Aligned olivine in the Springwater pallasite

The mechanism by which olivine grains became embedded within iron‐nickel alloy in pallasite meteorites continues to be a matter of scientific debate. Geochemical and textural observations have failed to fully elucidate the origin and history of the olivine crystals; however, little research attention has been devoted to their crystallographic orientations within the metal matrix. Using electron backscatter diffraction, we have collected crystallographic orientation data for 296 crystals within ～65 cm² sample surface from Springwater. Though no global crystallographic preferred orientation exists, very low misorientations are observed among [100] axes of olivine crystals within specific texturally defined domains. Combined with a thorough characterization of large‐scale Springwater textures, the definitively nonrandom spatial distribution of olivine orientations provides clues regarding the nature of the olivine's initial formation environment as well as the sequence of events subsequent to metal incorporation.     

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.     

The Fukang pallasite: Characterization and implications for the history of the Main‐group parent body

We report the results of a study of the Fukang pallasite that includes measurements of bulk composition, mineral chemistry, mineral structure, and petrology. Fukang is a Main‐group pallasite that consists of semiangular olivine grains (Fo 86.3) embedded in an Fe‐Ni matrix with 9–10 wt% Ni and low‐Ir (45 ppb). Olivine grains sometimes occur in large clusters up to 11 cm across. The Fe‐Ni phase is primarily kamacite with accessory taenite and plessite. Minor phases include schreibersite, chromite, merrillite, troilite, and low‐Ca pyroxene. We describe a variety of silicate inclusions enclosed in olivine that contain phases rarely or not previously reported in Main‐group pallasites, including clinopyroxene (augite), tridymite, K‐rich felsic glass, and an unknown Ca‐Cr silicate. Pressure constraints determined from tridymite (<0.4 GPa), two‐pyroxene barometry (0.39 ± 0.07 GPa), and geophysical calculations that assume pallasite formation at the core–mantle boundary (CMB), provide an upper estimate on the size of the Main‐group parent body from which Fukang originated. We conclude that Fukang originated at the CMB of a large differentiated planetesimal 400–680 km in radius.     

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.     

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.     

Cosmogenic nuclides in the Brenham pallasite

Abstract— Cosmic‐ray‐produced (cosmogenic) nuclides were studied in fragments of the Brenham pallasite, a large stony iron meteorite. The contents of light noble gases (He, Ne, and Ar) and long‐lived radionuclides (¹⁰Be, ²⁶Al, ³⁶Cl, and ⁵³Mn), produced by nuclear reactions with cosmic rays, were measured in the separated metal and olivine phases from numerous samples representing a wide range of shielding conditions in the meteoroid. The distribution of cosmogenic nuclide concentrations in the metal follows patterns similar to that observed in large iron meteorites. Shielding effects were estimated from the relative proportions of low‐ and high‐energy reaction products. The production rates varied, from surface to interior, by a factor of more than several hundred. The ³⁶Cl‐³⁶Ar cosmic‐ray exposure age of Brenham is 156 ± 8 Myr. This determination is based on a multiple nuclide approach that utilizes cosmogenic nuclide pairs. This approach not only yields a “shielding independent” exposure age but also demonstrates that the production of cosmogenic nuclides occurred in a single stage. The depth profiles of ¹⁰Be in the stone phase and ⁵³Mn in the metal phase are shown superimposed on corresponding profiles from the Apollo 15 long drill core. Surprisingly low abundances of lithophile elements, such as K, U, and Th, provided a unique opportunity to examine the production systematics of those nuclides whose inventories typically have significant contributions from non‐cosmogenic sources, particularly radiogenic contributions. The U and Th contents of the olivine samples are extremely low, allowing detection of cosmogenic ⁴He production from oxygen, magnesium, silicon, and iron.     

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.     

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.     

Symplectic exsolution in olivine from the Nakhla martian meteorite

Abstract— The Nakhla meteorite, commonly accepted to have originated from Mars, is a cumulus clinopyroxenite with ～10 vol% of Fe‐rich olivine. Almost all olivine grains in Nakhla contain dark lamellar inclusions (less than 2–3 μm wide). High‐resolution scanning and transmission electron microscopy revealed that the inclusions are complex intergrowths of augite and magnetite. Such a symplectic intergrowth of augite and magnetite in olivine was known in some terrestrial rocks, lunar rocks, and a few meteorites. The inclusion in Nakhla olivine is the first symplectite found in a martian rock. Apparently, the presence of Fe³+ in olivine under an oxidizing condition on Mars caused symplectic exsolution at high temperature (>900 °C) during cooling.     

