Alteration of calcium- and aluminium-rich inclusions in the Murray (CM2) carbonaceous chondrite

Abstract— Four different types of calcium- and aluminium-rich inclusions (CAIs) have been identified in the CM2 chondrite Murray, three of which contain alteration products. Two types of altered CAIs, spinel inclusions and spinel-pyroxene inclusions, contain primary spinel (± perovskite ± hibonite ± diopside) and secondary Fe-rich serpentine phyllosilicates (± tochilinite ± calcite). Original melilite in these CAIs is inferred to have been altered during aqueous activity in the parent body and Fe-rich serpentines, tochilinite and calcite were formed in its place. The other type of altered CAI is represented by one inclusion, here called MCA-1. This CAI contains primary spinel, perovskite, fassaite and diopside with secondary calcite, paragonite, Mg-Al-Fe phyllosilicates and a Mg-Al-Fe sulphate. Importantly, MCA-1 is similar in both primary and secondary mineralogy to a small number of altered CAIs described from other CM2 meteorites including Essebi, Murchison and a CM2 clast from Plainview. Features that these CAIs have in common include an unusually large size, a CV3-like primary mineralogy and the presence of secondary aluminosilicates and calcite. The Al-rich alteration products in MCA-1 are also reminiscent of secondary minerals in refractory inclusions from CV3 meteorites, which have previously been interpreted to form by interaction of the inclusions with solar nebula gases. In common with the other types of altered CAIs in Murray, MCA-1 is inferred to have experienced its main phase of alteration in a parent body environment. The Mg-Al-Fe phyllosilicates, calcite and the Mg-Al-Fe sulphate formed following aqueous alteration of an Al-rich precursor, possibly Ca dialuminate. This episode of parent body alteration may have overprinted an earlier phase of alteration in a solar nebula environment from which only paragonite remains.     

Primordial and cosmogenic noble gases in the Sutter's Mill CM chondrite

The Sutter's Mill (SM) CM chondrite fell in California in 2012. The CM chondrite group is one of the most primitive, consisting of unequilibrated minerals, but some of them have experienced complex processes occurring on their parent body, such as aqueous alteration, thermal metamorphism, brecciation, and solar wind implantation. We have determined noble gas concentrations and isotopic compositions for SM samples using a stepped heating gas extraction method, in addition to mineralogical observation of the specimens. The primordial noble gas abundances, especially the P3 component trapped in presolar diamonds, confirm the classification of SM as a CM chondrite. The mineralogical features of SM indicate that it experienced mild thermal alteration after aqueous alteration. The heating temperature is estimated to be <350 °C based on the release profile of primordial ³⁶Ar. The presence of a Ni‐rich Fe‐Ni metal suggests that a minor part of SM has experienced heating at >500 °C. The variation in the heating temperature of thermal alteration is consistent with the texture as a breccia. The heterogeneous distribution of solar wind noble gases is also consistent with it. The cosmic‐ray exposure (CRE) age for SM is calculated to be 0.059 ± 0.023 Myr based on cosmogenic ²¹Ne by considering trapped noble gases as solar wind, the terrestrial atmosphere, P1 (or Q), P3, A2, and G components. The CRE age lies at the shorter end of the CRE age distribution of the CM chondrite group.     

On the problem of Solar System origin: The regularities of noble gas fractionation in shock waves

The effects of the last supernova explosion before the formation of the Solar System are considered using noble gases as examples. Acceleration of generated supernova matter in the explosive shock wave led to its initial fractionation and to the formation of small-scale isotopic heterogeneity of primordial matter. This is fixed as some isotopic anomalies in high-temperature phases of the earliest condensates of carbonaceous chondrites, as well as in the isotopic systems of noble gases, and is the basis of the supernova phenomenon. Two main manifestations of shock-wave acceleration in noble gases are investigated: the change in the isotopic ratios of their cosmogenic components due to the increasing hardness of the spectrum of nuclear-active particles and the fractionation of gases, namely, the enrichment of their isotopic systems with heavier isotopes. The reality of the processes under consideration is demonstrated through the example of noble gases of solar corpuscular radiation in lunar ilmenites. The absence of r-process products among extinct radionuclides in Ca-and Al-rich inclusions (CAI) of carbonaceous chondrites with a formation interval of less than or equal to 1 Ma supports the idea that the last supernova was an Ia-type supernova, which possibly played an important role in the origin of the Solar System.     

