles, often in the shape of tubes along the dislocation lines, are thought to be due to the precipitation of a fluid migrating along the dislocations. The observations are compatible with crystallization of the refractory inclusions from the melt and with the existence of a later stage of metasomatism.     

Mineralogy of a refractory inclusion in the Allende (C3V) meteorite

Abstract— A spherical, 220-μm diameter, spinel-hibonite-perovskite inclusion from the Allende C3V meteorite contains a central hibonite cluster with an angular boundary. This central hibonite is enclosed within spinel that is zoned from Mg-rich at the hibonite boundary to more Fe-rich at the inclusion boundary. This spinel zone includes lath-shaped hibonites usually oriented subradial to the central hibonites. Two textural types of perovskites are present as exsolution from the central hibonite and as equidimensional grains within both the central hibonite and spinel. These second perovskites have exsolution lamellae of Al₂O₃. Within the central hibonite and adjacent to some equidimensional perovskites, a fine porous phase interpreted as alteration has a composition of nearly pure Al₂O₃ with minor amounts of Na and Si. This is possibly either an intergrowth of corundum and nepheline or a modified Al₂O₃, β-alumina. The central hibonites and equidimensional perovskites are considered relict grains on which the spinel-hibonite layer crystallized. The relict material had undergone slow cooling in a previous event to produce exsolution of original high-temperature compositions. Later alteration caused breakdown of hibonite to give an Al₂O₃-rich phase. This inclusion represents a composite body which formed in a Ca-Al-rich environment.     

A unique type B inclusion from Allende with evidence for multiple stages of melting

Abstract— A large (7 mm in diameter) Allende type B inclusion has a typical bulk composition and a unique structure: a fassaite‐rich mantle enclosing a melilite‐rich core. The core and mantle have sharply contrasting textures. In the mantle, coarse (?1 mm across), subhedral fassaite crystals enclose radially oriented melilite laths about 500 μm long that occur at the inclusion rim. The core consists of blocky melilite grains 20–50 μm across and poikilitically enclosed in anhedral fassaite grains that are optically continuous over ?1 mm. Another unique feature of this inclusion is that melilite laths also extend from the core into the mantle. Fassaite in both the core and mantle is very rich in fine‐grained (1–10 μm) spinel. The rim laths are normally zoned (Åk_30–70) inward from the rim of the inclusion with reverse zoning over the last ?200 μm to crystallize. A very wide range of melilite compositions is found in the core of the inclusion, where gehlenitic grains (Åk_5–12) occur. These grains are enclosed in strongly zoned (Åk_15–70) overgrowths. The gehlenitic cores and innermost parts of the overgrowths are Na₂O‐free, but the outer parts of the overgrowths are not. In the laths at the rim, Na₂O decreases inward from the rim, then increases. Fassaite in the core has the same range of Ti contents as that in the mantle: 2–9 wt% TiO₂ + Ti₂O₃. Two melting events are required to account for the features of this inclusion. In the first event, the precursor assemblage is heated to ?1400 °C and melts except for gehlenitic (Åk_5–12) melilite and some spinel. These grains become concentrated in the core. During cooling, Na₂O‐free melilite nucleates at the rim of the inclusion and on the relict grains in the core. After open system secondary alteration, the inclusion is heated again, but only to ?1260 °C. Melilite more gehlenitic than Åk₄₀ does not melt. During cooling, Na₂O‐bearing melilite crystallizes as small, blocky grains and laths in the core and as overgrowths on relict grains in the core and at the rim. Eventually melilite co‐crystallizes with fassaite, leading to the reverse zoning observed in the laths. The coexistence in this inclusion of Na‐free and Na‐bearing melilite, plus a positive correlation between Na₂O and åkermanite contents in melilite in an inclusion with a bulk Mg isotopic composition that is mass‐fractionated in favor of the heavy isotopes, are both consistent with at least two melting events. Several other recently described coarse‐grained inclusions also have features consistent with a sequence of early, high‐temperature melting, secondary alteration, and remelting at a lower temperature, suggesting that remelting of refractory inclusions was a common occurrence in the solar nebula.     