Chemical and genetic characterization of the ungrouped pallasite Lieksa

The meteorite Lieksa was found in 2017 in Löpönvaara, Finland, and later donated to the Finnish Museum of Natural History. Here, we report siderophile element concentrations, genetic isotopic data, and a metal–silicate segregation age for the meteorite. The ~280 g Lieksa is ~80% metal and ~20% silicate and oxide inclusions by volume, with the inclusions consisting primarily of Fe-rich olivine. Due to Lieksa's silicate content, coupled with a texture characterized by metal enclosing the silicates, it has been classified as a pallasite. Lieksa's olivine and bulk chemical characteristics are distinct from those of the known pallasite and iron meteorite groups, consistent with its classification as ungrouped. The meteorite exhibits a flat, chondrite-normalized highly siderophile element pattern, consistent with an origin as an early crystallization product from a metallic melt with chondritic relative abundances. Molybdenum, Ru, and ¹⁸³W isotopic data indicate that Lieksa formed in the non-carbonaceous (NC) domain of the solar nebula. Radiogenic ¹⁸²W abundances for Lieksa yield a model metal–silicate segregation age of 1.5 ± 0.8 Myr after calcium-aluminum-rich inclusion formation, which is within the range established for other NC-type pallasite and iron meteorite parent bodies.     

Oxygen isotopes in chondrule olivine and isolated olivine grains from the CO3 chondrite Allan Hills A77307

Abstract— We have measured O‐isotopic ratios in a variety of olivine grains in the CO3 chondrite Allan Hills (ALH) A77307 using secondary ion mass spectrometry in order to study the chondrule formation process and the origin of isolated olivine grains in unequilibrated chondrites. Oxygen‐isotopic ratios of olivines in this chondrite are variable from δ¹⁷O = ?15.5 to +4.5% and δ¹⁸O = ?11.5 to +3.9%, with Δ¹⁷O varying from ?10.4 to +3.5%. Forsteritic olivines, Fa_<1, are enriched in ¹⁶O relative to the bulk chondrite, whereas more FeO‐rich olivines are more depleted in ¹⁶O. Most ratios lie close to the carbonaceous chondrite anhydrous minerals (CCAM) line with negative values of Δ¹⁷O, although one grain of composition Fa₄ has a mean Δ¹⁷O of +1.6%. Marked O‐isotopic heterogeneity within one FeO‐rich chondrule is the result of incorporation of relic, ¹⁶O‐rich, Mg‐rich grains into a more ¹⁶O‐depleted host. Isolated olivine grains, including isolated forsterites, have similar O‐isotopic ratios to olivine in chondrules of corresponding chemical composition. This is consistent with derivation of isolated olivine from chondrules, as well as the possibility that isolated grains are chondrule precursors. The high ¹⁶O in forsteritic olivine is similar to that observed in forsterite in CV and CI chondrites and the ordinary chondrite Julesburg and suggests nebula‐wide processes for the origin of forsterite that appears to be a primitive nebular component.     

A neutron tomography study of the Seymchan pallasite

The internal structure of an olivine-rich fragment of the Seymchan meteorite was studied using neutron tomography. The differences in the neutron attenuation coefficients of constituent elements of the studied pallasite and the application of modern mathematical algorithms for a three-dimensional (3-D) imaging data analysis allow us to obtain a spatial distribution of different meteorite components. The heterogeneous distribution of the nickel in the metal phase was observed. In addition to the resulting 3-D model of the Seymchan meteorite, the distributions of the volumes, average sizes, and shape-related parameters of the FeNi metal component and olivine grains were obtained.     

Glass‐bearing inclusions in olivine of the Chassigny achondrite: Heterogeneous trapping at sub‐igneous temperatures

Abstract— Three types of glass‐bearing inclusions are present in olivine and chromite of the Chassigny achondrite: pure glass, monocrystal (glass plus a single mineral grain), and multiphase (glass plus a variety of minerals) inclusions. The occurrence, texture, and mineralogy of these inclusions and the chemical composition of the glass suggest an origin by heterogeneous trapping of these phases. The glass is rich in SiO₂, Al₂O₃, Na₂O, K₂O; and poor in MgO, FeO, and CaO; and contains appreciable amounts of Cl. The compositional variability of the glass is independent of the mineral content of the inclusions. Heating experiments with final temperatures of 900, 1000, and 1200 °C were performed with Chassigny inclusions for the first time. The glass of the heated inclusions has a chemical composition similar to that of unheated inclusions. This situation suggests that the glass cannot be a residual melt but rather is an independent component that was trapped with or without mineral phases. The extreme heterogeneity in alkali contents, and in particular Rb and Sr contents, also suggests precipitation and mixing of solid precursors. The most Rb‐rich glasses have near‐chondritic Rb/Sr ratios, possibly indicating a chondritic source for their precursor(s). None of the inclusions contain bubbles like those of typical melt inclusions in terrestrial igneous minerals. Furthermore, many inclusions are at the center of radial cracks in the host olivine, which indicates development of an overpressure within the inclusions at some time. A volume increase of the inclusions could have been achieved by differential thermal expansion of the content of the inclusion during a heating event. That mechanism requires bubble‐free and solid preheating inclusion contents. These features are incompatible with an origin of the inclusions by trapping of a silicate melt and point toward heterogeneous trapping of solid phases. The first N analyses performed in Chassigny glass‐bearing inclusions by nuclear reaction analysis (NRA) revealed high and variable N contents of the glass, which suggests trapping of a solid precursor (presumably at relatively low temperatures) from a fluid rather than a melt. In conclusion, the glass‐bearing inclusions in Chassigny olivine are not residuals after a closed‐system evolution of a trapped melt, but rather heterogeneously trapped precipitates of a fluid that existed during formation of Chassigny constituents. Consequently, it is very unlikely that the host olivine has an igneous origin.     