Synthesis of refractory metal nuggets and constraints on the thermal histories of nugget‐bearing Ca,Al‐rich inclusions

Tiny refractory metal nuggets are mainly observed inside Ca, Al‐rich inclusions (CAIs) from chondritic meteorites and are commonly assumed to be condensates from a solar composition gas. However, recent detailed studies of metal nugget compositions and their comparison with predictions from condensation show that the observed abundance patterns are extremely difficult to achieve in this way. As a test for the proposed alternative, precipitation from a silicate liquid, we conducted melting experiments, in which nine different refractory metals (nugget components) were equilibrated with each other along with a CAI‐like liquid at reducing conditions. When quenched, minerals similar to those in CAIs formed from such liquids including refractory metal nuggets exhibiting compositions and appearances similar to those of the meteoritic nuggets. The run products and their comparison with a meteoritic nugget‐bearing CAI is evidence for formation of refractory metal nuggets during cooling of Ca, Al‐rich liquids at rates about 1000°/40 s (in the interval from 1900 to 900 °C). To achieve the formation of refractory metal nuggets and the textures observed in the host inclusions, during cooling the rate probably changed. Refractory metal nuggets apparently formed during quenching before spinel crystallized.     

Searching for calcium‐aluminum‐rich inclusions in cometary particles with Rosetta/COSIMA

The calcium‐aluminum‐rich inclusions (CAIs) found in chondritic meteorites are probably the oldest solar system solids, dating back to 4567.30 ± 0.16 million years ago. They are thought to have formed in the protosolar nebula within a few astronomical units of the Sun, and at a temperature of around 1300 K. The Stardust mission found evidence of CAI‐like material in samples recovered from comet Wild 2. The appearance of CAIs in comets, which are thought to be formed at lower temperatures and larger distances from the Sun, is only explicable if some mechanism allows the efficient transfer of such objects from the inner solar nebula to the outer solar nebula. Such mechanisms have been proposed such as an X‐wind or turbulence. In this work, particles collected from within the coma of comet 67P/Churyumov–Gerasimenko are examined for compositional evidence of the presence of CAIs. COSIMA (the Cometary Secondary Ion Mass Analyzer) uses secondary ion mass spectrometry to analyze the composition of cometary dust captured on metal targets. While CAIs can have a radius of centimeters, they are more typically a few hundred microns in size, and can be smaller than 1 μm, so it is conceivable that particles visible on COSIMA targets (ranging in size from about 10 μm to hundreds of microns) could contain CAIs. Using a peak fitting technique, the composition of a set of 13 particles was studied, looking for material rich in both calcium and aluminum. One such particle was found.     

Mineralogy,petrography, bulk chemical,iodine-xenon,and oxygen-isotopic compositions of dark inclusions in the reduced CV3 chondrite Efremovka