Formation of orange hibonite,as inferred from some Allende inclusions

Abstract— We studied three fluffy Type A refractory inclusions from Allende that contain orange hibonite. The melilite in the present samples is very Al‐rich, averaging Åk₆, Åk₁₄, and Åk₁₂ in the three samples studied. Hibonite in two inclusions, unlike that in Murchison, has low rare earth element abundances of <10 × CI; in the other inclusion, the hibonite, melilite and perovskite have Group II‐like patterns. The hibonite and melilite in all three inclusions studied have excess ²⁶Mg consistent with (²⁶Al/²⁷Al)_I = 5 × 10^?5. Much of the hibonite and some of the spinel in these inclusions is corroded. These phases are found enclosed in melilite, but based on bulk compositions and phase equilibria, hibonite should not be an early‐crystallizing phase in these inclusions. We conclude that the hibonite and probably some of the spinel is relic. Reversely zoned melilite, rounded spinel and isotopically heavy Mg in the inclusions probably reflect reheating events that involved melting and evaporation. Alteration of the gehlenitic melilite gave rise to some rare phases, including corundum and nearly pure CaTs pyroxene. Studies have shown that blue hibonite contains Ti³⁺ while orange hibonite does not (Ihinger and Stolper, 1986; Beckett et al., 1988). Orange hibonite formed either under oxidizing conditions (such as at oxygen fugacities at least seven orders of magnitude greater than that of a solar gas at 1700 K), or under conditions reducing enough (e.g., solar) that it contained Ti³⁺, which was later oxidized in situ. Although V and Ce oxides are volatile at the temperature and range of oxygen fugacities at which orange hibonite is known to be stable, we find that (a) the hibonite is V‐rich (～1 wt% V₂O₃) and (b) there are no negative Ce anomalies in Allende hibonite. This indicates that the hibonite did not form by condensation under oxidizing conditions. In addition, there are slight excesses of Ti + Si cations relative to Mg + Fe cations (up to 0.1 of 0.8 cations per 19 oxygen anions), probably reflecting the original presence of Ti³⁺. The results of this study strongly support the suggestion (Ihinger and Stolper, 1986) that Allende hibonite originally formed under reducing conditions and was later oxidized. Oxygen fugacities within ～2–3 orders of magnitude of that of a solar gas are implied; otherwise, strong Ce and V depletions would be observed.     

A refractory inclusion returned by Stardust from comet 81P/Wild 2

Abstract— Among the samples returned from comet 81P/Wild 2 by the Stardust spacecraft is a suite of particles from one impact track (Track 25) that are Ca‐, Al‐rich and FeO‐free. We studied three particles from this track that range in size from 5.3 × 3.2 μ to 15 × 10 μ. Scanning and transmission electron microscopy show that they consist of very fine‐grained (typically from ?0.5 to ?2 μ) Al‐rich, Ti‐bearing and Ti‐free clinopyroxene, Mg‐Al spinel and anorthite, with trace amounts of fine perovskite, FeNi metal and osbornite (TiN) grains. In addition to these phases, the terminal particle, named “Inti”, also contains melilite. All of these phases, with the exception of osbornite, are common in refractory inclusions and are predicted to condense at high temperature from a gas of solar composition. Osbornite, though very rare, has also been found in meteoritic refractory inclusions, and could have formed in a region of the nebula where carbon became enriched relative to oxygen compared to solar composition. Compositions of Ti‐pyroxene in Inti are similar, but not identical, to those of fassaite from Allende inclusions. Electron energy loss spectroscopy shows that Ti‐rich pyroxene in Inti has Ti³⁺/Ti⁴⁺within the range of typical meteoritic fassaite, consistent with formation under reducing conditions comparable to those of a system of solar composition. Inti is ¹⁶O‐rich, with δ₁₈O?δ¹⁷O?‐40%0, like unaltered phases in refractory inclusions and refractory IDPs. With grain sizes, mineralogy, mineral chemistry, and an oxygen isotopic composition like those of refractory inclusions, we conclude that Inti is a refractory inclusion that formed in the inner solar nebula. Identification of a particle that formed in the inner solar system among the comet samples demonstrates that there was transport of materials from the inner to the outer nebula, probably either in a bipolar outflow or by turbulence.     