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.     

Bulk Mg isotopic compositions of Ca‐Al‐rich inclusions and amoeboid olivine aggregates

Abstract— High‐precision Mg isotopic compositions of Ca‐Al‐rich inclusions (CAIs) from both Ningqiang (ungrouped) and Allende (CV3) carbonaceous chondrites and amoeboid olivine aggregations (AOAs) from Allende were analyzed by multicollector inductively coupled plasma mass spectrometry (MC‐ICP‐MS). The CAIs from Allende plot on a line, with an inferred initial ²⁶Al/²⁷Al ratio of (4.77 ± 0.39) × 10^?5 close to the canonical value. This indicates a relatively closed Al‐Mg system in the CAIs and no significant Mg isotope exchange with ambient materials, although two of the CAIs are severely altered. The AOAs contain excess ²⁶Mg and plot close to the CAI regression line, which is suggestive of their contemporary formation. The CAIs from Ningqiang define a different line with a lower inferred (²⁶Al/²⁷Al)₀ ratio of (3.56 ± 0.08) × 10^?5. None of the CAIs and AOAs studied in this work shows significant mass fractionation with enrichment of the heavier Mg isotopes, arguing against an evaporation origin.     

Mineralogy and petrology of amoeboid olivine inclusions in CO3 chondrites: Relationship to parent‐body aqueous alteration

Abstract— Petrographic and mineralogic studies of amoeboid olivine inclusions (AOIs) in CO3 carbonaceous chondrites reveal that they are sensitive indicators of parent‐body aqueous and thermal alteration. As the petrologic subtype increases from 3.0 to 3.8, forsteritic olivine (Fa_0–1) is systematically converted into ferroan olivine (Fa_60–75). We infer that the Fe, Si and O entered the assemblage along grain boundaries, forming ferroan olivine that filled fractures and voids. As temperatures increased, Fe⁺² from the new olivine exchanged with Mg⁺² from the original AOI to form diffusive haloes around low‐FeO cores. Cations of Mn⁺², Ca⁺² and Cr⁺³ were also mobilized. The systematic changes in AOI textures and olivine compositional distributions can be used to refine the classification of CO3 chondrites into subtypes. In subtype 3.0, olivine occurs as small forsterite grains (Fa_0–1), free of ferroan olivine. In petrologic subtype 3.2, narrow veins of FeO‐rich olivine have formed at forsterite grain boundaries. With increasing alteration, these veins thicken to form zones of ferroan olivine at the outside AOI margin and within the AOI interior. By subtype 3.7, there is a fairly broad olivine compositional distribution in the range Fa_63–70, and by subtype 3.8, no forsterite remains and the high‐Fa peak has narrowed, Fa_64–67. Even at this stage, there is incomplete equilibration in the chondrite as a whole (e.g., data for coarse olivine grains in Isna (CO3.8) chondrules and lithic clasts show a peak at Fa₃₉). We infer that the mineral changes in AOI identified in the low petrologic types required aqueous or hydrothermal fluids whereas those in subtypes ?3.3 largely reflect diffusive exchange within and between mineral grains without the aid of fluids.     

Non-canonical perturbations in symplectic integration

The inclusion of non-canonical perturbations in symplectic integration schemes has been discussed. A rigorous derivation of an analog for theWisdom–Holman (1991) method, such that velocity dependent forces can be included, has been outlined. This is done both by using the δfunction formalism and also by means of formal Hamiltonization. Application to the relativistic corrections in Solar System integrations is discussed as an example. Numerical experiments confirm the usefulness of the method. This revised version was published online in July 2006 with corrections to the Cover Date.     

