Origin of fayalitic olivine rims and lath-shaped matrix olivine in the CV3 chondrite Allende and its dark inclusions

Abstract— We have studied an Allende dark inclusion by optical microscopy, scanning electron microscopy, electron microprobe analysis and transmission electron microscopy. The inclusion consists of chondrules, isolated olivines and matrix, which, as in the Allende host, is mainly composed of 5–20 μm long lath-shaped fayalitic grains with a narrow compositional range (Fa_{42 ± 2}) and nepheline. Olivine phenocrysts in chondrules and isolated olivine grains show various degrees of replacement by 5–10 μm wide fayalitic rims (Fa_{39 ± 2}) and 100–1000 μm wide translucent zones, which consist of 5–20 μm long lath-shaped fayalitic grains (Fa_{41 ± 1}) intergrown with nepheline. These fayalitic olivines, like those in the matrix of the dark inclusion, contain 10–20 nm sized inclusions of chromite, hercynite, and Fe-Ni sulfides. The fayalitic rims around remnant olivines are texturally and compositionally identical to those in Allende host, suggesting that they have similar origins. Chondrules are surrounded by opaque rims consisting of tiny lath-shaped fayalitic olivines (<1–3 μm long) intergrown with nepheline. As in the Allende host, fayalitic olivine veins may crosscut altered chondrules, fine-grained chondrule rims and extend into the matrix, indicating that alteration occurred after accretion. We infer that fayalitic olivine rims and lath-shaped fayalites in Allende and its dark inclusions formed from phyllosilicate intermediate phases. This explanation accounts for (1) the similarity of the replacement textures observed in the dark inclusion and Allende host to aqueous alteration textures in CM chondrites; (2) the anomalously high abundances of Al and Cr and the presence of tiny inclusions of spinels and sulfides in fayalitic olivines in Allende and Allende dark inclusions; (3) abundant voids and defects in lath-shaped fayalites in the Allende dark inclusion, which may be analogous to those in partly dehydrated phyllosilicates in metamorphosed CM/CI chondrites. We conclude that the matrix and chondrule rims in Allende were largely converted to phyllosilicates and then completely dehydrated. The Allende dark inclusions experienced diverse degrees of aqueous/hydrothermal alteration prior to complete dehydration. The absence of low-Ca pyroxene in the dark inclusion and its significant replacement by fayalitic olivine in Allende is consistent with the lower resistance of low-Ca pyroxene to aqueous alteration relative to forsteritic olivine. Hydro-thermal processing of Allende probably also accounts for the low abundance of planetary noble gases and interstellar grains, and the formation of nepheline, sodalite, salite-hedenbergite pyroxenes, wollastonite, kirschsteinite and andradite in chondrules and Ca,Al-rich inclusions.     

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.     

Magnesium isotopic fractionations in barred olivine chondrules from the Allende meteorite

Abstract— The Mg‐isotopic compositions in five barred olivine (BO) chondrules, one coarse‐grained rim of a BO chondrule, a relic spinel in a BO chondrule, one skeletal olivine chondrule similar to BO chondrules in mineralogy and composition, and two non‐BO chondrules from the Allende meteorite have been measured by thermal ionization mass spectrometry. The Mg isotopes are not fractionated and are within terrestrial standard values (±2.0%o per amu) in seven of the eight analyzed ferromagnesian chondrules. A clump of relic spinel grain and its host BO chondrule R‐11 give well‐resolvable Mg fractionations that show an enrichment of the heavier isotopes, up to +2.5%‰ per amu. The Mg‐isotopic compositions of coarse‐grained rim are identical to those of the host chondrule with BO texture. The results imply that ferromagnesian and refractory precursor components of the Allende chondrule may have been formed from isotopically heterogeneous reservoirs. In the nebula region where Allende chondrules formed, recycling of chondrules and multiple high‐temperature heating did not significantly alter the chemical and isotopic memory of earlier generations. Chemical and isotopic characteristics of refractory precursors of carbonaceous chondrite chondrules and CAIs are more closely related than previously thought. One of the refractory chondrule precursors of CV Allende is enriched in the heavier Mg isotopes and different from those of more common ferromagnesian chondrule precursors. The most probable scenario at the location where chondrule R‐11 formed is as follows. Before chondrule formation, several high‐temperature events occurred and then RPMs, refractory oxides, and silicates condensed from the nebular gas in which Mg isotopes were fractionated. Then, this CAI was transported into the chondrule formation region and mixed with more common, ferromagnesian precursors with normal Mg isotopes, and formed the BO chondrule. Because Mg isotope heterogeneity among silicates and spinel are found in some CAIs (Esat and Taylor, 1984), we cannot rule out the possibility that Mg isotopes of a melted portion of the refractory precursor (i.e., outer portion of CAI) are normal or enriched in the light isotope. Magnesium isotopes in the R‐11 host are also enriched in the heavier isotopes, +2.5%o per amu, which suggests that effects of isotopic heterogeneity among silicates and spinel, if they existed, are not considered to be large. It is possible that CAI precursor silicates partially dissolved during the chondrule forming event, contributing Mg to the melt and producing a uniform Mg‐isotopic signature but enriched in the heavier Mg isotopes, +2.5%‰ per amu. Most Mg isotopes in more common ferromagnesian chondrules represent normal chondritic material. Chemical and Mg‐isotopic signatures formed during nebular fractionations were not destroyed during thermal processes that formed the chondrule, and these were partly preserved in relic phases. Recycling of Allende chondrules and multiple heating at high temperature did not significantly alter the chemical and Mg‐isotopic memory of earlier generations.     