Mineralogical alteration of a type A CAI from Allende CV3 chondrite: Formation of secondary dmisteinbergite and its phase transition to anorthite

Dmisteinbergite, a hexagonal form of CaAl₂Si₂O₈, was found in a compact type A Ca-Al-rich inclusion (CAI) in the Allende CV3 chondrite. Scanning and transmission electron microscopic observations show that dmisteinbergite was always in contact with grossular and grossular was in contact with melilite. In addition, there is a crystallographic relationship between dmisteinbergite and anorthite. Based on the textural and crystallographic evidence, the following mineralogical alteration processes are proposed to have occurred in the CAI. (1) Melilite was replaced by grossular. High densities of vesicles in the grossular indicate that hydrogrossular might have been the primary alteration phase and dehydrated by later metamorphism. (2) Dmisteinbergite formed from (hydro)grossular through a reaction with Si-rich fluid. (3) Nano-sized minerals are formed within dmisteinbergite. (4) Dmisteinbergite was transformed to anorthite. (5) Both anorthite and dmisteinbergite were altered to nepheline. (6) Hydrogrossular was dehydrated to grossular. (Hydro)grossular, dmisteinbergite, anorthite, and nepheline in the CAI seem to have formed in the course of metasomatism that occurred in the Allende parent body. Except for the hydrogrossular dehydration, these reactions could have occurred at moderate temperature (200–250°C) in high pH fluids (pH 13–14) according to past experimental studies. Episodic changes in fluid composition seem to have occurred before reactions (2), (4), and (5), because these reactions were not completed before the next reaction started. Higher temperature is required for reactions (5) and (6) to occur. Our observation of the CAI suggests that it experienced multiple episodes of metasomatism as temperatures were rising in the Allende parent asteroid.     

Atom‐probe analyses of nanodiamonds from Allende

Atom‐probe tomography (APT) is currently the only analytical technique that, due to its spatial resolution and detection efficiency, has the potential to measure the carbon isotope ratios of individual nanodiamonds. We describe three different sample preparation protocols that we developed for the APT analysis of meteoritic nanodiamonds at sub‐nm resolution and present carbon isotope peak ratios of meteoritic and synthetic nanodiamonds. The results demonstrate an instrumental bias associated with APT that needs to be quantified and corrected to obtain accurate isotope ratios. After this correction is applied, this technique should allow determination of the distribution of ¹²C/¹³C ratios in individual diamond grains, solving the decades‐old question of the origin of meteoritic nanodiamonds: what fraction, if any, formed in the solar system and in presolar environments? Furthermore, APT could help us identify the stellar sources of any presolar nanodiamonds that are detected.     

Chemical and isotopic constraints on the formation and crystallization of SA-1, a basaltic Allende plagioclase-olivine inclusion

Abstract— SA-1, an unusual basaltic plagioclase-olivine inclusion (POI) in Allende, has concentric textural and mineralogic zones, a fine-grained, 100μm outer border and a coarse-grained interior with subophitic texture. Fassaite, diopside and olivine from the exterior border and interior of SA-1 have uniform intrinsic mass fractionation with isotopically heavy Mg (F_Mg = 3.6 ± 1.8‰/amu). In contrast, spinels from the spinel-rich regions adjacent to the fine-grained border have normal Mg isotopic composition (F_Mg = 0.1 ± 1.5‰/amu). The cores of large calcic (An_90,99) plagioclase have no excess ²⁶Mg, corresponding to ²⁶Mg*/ ²⁷Al < 3.7 × 10^?6. The Mg isotopic heterogeneity in SA-1 requires initial cooling rates of spinel-rich regions adjacent to the fine-grained border to be greater than ～75 °C/hr. In contrast, the subophitic texture of the interior suggests cooling rates of 5–20 °C/ hr. The minerals in SA-1 exhibit a wide range of REE abundances. Lanthanum concentrations vary from 1 × chondritic (ch) in early crystallizing diopside to 100 × ch in late crystallizing fassaite. Nepheline has 18–20 × ch LREE and 11–25 × ch HREE and iron-rich mesostasis is highly enriched in the REE with 270–400 × ch LREE and 230–280 × ch HREE. The complementary REE patterns of clinopyroxene and plagioclase and the enrichment of incompatible trace elements in the mesostasis and late crystallizing phases is consistent with closed system crystallization. The REE data for nepheline and the iron-rich mesostasis indicate these phases are in equilibrium and that nepheline crystallized from a melt. Influx of alkalies, minor Fe and halogens must have occurred during the last stages of crystallization or the inclusion must have been partially molten during Na influx as both anorthite (An₉₉) and nepheline are present in this inclusion. The preservation of isotopic heterogeneity in an inclusion that crystallized from a melt implies that melting was incomplete, allowing for survival of the relict spinels. The major and trace element abundances in SA-1 are inconsistent with formation as a mixture of nebular materials and suggest that SA-1 contains a chemically fractionated component produced by igneous differentiation.     