An American on Paris: Extent of aqueous alteration of a CM chondrite and the petrography of its refractory and amoeboid olivine inclusions

Paris is the least aqueously altered CM chondrite identified to date, classified as subtype 2.7; however, literature data indicate that some regions of this apparently brecciated meteorite may be subtype 2.9. The suite of CAIs in Paris includes 19% spinel–pyroxene inclusions, 19% spinel inclusions, 8% spinel–pyroxene–olivine inclusions, 43% pyroxene inclusions, 8% pyroxene–olivine inclusions, and 3% hibonite‐bearing inclusions. Both simple and complex inclusions are present; some have nodular, banded, or distended structures. No melilite was identified in any of the inclusions in the present suite, but other recent studies have found a few rare occurrences of melilite in Paris CAIs. Because melilite is highly susceptible to aqueous alteration, it is likely that it was mostly destroyed during early‐stage parent‐body alteration. Two of the CAIs in this study are part of compound CAI–chondrule objects. Their presence suggests that there were transient heating events (probably associated with chondrule formation) in the nebula after chondrules and CAIs were admixed. Also present in Paris are a few amoeboid olivine inclusions (AOI) consisting of relatively coarse forsterite rims surrounding fine‐grained, porous zones containing diopside and anorthite. The interior regions of the AOIs may represent fine‐grained rimless CAIs that were incorporated into highly porous forsterite‐rich dustballs. These assemblages were heated by an energy pulse that collapsed and coarsened their rims, but failed to melt their interiors.     

26Al‐26Mg systematics of Ca‐Al‐rich inclusions,amoeboid olivine aggregates,and chondrules from the ungrouped carbonaceous chondrite Acfer 094

Abstract— We report in situ magnesium isotope measurements of 7 porphyritic magnesium‐rich (type I) chondrules, 1 aluminum‐rich chondrule, and 16 refractory inclusions (14 Ca‐Al‐rich inclusions [CAIs] and 2 amoeboid olivine aggregates [AOAs]) from the ungrouped carbonaceous chondrite Acfer 094 using a Cameca IMS 6f ion microprobe. Both AOAs and 9 CAIs show radiogenic ²⁶Mg excesses corresponding to initial ²⁶Al/²⁷Al ratios between ～5 × 10^?5 ～7 × 10^?5 suggesting that formation of the Acfer 094 CAIs may have lasted for ～300,000 years. Four CAIs show no evidence for radiogenic ²⁶Mg; three of these inclusions (a corundum‐rich, a grossite‐rich, and a pyroxene‐hibonite spherule CAI) are very refractory objects and show deficits in ²⁶Mg, suggesting that they probably never contained ²⁶Al. The fourth object without evidence for radiogenic ²⁶Mg is an anorthite‐rich, igneous (type C) CAI that could have experienced late‐stage melting that reset its Al‐Mg systematics. Significant excesses in ²⁶Mg were observed in two chondrules. The inferred ²⁶Al/²⁷Al ratios in these two chondrules are (10.3 ± 7.4) × 10^?6 (6.0 ± 3.8) × 10^?6 (errors are 2σ), suggesting formation 1.6^+1.2_‐0.6 and 2.2^+0.4_‐0.3 Myr after CAIs with the canonical ²⁶Al/²⁷Al ratio of 5 × 10^?5. These age differences are consistent with the inferred age differences between CAIs and chondrules in primitive ordinary (LL3.0–LL3.1) and carbonaceous (CO3.0) chondrites.     

The cooling history of the Lewis Cliff 86010 angrite as inferred from kirschsteinite lamellae in olivine

Abstract— Olivine in the angritic meteorite Lewis Cliff (LEW) 86010 contains abundant exsolution lamellae of kirschsteinite. Compositional gradients adjacent to the interface in both host and lamellae were formed by diffusion of chemical components into and out of the lamellae during cooling and growth. We have compared these gradients with compositional profiles calculated from diffusion and heat flow equations to estimate the cooling rate and burial depth of the sample. The resulting values for cooling rate and burial depth depend on which values are used for the diffusion rate of Ca in olivine, and how measured diffusivities are extrapolated to the lower temperatures at which the lamellae grew. If the highest diffusion coefficients are used, the cooling rates obtained from seven different lamellae range from 30 to 52 °C/year, with an average of 42 °C/year, and burial depths (assuming an overburden with a thermal diffusivity typical of solid rock) range from 14 to 17 m, with an average of 15 m. If the lowest reasonable diffusion coefficients are used, the cooling rates range from 1.4 to 2.2 °C/year, with an average of 1.7 °C/year, and the depths range from 68 to 83 m, with an average of 75 m. For the highest Ca diffusivities, details of the compositional profiles near the olivine/kirschsteinite interface suggest that continuous cooling was greatly accelerated at a temperature near 600–700 °C. The simplest physical explanation for such an acceleration is excavation of the sample from its original burial depth by an impact event. If Ca diffusivities are lower, a two-stage cooling history is not required.