Secondary calcium-iron-rich minerals in the Bali-like and Allende-like oxidized CV3 chondrites and Allende dark inclusions

Abstract— We have characterized Ca-Fe-rich silicates (salite-hedenbergite pyroxenes (Fs_10–50Wo_45–50), andradite (Ca₃Fe₂Si₃O₁₂), kirschsteinite (CaFeSiO₄), and wollastonite (Ca₃Si₃O₉)) in the type I chondrules and matrices in the Bali-like and Allende-like oxidized CV3 chondrites and Allende dark inclusions. In type I chondrules in the Bali-like CV3 chondrites, metal is oxidized to magnetite; magnetite-sulfide nodules are replaced by Ca-Fe-rich pyroxenes with minor andradite and pure fayalite. We infer that Ca-Fe-rich pyroxenes, andradite, fayalite, magnetite, and phyllosilicates (which occur in mesostases) formed at relatively low temperatures (<300 °C) in the presence of aqueous solutions. Thermodynamic analysis of phase relations in the Si-Fe-Ca-O-H system and large O isotopic fractionation of the coexisting magnetite and fayalite (～20%) (Krot et al., 1998) are consistent with this interpretation. In type I chondrules in the Allende-like CV3 chondrites and dark inclusions, magnetite-sulfide nodules are replaced by Ca-Fe-rich pyroxenes and ferrous olivine; low-Ca pyroxene and forsterite phenocrysts are rimmed and veined by ferrous olivine. It appear that the Ca-Fe-rich pyroxenes predate formation of ferrous olivine; the latter postdates formation of talc and biopyriboles (Brearley, 1997). The Allende dark inclusions are crosscut by Ca-Fe-pyroxene-andradite veins and surrounded by Ca-rich rims that consist of Ca-Fe-rich pyroxenes, andradite, wollastonite, and kirschsteinite. Calcium-rich veins and rims formed after aggregation and lithification of the dark inclusions. The rimmed dark inclusions show zoned depletion in Ca, which is due to a lower abundance of Ca-Fe-rich pyroxenes close to the rim. Calcium was probably leached from the inclusions and redeposited along their edges. We infer that the Allende-like chondrites and dark inclusions experienced similar aqueous alteration to the Bali-like chondrites and were metamorphosed subsequently, which resulted in loss of aqueous solutions and dehydration of phyllosilicates. We conclude that Ca-Fe-rich silicates in the oxidized CV3 chondrites and Allende dark inclusions are secondary and resulted from aqueous fluid-rock interactions during progressive metamorphism of a heterogeneous mixture of hydrous (ices?) and anhydrous materials; the latter were possibly mineralogically similar to the reduced CV3 chondrites.     