Mineralogy and possible origin of an unusual Cr-rich inclusion in the Los Martinez (L6) chondrite

Abstract— During a petrological study of the previously unclassified ordinary chondrite Los Martínez, we discovered a highly unusual Cr-rich inclusion which we believe is unique in both extraterrestrial and terrestrial mineralogy. The inclusion is highly zoned both compositionally and optically, with a Ca-Al rich, cloudy core and an opaque, Cr-Na-rich rim (up to 24 wt.% Cr₂O₃). Detailed SEM and TEM studies show that the inclusion now consists of a highly zoned, single crystal of plagioclase intergrown with chromium-rich spinel. The spinel has a well-developed crystallographic orientation relationship with the host plagioclase, which indicates that it is the product of exsolution. Although superficially similar to a plagioclase feldspar in composition, in detail the inclusion is Si-deficient and Al-enriched relative to a stoichiometric feldspar. We have not been able to identify a viable precursor mineral phase to the plagioclase-chromite intergrowth and suggest that it may be an unknown metastable phase. The Cr-rich precursors of the inclusion probably have close affinities to the chromite-plagioclase chondrules observed by Ramdohr (1967) in several ordinary chondrites. Based on the zoning in the inclusion, we suggest that it is the product of fractional crystallization from a melt, which may have formed as a liquid condensate, or by melting of solid condensates, in the solar nebula. Subsequent cooling of this melt condensate resulted in crystallization of the, as yet, unidentified phase. After crystallization, the inclusion was probably incorporated into a parent body where it underwent metamorphism and was probably shocked to some degree. During this period of parent body metamorphism, exsolution and decomposition of the unknown precursor occurred to produce the observed intergrowth of plagioclase and chromite. Finally, we have classified Los Martínez as an L6 ordinary chondrite breccia.     

Formation of relativistic electron-photon showers in compact X-ray sources

The possibility of relativistic electron-photon shower formation in compact X-ray sources is discussed and the results of Monte-Carlo calculations are reported.     

A refractory inclusion with solar oxygen isotopes and the rarity of such objects in the meteorite record

NASA's Genesis mission revealed that the Sun is enriched in ¹⁶O compared to the Earth and Mars (the Sun's Δ¹⁷O, defined as δ¹⁷O–0.52×δ¹⁸O, is –28.4 ± 3.6‰; McKeegan et al. 2011). Materials as ¹⁶O‐rich as the Sun are extremely rare in the meteorite record. Here, we describe a Ca‐Al‐rich inclusion (CAI) from a CM chondrite that is as ¹⁶O‐enriched as the Sun (Δ¹⁷O = –29.1 ± 0.7‰). This CAI also has large nucleosynthetic anomalies in ⁴⁸Ca and ⁵⁰Ti (δ‐values are –8.1 ± 3.3 and –11.7 ± 2.4‰, respectively) and shows no clear evidence for incorporation of live ²⁶Al; (²⁶Al/²⁷Al)₀ = (0.03 ± 0.11) × 10^–5. Due to their anomalous isotopic characteristics, the rare CAIs consistent with the Genesis value could be among the first materials that formed in the solar system. In contrast to the CAI studied here, the majority of CAIs formed in or interacted with a reservoir characterized by a Δ¹⁷O value near –23.5‰. Combined with ²⁶Al‐²⁶Mg systematics, the oxygen isotopic compositions of FUN (fractionation and unidentified nuclear effects), UN, and normal CAIs suggest that nebular conditions were favorable for solids to inherit this value for an extended period of time. Many later‐formed materials, such as chondrules, planetesimals, and terrestrial planets, formed in reservoirs with Δ¹⁷O near 0‰. The distribution could be easier to explain if the common CAI value of –23.5‰, which is consistent with the Genesis value within 3σ, represented the average composition of the protoplanetary disk.     