Abstract— We studied the petrography, mineralogy, bulk chemical, I-Xe, and O-isotopic compositions of three dark inclusions (E39, E53, and E80) in the reduced CV3 chondrite Efremovka. They consist of chondrules, calcium-aluminum-rich inclusions (CAIs), and fine-grained matrix. Primary minerals in chondrules and CAIs are pseudomorphed to various degrees by a mixture largely composed of abundant (>95%), fine-grained (>0.2 μm) fayalitic olivine (Fa_35–42) and minor amounts of chlorite, poorly-crystalline Si-Al-rich material, and chromite; chondrule and CAI shapes and textures are well-preserved. Secondary Ca-rich minerals (Ti-andradite, kirschsteinite, Fe-diopside) are common in chondrule pseudomorphs and matrices in E39 and E80. The degree of replacement increases from E53 to E39 to E80. Fayalitic olivines are heavily strained and contain abundant voids similar to those in incompletely dehydrated phyllosilicates in metamorphosed CM and CI chondrites. Opaque nodules in chondrules consist of Ni- and Co-rich taenite, Co-rich kamacite, and wairauite; sulfides are rare; magnetite is absent. Bulk O-isotopic compositions of E39 and E53 plot in the field of aqueously altered CM chondrites, close to the terrestrial fractionation line; the more heavily altered E39 is isotopically heavier than the less altered E53. The apparent I-Xe age of E53 is 5.4 Ma earlier than Bjurböle and 5.7 ± 2.0 Ma earlier than E39. The I-Xe data are consistent with the most heavily altered dark inclusion, E39 having experienced either longer or later alteration than E53. Bulk lithophile elements in E39 and E53 most closely match those of CO chondrites, except that Ca is depleted and K and As are enriched. Both inclusions are depleted in Se by factors of 3–5 compared to mean CO, CV, CR, or CK chondrites. Zinc in E39 is lower than the mean of any carbonaceous chondrite groups, but in E53 Zn is similar to the means in CO, CV, and CK chondrites. The Efremovka dark inclusions experienced various degrees of aqueous alteration, followed by low degree thermal metamorphism in an asteroidal environment. These processes resulted in preferential oxidation of Fe from opaque nodules and formation of Ni- and Co-rich metal, metasomatic alteration of primary minerals in chondrules and CAIs, and the formation of fayalitic olivine and secondary Ca-Fe-rich minerals. Based on the observed similarities of the alteration mineralization in the Efremovka and Allende dark inclusions, we infer that the latter may have experienced similar alteration processes.     

Unmelted cosmic metal particles in the Indian Ocean

Fe‐Ni metal is a common constituent of most meteorites and is an indicator of the thermal history of the respective meteorites, it is a diagnostic tool to distinguish between groups/subgroups of meteorites. In spite of over a million micrometeorites collected from various domains, reports of pure metallic particles among micrometeorites have been extremely rare. We report here the finding of a variety of cosmic metal particles such as kamacite, plessite, taenite, and Fe‐Ni beads from deep‐sea sediments of the Indian Ocean, a majority of which have entered the Earth unaffected by frictional heating during atmospheric entry. Such particles are known as components of meteorites but have never been found as individual entities. Their compositions suggest precursors from a variety of meteorite groups, thus providing an insight into the metal fluxes on the Earth. Some particles have undergone heating and oxidation to different levels during entry developing features similar to I‐type cosmic spherules, suggesting atmospheric processing of individual kamacites/taenite grains as another hitherto unknown source for the I‐type spherules. The particles have undergone postdepositional aqueous alteration transforming finally into the serpentine mineral cronstedtite. Aqueous alteration products of kamacite reflect the local microenvironment, therefore they have the potential to provide information on the composition of water in the solar nebula, on the parent bodies or on surfaces of planetary bodies. Our observations suggest it would take sustained burial in water for tens of thousands of years under cold conditions for kamacites to alter to cronstedtite.     

Alkali‐halogen metasomatism of the CM carbonaceous chondrites

Meteorite Hills (MET) 01075 is unique among the CM carbonaceous chondrites in containing the feldspathoid mineral sodalite, and hence it may provide valuable evidence for a nebular or parent body process that has not been previously recorded by this meteorite group. MET 01075 is composed of aqueously altered chondrules and calcium‐ and aluminum‐rich inclusions (CAIs) in a matrix that is predominantly made of serpentine‐ and tochilinite‐rich particles. The chondrules have been impact flattened and define a foliation petrofabric. Sodalite occurs in a 0.6 mm size CAI that also contains spinel, perovskite, and diopside together with Fe‐rich phyllosilicate and calcite. By analogy with feldspathoid‐bearing CAIs in the CV and CO carbonaceous chondrites, the sodalite is interpreted to have formed by replacement of melilite or anorthite during alkali‐halogen metasomatism in a parent body environment. While it is possible that the CAI was metasomatized in a precursor parent body, then excavated and incorporated into the MET 01075 parent body, in situ metasomatism is the favored model. The brief episode of relatively high temperature water–rock interaction was driven by radiogenic or impact heating, and most of the evidence for metasomatism was erased by subsequent lower temperature aqueous alteration. MET 01075 is very unusual in sampling a CM parent body region that underwent early alkali‐halogen metasomatism and has retained one of its products.     