Tubular symplectic inclusions in olivine from the Fukang pallasite

Abstract– Olivine from the Fukang meteorite, like that from many other pallasites, contains distinctive arrays of parallel, straight, tubular inclusions. They differ in their extension and linearity from those in terrestrial olivines. They comprise approximately 1% of the total volume. Most have lens‐shaped cross‐sections, but some are rounded. The major axis of the lens‐shaped inclusions is rigorously oriented along olivine [001], and the rounded ones lie along olivine [010] and a few along [100]. The linear nature and orientations of the inclusions suggest that they nucleated on screw dislocations, perhaps formed through shock triggering. High‐resolution transmission electron microscopy (TEM) and energy‐dispersive x‐ray spectroscopy show that the inclusions consist of symplectic intergrowths of chromite, diopside, and silica that appear to have formed by exsolution from the host olivine. The symplectites consist of chromite lamellae with approximately 35‐nm spacings that grew outward from a central plane, with interstitial diopside and silica. Contrast modulations having an average spacing of 4.4 nm occur within the chromite lamellae. Using a reaction‐front model, we estimate that exsolution occurred over a period of 30 to 100 min, suggesting rapid cooling at high temperature. The crystallographic observations and inferences on growth rate are consistent with the hypothesis that the inclusions nucleated during heating following dislocation formation in a shock event, perhaps concurrent with that proposed to have disrupted the pallasite parent body.     

Fayalitic olivine in CV3 chondrite matrix and dark inclusions: A nebular origin

Abstract— Fayalitic olivine (Fa₃₂) is the major component of the matrices and dark inclusions of CV3 and other unequilibrated chondrites. It occurs most commonly as rims, veins and halos in and around chondrule silicates in the Allende-type (CV3_OXA) chondrites and, to a much lesser extent, in the reduced (CV3R) and Bali-type (CV3OXB) chondrites. The olivines have distinctive platy, tabular and lath- or irregular-shaped crystals, with the ratio of the two types varying widely. In CV3_OXB chondrites, matrix fayalitic olivines range up to Fag_99.9; whereas, in the other CV3 chondrites, the range is much smaller. The platy and tabular anisotropic forms of the fayalitic olivines strongly suggest growth from a vapor, and the nature of the occurrences suggests that CV3 matrices are unequilibrated mixtures of nebular materials. We argue that the parent body hydration/dehydration model has numerous inconsistencies that make this hypothesis highly unlikely. These include: (1) There is no direct evidence linking fayalitic olivine to precursor phyllosilicates. (2) Dehydration of phyllosilicates cannot explain the wide range of morphologies of the fayalitic olivines. (3) Fayalitic olivine clearly predates the formation of the hydrous phases in CV3 chondrites and is one of the phases that breaks down to form phyllosilicates (Keller et al., 1994). (4) The unequilibrated nature of the matrix, including fine-scale zoning in 10 μm sized fayalitic olivine crystals, would not survive the parent body metamorphism required in the dehydration model. (5) A dark inclusion in the Ningqiang chondrite contains fayalitic olivine rimmed by glassy and microcrystalline material (Zolensky et al., 1997), which probably formed by radiation damage. This indicates that the fayalitic olivine was exposed to solar radiation in a nebular setting. (6) Some Allende chondrules contain unaltered primary, anhydrous glassy mesostasis in contact with the host matrix (e.g., Ikeda and Kimura, 1995). Chondrule mesostases would not have survived parent body hydration without becoming hydrated and would probably not survive the metamorphic heating required in the dehydration scenario. (7) Single platy and barrel-shaped crystals of fayalitic olivine are present in accretionary rims in calcium-aluminum-rich inclusions (CAIs) (MacPherson and Davis, 1997), which developed in the nebula. (8) Matrix lumps completely encased in chondrules in ordinary chondrites contain mainly fayalitic olivine (Scott et al., 1984), which indicates a nebular origin. (9) Oxygen isotopic compositions of Allende matrix and dark inclusions strongly indicate little or no hydration for Allende and its components (Clayton, 1997). We favor a nebular vaporization/recondensation model in which vaporization of chondritic dust produced a fayalite-rich vapor, followed by formation of the fayalitic olivine by direct recondensation from the vapor, epitactic growth on surfaces of existing forsterite and enstatite in chondrules, and replacement of existing forsterite and enstatite by gas-solid exchange.     