A unique ultrarefractory inclusion from the Murchison meteorite

Abstract— Through freeze-thaw disaggregation of the Murchison meteorite, we have recovered a refractory inclusion, HIB-11, that is unique in terms of its texture, mineral compositions, and bulk composition. It consists of anhedral, Y-rich (1.6 wt% Y₂O₃) perovskite and lathlike spinel grains enclosed in a matrix of fine-grained, Sc-rich (10.5 wt% SC₂O₃ avg.), Ti-rich (12.6 wt% TiO₂ avg., reporting all Ti as TiO₂) clinopyroxene. The chondrite-normalized rare earth element (REE) pattern is complex, with light REE (LREE) at ～10× C1, abundances increasing from Gd through Ho (the latter at ～10⁴× C1), decreasing through Yb at 200× C1, and Lu at ～400× C1. The pattern reflects several stages of high-temperature volatility fractionation. Removal of Lu and Er from the source gas in the first condensation event was followed by partial to complete removal of the somewhat less refractory heavy REE, Gd through Ho, in the HIB-11 precursors by condensation from the fractionated residual gas in a second event. Both of these events probably reflect condensation of REE into ZrO₂ or a mixed Zr-, Sc-, Ti-, Y-oxide at temperatures too high for hibonite stability. A second, lower-temperature component, which was subsequently added, had fractionated (Nd-poor, Ce-rich) LREE abundances that resulted from condensation from a gas that had undergone prior removal of the more refractory LREE, resulting in enrichment in Ce and the most volatile REE, Eu and Yb. The aggregate was then melted and quickly cooled, forming a fine-grained spherule. This is the first reported inclusion in which the two most refractory REE, Lu and Er, are strongly fractionated from the other REE. An absence of mass fractionation among the Ti isotopes indicates that HIB-11 is not an evaporative residue, implying that volatility fractionation of trace elements took place during condensation. The fact that the two most refractory heavy REE could be separated from the other, only slightly less refractory heavy REE suggests that a wide variety of REE patterns is possible, and that ultrarefractory inclusions with other unusual REE patterns, important recorders of nebular condensation, may yet be discovered.     

Transmission electron microscope study of compact Type A calcium-aluminum-rich inclusions from CV3 chondrites: Clues to their origin

Abstract— A transmission electron microscope (TEM) study of three coarse-grained Type A Ca, Al-rich inclusions (CAIs) from Allende, Acfer 082 and Acfer 086 (all CV3 chondrites) was performed in order to decipher their origin and effects of possible metamorphism. The constituent minerals of the CAIs are found to exhibit very similar microstructural characteristics in each of the inclusions studied. In general, the minerals show a well-developed equilibrium texture with typical 120° triple junctions. Melilites are clearly considerably strained and characterized by high dislocation densities up to 3 × 10¹¹ cm^?2. The dislocations have Burgers vectors of [001], [110] or [011] and often form subgrain boundaries subparallel {100}. Melilite in the Allende CAI additionally contains thin amorphous lamellae mostly oriented parallel to {001}. Fassaite (Al-Ti-diopside) is almost featureless even on the TEM scale. Only a few subplanar dislocation walls composed of dislocations with Burgers vectors [001] and 1/2 [110] were detected. Although enclosed within the highly strained melilites, the euhedral spinels contain only low dislocation densities (<2 × 10⁴ cm^?2). In the Allende CAI, spinels were found twinned on {111}. Perovskite is also characterized by a low number of linear lattice defects. All grains possess orthorhombic symmetry and are commonly twinned according to a 90° rotation around [101]. Many crystals exhibit typical domain structures as well as curved twin walls where two orthogonal sets intersect. In addition to the mineral phases described above, tiny inclusions of the simple oxides CaO and TiO₂ were found within melilite (CaO), spinel (CaO, TiO₂) and perovskite (CaO, TiO₂). Based on these observations, it is assumed that at the beginning of the formation of the CAIs a condensed solid precursor was present. Euhedral spinels poikilitically enclosed within melilites suggest that this solid aggregate was then molten. If the pure oxides represent relict condensates, their presence proves that this melting was incomplete. While still plastic, the CAIs were shocked by microimpacts causing the high dislocation densities in melilite as well as diaplectic melilite glass and twinned spinels in the Allende CAI. In Acfer 082 and 086, the deformation took place at elevated temperatures, preventing the solid phase transition and mechanical twinning. The absence of linear lattice defects in spinel, fassaite and perovskite most probably reflects inhomogeneous pressure distribution in the polycrystalline CAI as well as the different strengths of the minerals. According to cooling-rate experiments on perovskite by Keller and Buseck (1994), the dominating (101) twins in the CAI perovskites point to cooling rates ≤50 °C/min. Finally, after crystallization of the CAI was complete, mild thermal metamorphism caused the formation of subgrain boundaries, 120° triple junctions and chemical homogenization of the melilites.     