Compositions and microstructures of CB sulfides: Implications for the thermal history of the CB chondrite parent body

We studied textures and compositions of sulfide inclusions in unzoned Fe,Ni metal particles within CBa Gujba, CBa Weatherford, CBb HH 237, and CBb QUE 94411 in order to constrain formation conditions and secondary thermal histories on the CB parent body. Unzoned metal particles in all four chondrites have very similar metal and sulfide compositions. Metal particles contain different types of sulfides, which we categorize as: homogeneous low‐Cr sulfides composed of troilite, troilite‐containing exsolved daubreelite lamellae, arcuate sulfides that occur along metal grain boundaries, and shock‐melted sulfides composed of a mixture of troilite and Fe, Ni metal. Our model for formation proposes that the unzoned metal particles were initially metal droplets that formed from splashing by a partially molten impacting body. Sulfide inclusions later formed as a result of precipitation of excess S from solid metal at low temperatures, either during single stage cooling or during a reheating event by impacts. Sulfides containing exsolution lamellae record temperatures of ?600 °C, and irregular Fe‐FeS intergrowth textures suggest localized shock melting, both of which are indicative of heterogeneous heating by impact processes on the CB parent body. Our study shows that CBa and CBb chondrites formed in a similar environment, and also experienced similar secondary impact processing.     

Forsterite‐rich accretionary rims around calcium‐aluminum‐rich inclusions from the reduced CV3 chondrite Efremovka

Abstract— It was suggested that multilayered accretionary rims composed of ferrous olivine, andradite, wollastonite, salite‐hedenbergitic pyroxenes, nepheline, and Ni‐rich sulfides around Allende calcium‐aluminum‐rich inclusions (CAIs) are aggregates of gas‐solid condensates which reflect significant fluctuations in physico‐chemical conditions in the slowly cooling solar nebula and grain/gas separation processes. In order to test this model, we studied the mineralogy of accretionary rims around one type A CAI (E104) and one type B CAI (E48) from the reduced CV3 chondrite Efremovka, which is less altered than Allende. In contrast to the Allende accretionary rims, those in Efremovka consist of coarse‐grained (20–40 μm), anhedral forsterite (Fa_1–8), Fe, Ni‐metal nodules, amoeboid olivine aggregates (AOAs) and fine‐grained CAIs composed of Al‐diopside, anorthite, and spinel, ± forsterite. Although the fine‐grained CAIs, AOAs and host CAIs are virtually unaltered, a hibonite‐spinel‐perovskite CAI in the E48 accretionary rim experienced extensive alteration, which resulted in the formation of Fe‐rich, Zn‐bearing spinel, and a Ca, Al, Si‐hydrous mineral. Forsterites in the accretionary rims typically show an aggregational nature and consist of small olivine grains with numerous pores and tiny inclusions of Al‐rich minerals. No evidence for the replacement of forsterite by enstatite was found; no chondrule fragments were identified in the accretionary rims. We infer that accretionary rims in Efremovka are more primitive than those in Allende and formed by aggregation of high‐temperature condensates around host CAIs in the CAI‐forming regions. The rimmed CAIs were removed from these regions prior to condensation of enstatite and alkalies. The absence of andradite, wollastonite, and hedenbergite from the Efremovka rims may indicate that these rims sampled different nebular regions than the Allende rims. Alternatively, the Ca, Fe‐rich silicates rimming Allende CAIs may have resulted from late‐stage metasomatic alteration, under oxidizing conditions, of original Efremovka‐like accretionary rims. The observed differences in O‐isotope composition between forsterite and Ca, Fe‐rich minerals in the Allende accretionary rims (Hiyagon, 1998) suggest that the oxidizing fluid had an ¹⁶O‐poor oxygen isotopic composition.     