Carbon in glass inclusions in the Allende meteorite

Abstract— Olivines in chondrules of the Allende CV3 carbonaceous chondrite contain two types of micron sized silicate melt inclusions: clear glass (type I) and devitrified glass (type II) inclusions. Many of the type I inclusions contain a gas bubble of variable size. Type II inclusions can be transparent (IIa), with or without a gas bubble, and brown (type IIb), with a gas bubble. A number of inclusions were measured with the Raman microprobe to detect possible presence of carbon. Carbon in the form of graphite was detected only in type II inclusions. Compositions of 11 inclusions were determined with the electron microprobe and proton microprobe in search for possible explanation of this preference of carbon for devitrified inclusions. All of the measured inclusions are rich in Si, Al and Ca. No significant differences between the compositions of the two types of inclusions were found. The data suggest that the inclusions formed from the melt trapped in growing olivine crystals, which themselves crystallized from a silicon rich, gas bearing melt. There is no coherent relation between the occurrence of graphite and the gas abundance in the original melt, as indicated by the sizes of gas bubbles. Therefore, carbon was not combined in a gaseous species (e.g., CO). It must have been preferentially dissolved in some domains of the melt.     

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.     

Rare-earth elements in matrix,inclusions, and chondrules of the Allende meteorite

Rare-earth elements in a whole-rock sample and in major components of the Allende meteorite were investigated; for a few samples, abundances of Ba, Sr, Ca, and Al were also determined. Of the materials investigated in the present work, CaAl-rich inclusions G and O seem to be of the greatest significance. In spite of the minor difference in mineralogy between them, the apparent chondrite-normalized RE pattern is much different between these two inclusions. (Yb and Eu in inclusion G appear exceptionally irregular). This observation is inferred to reflect a rather subtle difference in condition of condensation. It is also worthwhile to note that, while two portions (pink and white) of the inclusion G show similar aspects in the abundances of lithophile trace elements investigated, they show a remarkable difference at the same time. The white portion (G_w) of inclusion G can be considered to be a mixture of chondritic material and highly fractionated material like the faintly pink portion (G_p) picked from the same inclusion. This would suggest the possibility that the G_p-like material was produced from chondritic dust.The “matrix” separated from Allende was found to be fractionated with respect to the RE abundances relative to representative chondrite. It has also a very high value for the Ba abundance.     

A study of the minor element concentrations of spinels from two type B calcium-aluminum-rich inclusions: An investigation into potential formation conditions of calcium-aluminum-rich inclusions

Abstract— We have conducted an electron microprobe study of minor element distributions among spinels from two type B1 calcium-aluminum-rich inclusions (CAIs): Allende TS-23 and Leoville 3537–2. We show that by maintaining the petrologic context (edge, middle, and center of the inclusion plus their host silicate phase), four populations of spinels are resolvable based on their minor element contents. One population resides within the edge area (mainly mantle melilite) and is characterized by the highest V contents. Unlike Leoville 3537–2, many edge grains from Allende TS-23 also have high-Fe contents (up to 4.0 wt%) and low-Cr values. Based on their V and Ti concentrations (which is positively correlated), middle and center grains define a trend that is divided into three populations: spinels enclosed by melilite, fassaite, and anorthite. The overall range in Ti concentration based on fractional crystallization should be much less than a factor of 2; however, the observed range is considerably larger. The minor element contents of these grains are interpreted as recording alteration, primary fractional crystallization, and a complex igneous history that may involve remelting and recrystallization. From our data, Allende TS-23 has experienced more alteration than Leoville 3537–2, which is consistent with previous petrologic studies of silicates within these objects; yet both objects have likely been remelted (at least one additional melting event, possibly two, postdating the initial formation of these CAIs). By invoking a remelting history, the large range ir Ti concentrations and the different populations of spinels can be explained. Although our data suggest that more than one generation of spinels exist within these objects, we are unable to establish any population of relic spinel grains that predate the initial melting event.     