The Leedey,Oklahoma, chondrite: Fall,petrology, chemistry and an unusual Fe,Ni-FeS inclusion

Abstract— The Leedey, Oklahoma, meteorite shower fell on 1943 November 25, following a fireball which was visible across much of southwestern Oklahoma and northcentral Texas. The shower produced 24 stones with a total mass of ～51.5 kg. The stones formed a strewnfield ～18 km in length in the same direction as the observed path of the meteor (N50°W). Leedey is classified as an L6(S3) ordinary chondrite. We report bulk major element chemical analyses from four separate laboratories. Leedey contains an unusual 6 by 8 mm composite Fe,Ni-FeS grain, which is composed of a 3 mm kamacite grain adjacent to a 5 mm troilite grain. A 50–100 μm rim of high-Ni (45–55 wt%) taenite (tetrataenite) occurs at the boundary between kamacite and troilite. A single, zoned pyrophanite grain is observed at the boundary between the inclusion troilite and host silicates. An origin as a foreign particle incorporated after metamorphism or during impact melting appears unlikely. This particle likely formed by a complex set of processes, including melting in the nebula, parent body metamorphism and reheating by later shock, mirroring the history of the host chondrite.     

New insights into the formation of fayalitic olivine from Allende dark inclusions

Abstract– Although considerable progress has been made in unraveling the origin(s) of fayalitic olivines in dark inclusions (DIs), many questions remain still unresolved and/or controversial. We combine a chemical and petrographic study of the Allende dark inclusion 4884‐2B (AMNH, New York) and ATEM studies of a fragment of the dark inclusion Allende AF (NHM, Vienna) and discuss an alternative way in which fayalitic olivines could have formed. Allende dark inclusion 4884‐2B contains a few aggregates with variable proportions of transparent and feathery olivine. Two such objects (aggregates A and B) are the focus of this study as they preserve glasses that can help in deciphering the nature of the processes involved during olivine growth and subsequent olivine transformation. The petrographic and chemical characteristics of aggregates A and B indicate that the forsteritic stack olivines may be pseudomorphs of clear olivine crystals. The ATEM studies in All‐AF suggest that fayalitic olivines may be the result of secondary processes (e.g., metasomatic exchange reactions) operating in the solar nebula. Transformation may have occurred through the mediation of a dry gas phase involving nonvolatile major elements, such as Mg and Fe (e.g., Dohmen et al. 1998 ). This mechanism could reveal olivine growth patterns (e.g., stacked platelets due to a rapid growth regime) and may have contributed to the development of their fibrous aspect while preserving the shape (i.e., volume) of the crystals. This highly selective process did completely or partially transform ferromagnesian minerals, but affected the fine‐grained mesostasis only slightly.     