Primary iron sulfides in CM and CR carbonaceous chondrites: Insights into nebular processes

We have carried out a systematic study involving SEM, EPMA, and TEM analyses to determine the textures and compositions of sulfides and sulfide–metal assemblages in a suite of minimally to weakly altered CM and CR carbonaceous chondrites. We have attempted to constrain the distribution and origin of primary sulfides that formed in the solar nebula, rather than by secondary asteroidal alteration processes. Our study focused primarily on sulfide assemblages associated with chondrules, but also examined some occurrences of sulfides within the matrices of these meteorites. Although sulfides are a minor phase in carbonaceous chondrites, we have determined that primary sulfide grains are actually a major proportion of the sulfide grains in weakly altered CM chondrites and have survived aqueous alteration relatively unscathed. In minimally altered CR chondrites, we have determined that essentially all of the sulfides are of primary origin, confirming the observations of Schrader et al. ( 2015 ). The pyrrhotite–pentlandite intergrowth (PPI) grains formed from crystallization of monosulfide solid solution (mss) melts, while sulfide-rimmed metal (SRM) grains formed from sulfidization of Fe,Ni metal. Micron-sized metal inclusions in some PPI grains may have formed by co-crystallization of metal and sulfide from a sulfide melt that experienced S volatilization during the chondrule formation event, or alternatively, may be a remnant of sulfidization of Fe,Ni metal that also occurred during chondrule formation. Sulfur fugacity for SRM grains ranged from −18 to −10 (log units) largely in agreement with predicted solar nebular values. Our observations show that understanding the formation mechanisms of primary sulfide grains provides clues to solar nebular conditions, such as the sulfur fugacity during chondrule formation.     

Oxygen isotopic and chemical zoning of melilite crystals in a type A Ca‐Al‐rich inclusion of Efremovka CV3 chondrite

Abstract– Different oxygen isotopic reservoirs have been recognized in the early solar system. Fluffy type A Ca‐Al‐rich inclusions (CAIs) are believed to be direct condensates from a solar nebular gas, and therefore, have acquired oxygen from the solar nebula. Oxygen isotopic and chemical compositions of melilite crystals in a type A CAI from Efremovka CV3 chondrite were measured to reveal the temporal variation in oxygen isotopic composition of surrounding nebular gas during CAI formation. The CAI is constructed of two domains, each of which has a core‐mantle structure. Reversely zoned melilite crystals were observed in both domains. Melilite crystals in one domain have a homogeneous ¹⁶O‐poor composition on the carbonaceous chondrite anhydrous mineral (CCAM) line of δ¹⁸O = 5–10‰, which suggests that the domain was formed in a ¹⁶O‐poor oxygen isotope reservoir of the solar nebula. In contrast, melilite crystals in the other domain have continuous variations in oxygen isotopic composition from ¹⁶O‐rich (δ¹⁸O = ?40‰) to ¹⁶O‐poor (δ¹⁸O = 0‰) along the CCAM line. The oxygen isotopic composition tends to be more ¹⁶O‐rich toward the domain rim, which suggests that the domain was formed in a variable oxygen isotope reservoir of the solar nebula. Each domain of the type A CAI has grown in distinct oxygen isotope reservoir of the solar nebula. After the domain formation, domains were accumulated together in the solar nebula to form a type A CAI.     

The CR chondrite clan: Implications for early solar system processes

Abstract— In this paper, we review the mineralogy and chemistry of calcium‐aluminum‐rich inclusions (CAIs), chondrules, FeNi‐metal, and fine‐grained materials of the CR chondrite clan, including CR, CH, and the metal‐rich CB chondrites Queen Alexandra Range 94411, Hammadah al Hamra 237, Bencubbin, Gujba, and Weatherford. The members of the CR chondrite clan are among the most pristine early solar system materials, which largely escaped thermal processing in an asteroidal setting (Bencubbin, Weatherford, and Gujba may be exceptions) and provide important constraints on the solar nebula models. These constraints include (1) multiplicity of CAI formation; (2) formation of CAIs and chondrules in spatially separated nebular regions; (3) formation of CAIs in gaseous reservoir(s) having ¹⁶O‐rich isotopic compositions; chondrules appear to have formed in the presence of ¹⁶O‐poor nebular gas; (4) isolation of CAIs and chondrules from nebular gas at various ambient temperatures; (5) heterogeneous distribution of ²⁶Al in the solar nebula; and (6) absence of matrix material in the regions of CAI and chondrule formation.     