Calcium‐aluminum‐rich inclusions and amoeboid olivine aggregates from the CR carbonaceous chondrites

Abstract— Calcium‐aluminum‐rich refractory inclusions (CAIs) in CR chondrites are rare (<1 vol%), fairly small (<500 μm) and irregularly‐shaped, and most of them are fragmented. Based on the mineralogy and petrography, they can be divided into grossite ± hibonite‐rich, melilite‐rich, and pyroxene‐anorthite‐rich CAIs. Other types of refractory objects include fine‐grained spinel‐melilite‐pyroxene aggregates and amoeboid olivine aggregates (AOAs). Some of the pyroxene‐anorthite‐rich CAIs have igneous textures, and most melilite‐rich CAIs share similarities to both the fluffy and compact type A CAIs found in CV chondrites. One major difference between these CAIs and those in CV, CM, and CO chondrites is that secondary mineral phases are rare. In situ ion microprobe analyses of oxygen‐isotopic compositions of 27 CAIs and AOAs from seven CR chondrites demonstrate that most of the CAIs are ¹⁶O‐rich (δ¹⁷O of hibonite, melilite, spinel, pyroxene, and anorthite < ?22‰) and isotopically homogeneous within 3–4‰. Likewise, forsterite, spinel, anorthite, and pyroxene in AOAs have nearly identical, ¹⁶O‐rich compositions (?24‰ < δ¹⁷O < ?20‰). In contrast, objects which show petrographic evidence for extensive melting are not as ¹⁶O‐rich (δ¹⁷O less than ?18‰). Secondary alteration minerals replacing ¹⁶O‐rich melilite in melilite‐rich CAIs plot along the terrestrial fractionation line. Most CR CAIs and AOAs are mineralogically pristine objects that largely escaped thermal metamorphism and secondary alteration processes, which is reflected in their relatively homogeneous ¹⁶O‐rich compositions. It is likely that these objects (or their precursors) condensed in an ¹⁶O‐rich gaseous reservoir in the solar nebula. In contrast, several igneous CAIs are not very enriched in ¹⁶O, probably as a result of their having melted in the presence of a relatively ¹⁶O‐poor nebular gas. If the precursors of these CAIs were as ¹⁶O‐rich as other CR CAIs, this implies either temporal excursions in the isotopic composition of the gas in the CAI‐forming regions and/or radial transport of some CAI precursors into an ¹⁶O‐poor gas. The absence of oxygen isotope heterogeneity in the primary minerals of melilite‐rich CAIs containing alteration products suggests that mineralogical alteration in CR chondrites did not affect oxygen‐isotopic compositions of their CAIs.     

Structural uplift and ejecta thickness of lunar mare craters: New insights into the formation of complex crater rims

We investigate the elevated crater rims of lunar craters. The two main contributors to this elevation are a structural uplift of the preimpact bedrock and the emplacement of ejecta on top of the crater rim. Here, we focus on five lunar complex mare craters with diameters ranging between 16 and 45 km: Bessel, Euler, Kepler, Harpalus, and Bürg. We performed 5281 measurements to calculate precise values for the structural rim uplift and the ejecta thickness at the elevated crater rim. The average structural rim uplift for these five craters amounts to S_RU = 70.6 ± 1.8%, whereas the ejecta thickness amounts to E_T = 29.4 ± 1.8% of the total crater rim elevation. Erosion is capable of modifying the ratio of ejecta thickness to structural rim uplift. However, to minimize the impact of erosion, the five investigated craters are young, pristine craters with mostly preserved ejecta blankets. To quantify how strongly craters were enlarged by crater modification processes, we reconstructed the dimensions of the transient crater. The difference between the transient crater diameter and the final crater diameter can extend up to 11 km. We propose reverse faulting and thrusting at the final crater rim to be one of the main contributing factors of forming the elevated crater rim.     

Whole‐rock 26Al‐26Mg systematics of amoeboid olivine aggregates from the oxidized CV3 carbonaceous chondrite Allende