Oxygen isotopes in forsterite grains from Julesburg and Allende: Oxygen-16-rich material in an ordinary chondrite

Abstract— We have used the Manchester ISOLAB 54 ion microprobe to make in situ measurements of the ¹⁷O/¹⁶O and ¹⁸O/¹⁶O ratios of olivine grains in the Julesburg (L3.6) and Allende (CV3) chondrites. We have discovered a population of olivines in Julesburg characterised by (1) the most ¹⁶O-rich compositions yet reported for olivine from an ordinary chondrite; (2) cores of low-Fa olivine, which frequently shows blue cathodoluminesce; (3) thick coats of more Fa-rich (Fa ～20) olivine, which is also ¹⁶O-enriched. In an O isotopic plot, the Julesburg ¹⁶O-rich grains form a roughly linear array that is offset from the Allende mixing line. The presence of very low Fa olivine and, sometimes, well-defined Fa-rich coats indicates that these grains experienced significantly less thermal metamorphism than most of the olivine in the meteorite. Some ¹⁶O-rich Julesburg grains are associated with minor feldspar or pyroxene and are probably chondrule fragments. They are isotopically indistinguishable from forsterite in Allende; however, Allende forsterite grains do not have the thick Fa20 coats typical of those in Julesburg. These ¹⁶O-rich forsterite grains appear to be related to the “blue olivine” of Steele (1986). Both cores and coats of ¹⁶O-rich grains in Julesburg are isotopically distinct from olivine in Semarkona group A and group B chondrules.     

Remelting of refractory inclusions in the chondrule‐forming regions: Evidence from chondrule‐bearing type C calcium‐aluminum‐rich inclusions from Allende

Abstract— We describe the mineralogy, petrology, oxygen, and magnesium isotope compositions of three coarse‐grained, igneous, anorthite‐rich (type C) Ca‐Al‐rich inclusions (CAIs) (ABC, TS26, and 93) that are associated with ferromagnesian chondrule‐like silicate materials from the CV carbonaceous chondrite Allende. The CAIs consist of lath‐shaped anorthite (An₉₉), Cr‐bearing Al‐Ti‐diopside (Al and Ti contents are highly variable), spinel, and highly åkermanitic and Na‐rich melilite (Åk_63–74, 0.4–0.6 wt% Na₂O). TS26 and 93 lack Wark‐Lovering rim layers; ABC is a CAI fragment missing the outermost part. The peripheral portions of TS26 and ABC are enriched in SiO₂ and depleted in TiO₂ and Al₂O₃ compared to their cores and contain relict ferromagnesian chondrule fragments composed of forsteritic olivine (Fa_6–8) and low‐Ca pyroxene/pigeonite (Fs₁Wo_1–9). The relict grains are corroded by Al‐Ti‐diopside of the host CAIs and surrounded by haloes of augite (Fs_0.5Wo_30–42). The outer portion of CAI 93 enriched in spinel is overgrown by coarse‐grained pigeonite (Fs_0.5–2Wo_5–17), augite (Fs_0.5Wo_38–42), and anorthitic plagioclase (An₈₄). Relict olivine and low‐Ca pyroxene/pigeonite in ABC and TS26, and the pigeonite‐augite rim around 93 are ¹⁶O‐poor (Δ17O ～ ?1‰ to ?8‰). Spinel and Al‐Ti‐diopside in cores of CAIs ABC, TS26, and 93 are ¹⁶O‐enriched (Δ17O down to ?20‰), whereas Al‐Ti‐diopside in the outer zones, as well as melilite and anorthite, are ¹⁶O‐depleted to various degrees (Δ17O = ?11‰ to 2‰). In contrast to typical Allende CAIs that have the canonical initial ²⁶Al/²⁷Al ratio of ～5 × 10^?5 ABC, 93, and TS26 are ²⁶Al‐poor with (²⁶Al/²⁷Al)₀ ratios of (4.7 ± 1.4) × 10^?6 (1.5 ± 1.8) × 10^?6 <1.2 × 10^?6 respectively. We conclude that ABC, TS26, and 93 experienced remelting with addition of ferromagnesian chondrule silicates and incomplete oxygen isotopic exchange in an ¹⁶O‐poor gaseous reservoir, probably in the chondrule‐forming region. This melting episode could have reset the ²⁶Al‐²⁶Mg systematics of the host CAIs, suggesting it occurred ～2 Myr after formation of most CAIs. These observations and the common presence of relict CAIs inside chondrules suggest that CAIs predated formation of chondrules.     