Microdistribution of primordial Ne and Ar in fine‐grained rims,matrices, and dark inclusions of unequilibrated chondrites—Clues on nebular processes

Abstract— The low temperature fine‐grained material in unequilibrated chondrites, which occurs as matrix, rims, and dark inclusions, carries information about the solar nebula and the earliest stages of planetesimal accretion. The microdistribution of primordial noble gases among these components helps to reveal their accretionary and alteration histories. We measured the Ne and Ar isotopic ratios and concentrations of small samples of matrix, rims, and dark inclusions from the unequilibrated carbonaceous chondrites Allende (CV3), Leoville (CV3), and Renazzo (CR2) and from the ordinary chondrites Semarkona (LL3.0), Bishunpur (LL3.1), and Krymka (LL3.1) to decipher their genetic relationships. The primordial noble gas concentrations of Semarkona, and—with certain restrictions—also of Leoville, Bishunpur, and Allende decrease from rims to matrices. This indicates a progressive accretion of nebular dust from regions with decreasing noble gas contents and cannot be explained by a formation of the rims on parent bodies. The decrease is probably due to dilution of the noble‐gas‐carrying phases with noble‐gas‐poor material in the nebula. Krymka and Renazzo both show an increase of primordial noble gas concentrations from rims to matrices. In the case of Krymka, this indicates the admixture of noble gas‐rich dust to the nebular region from which first rims and then matrix accreted. This also explains the increase of the primordial elemental ratio 36Ar/ 20Ne from rims to matrix. Larger clasts of the noble‐gas‐rich dust form macroscopic dark inclusions in this meteorite, which seem to represent unusually pristine material. The interpretation of the Renazzo data is ambiguous. Rims could have formed by aqueous alteration of matrix or—as in the case of Krymka—by progressive admixture of noble gas‐rich dust to the reservoir from which the Renazzo constituents accreted. The Leoville and Krymka dark inclusions, as well as one dark inclusion of Allende, show noble gas signatures different from those of the respective host meteorites. The Allende dark inclusion probably accreted from the same region as Allende rims and matrix but suffered a higher degree of alteration. The Leoville and Krymka dark inclusions must have accreted from regions different from those of their respective rims and matrices and were later incorporated into their host meteorites. The noble gas data imply a heterogeneous reservoir with respect to its primordial noble gas content in the accretion region of the studied meteorites. Further studies will have to decide whether these differences are primary or evolved from an originally uniform reservoir.     

Noble gas studies in CAIs from CV3 chondrites: No evidence for primordial noble gases

Abstract— Calcium‐aluminum‐rich inclusions (CAIs) were among the first solids in the solar system and were, similar to chondrules, created at very high temperatures. While in chondrules, trapped noble gases have recently been detected, the presence of trapped gases in CAIs is unclear but could have important implications for CAI formation and for early solar system evolution in general. To reassess this question, He, Ne, and Ar isotopes were measured in small, carefully separated and, thus, uncontaminated samples of CAIs from the CV3 chondrites Allende, Axtell, and Efremovka. The ²⁰Ne/²²Ne ratios of all CAIs studied here are <0.9, indicating the absence of trapped Ne as, e.g., Ne‐HL, Ne‐Q, or solar wind Ne. The ²¹Ne/²²Ne ratios range from 0.86 to 0.72, with fine‐grained, more altered CAIs usually showing lower values than coarse‐grained, less altered CAIs. This is attributed to variable amounts of cosmogenic Ne produced from Na‐rich alteration phases rather than to the presence of Ne‐G or Ne‐R (essentially pure ²²Ne) in the samples. Our interpretation is supported by model calculations of the isotopic composition of cosmogenic Ne in minerals common in CAIs. The ³⁶Ar/³⁸Ar ratios are between 0.7 and 4.8, with fine‐grained CAIs within one meteorite showing higher ratios than the coarse‐grained ones. This agrees with higher concentrations of cosmogenic ³⁶Ar produced by neutron capture on ³⁵Cl with subsequent β^?‐decay in finer‐grained, more altered, and thus, more Cl‐rich CAIs than in coarser‐grained, less altered ones. Although our data do not strictly contradict the presence of small amounts of Ne‐G, Ne‐R, or trapped Ar in the CAIs, our noble gas signatures are most simply explained by cosmogenic production, mainly from Na‐, Ca‐, and Cl‐rich minerals.     