Abstract– We report on mineralogy, petrography, and whole‐rock ²⁶Al‐²⁶Mg systematics of eight amoeboid olivine aggregates (AOAs) from the oxidized CV chondrite Allende. The AOAs consist of forsteritic olivine, opaque nodules, and variable amounts of Ca,Al‐rich inclusions (CAIs) of different types, and show evidence for alteration to varying degrees. Melilite and anorthite are replaced by nepheline, sodalite, and grossular; spinel is enriched in FeO; opaque nodules are replaced by Fe,Ni‐sulfides, ferroan olivine and Ca,Fe‐rich pyroxenes; forsteritic olivine is enriched in FeO and often overgrown by ferroan olivine. The AOAs are surrounded by fine‐grained, matrix‐like rims composed mainly of ferroan olivine and by a discontinuous layer of Ca,Fe‐rich silicates. These observations indicate that AOAs experienced in situ elemental open‐system iron‐alkali‐halogen metasomatic alteration during which Fe, Na, Cl, and Si were introduced, whereas Ca was removed from AOAs and used to form the Ca,Fe‐rich silicate rims around AOAs. The whole‐rock ²⁶Al‐²⁶Mg systematics of the Allende AOAs plot above the isochron of the whole‐rock Allende CAIs with a slope of (5.23 ± 0.13) × 10^?5 reported by Jacobsen et al. (2008) . In contrast, whole‐rock ²⁶Al‐²⁶Mg isotope systematics of CAIs and AOAs from the reduced CV chondrite Efremovka define a single isochron with a slope of (5.25± 0.01) × 10^?5 ( Larsen et al. 2011 ). We infer that the excesses in ²⁶Mg* present in Allende AOAs are due to their late‐stage open‐system metasomatic alteration. Thus, the ²⁶Al‐²⁶Mg isotope systematics of Allende CAIs and AOAs are disturbed by parent body alteration processes, and may not be suitable for high‐precision chronology of the early solar system events and processes.     

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.     

New insights into the photometric structure of Blue Compact Dwarf Galaxies from a deep Near-Infrared study

We present deep Near-Infrared (NIR) imaging of Blue Compact Dwarf Galaxies (BCDs), allowing for the first time to derive and systematize the NIR structural properties of their stellar low-surface brightness (LSB) host galaxies. Compared to optical data, NIR images, being less contamined by the extended stellar and ionized gas emission from the starburst, permit to study the LSB host galaxy closer to its center. We find that radial surface brightness profiles (SBPs) of the LSB hosts show at large radii a mostly exponential intensity distribution, in agreement with previous optical studies. At small to intermediate radii, however, the NIR data reveal an inwards flattening with respect to the outer exponential slope (`type V SBPs', Binggeli and Cameron, 1991) in the LSB component of more than one half of the sample BCDs. This result may constitute an important observational constraint to the dynamics and evolution of BCDs. We apply a modified exponential fitting function (Papaderos et al., 1996a) to parametrize and systematically study type V profiles in BCDs. A Sérsic law is found to be less suitable for studying the LSB component of BCDs, since it yields very uncertain solutions. This revised version was published online in August 2006 with corrections to the Cover Date.     

Al‐Mg isotopic evidence for episodic alteration of Ca‐Al‐rich inclusions from Allende

Abstract— Textures, mineral assemblages, and Al‐Mg isotope systematics indicate a protracted, episodic secondary mineralization history for Allende Ca‐Al‐rich inclusions (CAIs). Detailed observations from one type B1 CAI, one B2, one compact type A (CTA), and one fluffy type A (FTA) indicate that these diverse types of CAIs are characterized by two distinct textural and mineralogic types of secondary mineralization: (1) grossular‐rich domains, concentrated along melilite grain boundaries in CAI interiors, and (2) feldspathoid‐bearing domains, confined mostly to CAI margins just interior to the Wark‐Lovering rim sequence. The Al‐Mg isotopic compositions of most secondary minerals in the type B1 CAI, and some secondary minerals in the other CAIs, show no resolvable excesses of ²⁶Mg, whereas the primary CAI phases mostly yield correlated excesses of ²⁶Mg with increasing Al/Mg corresponding to “canonical” initial ²⁶Al/²⁷Al ～ 4.5–5 × 10^?5. These secondary minerals formed at least 3 Ma after the primary CAI minerals. All but two analyses of secondary minerals from the fluffy type‐A CAI define a correlated increase in ²⁶Mg/²⁴Mg with increasing Al/Mg, yielding (²⁶Al/²⁷Al)0 = (4.9 ± 2.8) × 10^?6. The secondary minerals in this CAI formed 1.8–3.2 Ma after the primary CAI minerals. In both cases, the timing of secondary alteration is consistent with, but does not necessarily require, alteration in an asteroidal setting. One grossular from the type B2 CAI, and several grossular and secondary feldspar analyses from the compact type A CAI, have excesses of ²⁶Mg consistent with initial ²⁶Al/²⁷Al ～ 4.5 × 10^?5. Especially in the compact type A CAI, where ²⁶Mg/²⁴Mg in grossular correlates with increasing Al/Mg, these ²⁶Mg excesses are almost certainly due to in situ decay of ²⁶Al. They indicate a nebular setting for formation of the grossular. The preservation of these diverse isotopic patterns indicates that heating on the Allende parent body was not pervasive enough to reset isotopic systematics of fine‐grained secondary minerals. Secondary mineralization clearly was not restricted to a short time interval, and at least some alteration occurred coincident with CAI formation and melting events (chondrule formation) in the nebula. This observation supports the possibility that alteration followed by melting affected the compositional evolution of CAIs.     