The White Angel: A unique wollastonite‐bearing,mass‐fractionated refractory inclusion from the Leoville CV3 carbonaceous chondrite

Abstract— We report the study of an unusual compact type A refractory inclusion, named the White Angel, from the Leoville CV3 meteorite. The petrologic, mineral chemical, isotopic, and trace‐element signatures of this once‐molten Ca‐Al‐rich inclusion (CAI), which contains large, equant wollastonite crystals, indicate a short multistage history that occurred very early, before substantial decay of ²⁶Al. Magnesium in the inclusion is isotopically heavy, with F_Mg reaching 18‰/amu, in the range of fractionated and with unidentified nuclear effects (FUN) inclusions. However, the absence of any nuclear anomalies in Ca and Ti and an inferred ²⁶Al/²⁷Al ratio of (5.5 ± 0.9) × 10^?5 indicate that the White Angel belongs to the F inclusions. Silicon and oxygen are also mass fractionated in favor of the heavy isotopes, but to a lesser extent. The O isotopes show a range in ¹⁶O excesses. On an O three‐isotope plot, data points lie on a line parallel and to the right of the carbonaceous chondrite anhydrous mineral mixing line, with wollastonite being the most ¹⁶O‐rich phase. The chondrite‐normalized rare earth and trace‐element pattern of the whole inclusion is the complement of an ultrarefractory pattern indicating that precursor phases of the CAI must have condensed in an Al‐, heavy rare earth element (HREE)‐depleted reservoir. Melting of those precursor phases in an ¹⁶O‐rich environment and evaporation led to mass‐dependent isotopic fractionation of Mg, Si, and O. Partial isotopic exchange with a reservoir containing unfractionated Mg took place at a later stage but before any measurable decay of ²⁶Al. Some minerals (melilite and perovskite) in the White Angel equilibrated oxygen isotopes with a relatively ¹⁶O‐poor reservoir that was also mass‐fractionated toward the heavy isotopes, different from that with which the normal or FUN inclusions interacted.     

A search for electron cyclotron maser emission from compact binaries

Unipolar induction (UI) is a fundamental physical process, which occurs when a conducting body transverses a magnetic field. It has been suggested that UI is operating in RX J0806+15 and RX J1914+24, which are believed to be ultracompact binaries with orbital periods of 5.4 and 9.6 min, respectively. The UI model predicts that those two sources may be electron cyclotron maser sources at radio wavelengths. Other systems in which UI has been predicted to occur are short period extrasolar terrestrial planets with conducting cores. If UI is present, circularly polarized radio emission is predicted to be emitted. We have searched for this predicted radio emission from short period binaries using the Very Large Array (VLA) and Australian Telescope Compact Array (ATCA). In one epoch, we find evidence for a radio source, coincident in position with the optical position of RX J0806+15. Although we cannot completely exclude that this is a chance alignment between the position of RX J0806+15 and an artefact in the data reduction process, the fact that it was detected at a significance level of 5.8σ and found to be transient suggests that it is more likely that RX J0806+15 is a transient radio source. We find an upper limit on the degree of circular polarization to be ∼50 per cent. The inferred brightness temperature exceeds 10¹⁸ K, which is too high for any known incoherent process, but is consistent with maser emission and UI being the driving mechanism. We did not detect radio emission from ES Cet, RX J1914+24 or Gliese 876.     

The origin of dark inclusions in Allende: New evidence from lithium isotopes

Abstract— Aqueous and thermal processing of primordial materials occurred prior to and during planet formation in the early solar system. A record of how solid materials were altered at this time is present in the carbonaceous chondrites, which are naturally delivered fragments of primitive asteroids. It has been proposed that some materials, such as the clasts termed “dark inclusions” found in type III chondrites, suggest a sequence of aqueous and thermal events. Lithium isotopes (⁶Li and ⁷Li) can reveal the role of liquid water in dark inclusion history. During aqueous alteration, ⁷Li passes preferentially into solution leaving ⁶Li behind in the solid phase and, consequently, any relatively extended periods of interaction with ⁷Li‐rich fluids would have left the dark inclusions enriched in the heavier isotope when compared to the meteorite as a whole. Our analyses of lithium isotopes in Allende and its dark inclusions reveal marked isotopic homogeneity and no evidence of greater levels of aqueous alteration in dark inclusion history.