The water content and parental magma of the second chassignite NWA 2737: Clues from trapped melt inclusions in olivine

NWA 2737, the second known chassignite, mainly consists of cumulate olivine crystals of homogeneous composition (Fo = 78.7 ± 0.9). These brown colored olivine grains exhibit two sets of perpendicular planar defects due to shock. Two forms of trapped liquids, interstitial melts and magmatic inclusions, have been examined. Mineral assemblages within the olivine‐hosted magmatic inclusions include low‐Ca pyroxene, augite, kaersutite, fluorapatite, biotite, chromite, sulfide, and feldspathic glass. The reconstructed parental magma composition (A^#) of the NWA 2737 is basaltic and resembles both the experimentally constrained parental melt composition of chassiginites and the Gusev basalt Humphrey, albeit with lower Al contents. A^# also broadly resembles the average of shergottite parent magmas or LAR 06319. However, we suggest that the mantle source for the chassignite parental magmas was distinct from that of the shergottite meteorites, particularly in CaO/Al₂O₃ ratio. In addition, based on the analysis of the volatile contents of kaersutite, we derived a water content of 0.48–0.67 wt% for the parental melt. Finally, our MELTS calculations suggest that moderate pressure (approximately 6.8 kb) came closest to reproducing the crystallized melt‐inclusion assemblages.     

The Chelyabinsk meteorite: New insights from a comprehensive electron microscopy and Raman spectroscopy study with evidence for graphite in olivine of ordinary chondrites

We present results of petrographic, mineralogical, and chemical investigations of three Chelyabinsk meteorite fragments. Three distinct lithologies were identified: light S3 LL5, dark S4–S5 LL5 material, and opaque fine-grained former impact melt. Olivine–spinel thermometry revealed an equilibration temperature of 703 ± 23 °C for the light lithology. All plagioclase seems to be secondary, showing neither shock-induced fractures nor sulfide-metal veinlets. Feldspathic glass can be observed showing features of extensive melting and, in the dark lithology, as maskelynite, lacking melt features and retaining grain boundaries of former plagioclase. Olivine of the dark lithology shows planar deformation features. Impact melt is dominated by Mg-rich olivine and resembles whole-rock melt. Melt veins (<2 mm) are connected to narrower veinlets. Melt vein textures are similar to pegmatite textures showing chilled margins, a zone of inward-grown elongated crystals and central vugs, suggesting crystallization from supercooled melt. Sulfide-metal droplets indicate liquid immiscibility of both silicate and sulfide as well as sulfide and metal melts. Impact melting may have been an important factor for differentiation of primitive planetary bodies. Graphite associated with micrometer-sized melt inclusions in primary olivine was detected by Raman mapping. Carbon isotopic studies of graphite could be applied to test a possible presolar origin.     

V-R diagram for spiral galaxies. New insights into the hubble sequence

We examine the relation between the global parameters of spiral galaxies and their morphological type. We have represented the rotational velocityV at a radius of 25 mag arc sec^–2 against this radiusR for a sample of 56 galaxies. Different types of galaxies appear separated on this diagram. We have plotted above theV-R diagram isolines of total mass, angular momentum and mean surface density for isolated galaxies with circular motion in centrifugal equilibrium showing: (1) That the parameter that represents the morphological type is the mean surface density or its observational parameterV ²/R. (2) From aV-R diagram we can easily see what kinds of correlations we expect to find between the global properties of galaxies and their morphological type. (3) For every morphological type of galaxies there is a critical mass above which there is a change in its dynamical behaviour. Finally, consideringV ²/R as representative of the morphological type, we can deduce theoretically the empirical Tully-Fisher relation and the dependence of the slope and the zero point on the morphological type. The separation in logV that we expect to find for different types of galaxies for a constant luminosity is the same order as the experimental errors in the determination of logV. This explains the tight power-law relationship observed between this and the luminosity.