Ca,Al‐rich inclusions in Rumuruti (R) chondrites

Abstract— Rumuruti chondrites (R chondrites) constitute a well‐characterized chondrite group different from carbonaceous, ordinary, and enstatite chondrites. Many of these meteorites are breccias containing primitive type 3 fragments as well as fragments of higher petrologic type. Ca,Al‐rich inclusions (CAIs) occur within all lithologies. Here, we present the results of our search for and analysis of Al‐rich objects in Rumuruti chondrites. We studied 20 R chondrites and found 126 Ca,Al‐rich objects (101 CAIs, 19 Al‐rich chondrules, and 6 spinel‐rich fragments). Based on mineralogical characterization and analysis by SEM and electron microprobe, the inclusions can be grouped into six different types: (1) simple concentric spinel‐rich inclusions (42), (2) fassaite‐rich spherules, (3) complex spinel‐rich CAIs (53), (4) complex diopside‐rich inclusions, (5) Al‐rich chondrules, and (6) Al‐rich (spinel‐rich) fragments. The simple concentric and complex spinel‐rich CAIs have abundant spinel and, based on the presence or absence of different major phases (fassaite, hibonite, Na,Al‐(Cl)‐rich alteration products), can be subdivided into several subgroups. Although there are some similarities between CAIs from R chondrites and inclusions from other chondrite groups with respect to their mineral assemblages, abundance, and size, the overall assemblage of CAIs is distinct to the R‐chondrite group. Some Ca,Al‐rich inclusions appear to be primitive (e.g., low FeO‐contents in spinel, low abundances of Na,Al‐(Cl)‐rich alteration products; abundant perovskite), whereas others were highly altered by nebular and/or parent body processes (e.g., high concentrations of FeO and ZnO in spinel, ilmenite instead of perovskite, abundant Na,Al‐(Cl)‐rich alteration products). There is complete absence of grossite and melilite, which are common in CAIs from most other groups. CAIs from equilibrated R‐chondrite lithologies have abundant secondary Ab‐rich plagioclase (oligoclase) and differ from those in unequilibrated type 3 lithologies which have nepheline and sodalite instead.     

Calcium-aluminum-rich inclusion found in the Ivuna CI chondrite: Are CI chondrites a good proxy for the bulk composition of the solar system?

Cosmochemists have relied on CI carbonaceous chondrites as proxies for chemical composition of the non-volatile elements in the solar system because these meteorites are fine-grained, chemically homogeneous, and have well-determined bulk compositions that agree with that of the solar photosphere, within uncertainties. Here we report the discovery of a calcium-aluminum-rich inclusion (CAI) in the Ivuna CI chondrite. CAIs are chemically highly fractionated compared to CI composition, consisting of refractory elements and having textures that either reflect condensation from nebular gas or melting in a nebular environment. The CAI we found is a compact type A CAI with typical ¹⁶O-rich oxygen. However, it shows no evidence of ²⁶Al, which was present when most CAIs formed. Finding a CAI in a CI chondrite raises serious questions about whether CI chondrites are a reliable proxy for the bulk composition of the solar system. Too much CAI material would show up as mismatches between the CI composition and the composition of the solar photosphere. Although small amounts of refractory material have previously been identified in CI chondrites, this material is not abundant enough to significantly perturb the bulk compositions of CI chondrites. The agreement between the composition of the solar photosphere and CI chondrites allows no more than ~0.5 atom% of CAI-like material to have been added to CI chondrites. As the compositions of CI chondrites, carbonaceous asteroids, and the solar photosphere are better determined, we will be able to reduce the uncertainties in our estimates of the composition of the solar system.     

Fe‐Ni metal and sulfide minerals in CM chondrites: An indicator for thermal history

Abstract– CM chondrites were subjected to aqueous alteration and, in some cases, to secondary metamorphic heating. The effects of these processes vary widely, and have mainly been documented in silicate phases. Herein, we report the characteristic features of Fe‐Ni metal and sulfide phases in 13 CM and 2 CM‐related chondrites to explore the thermal history of these chondrites. The texture and compositional distribution of the metal in CM are different from those in unequilibrated ordinary and CO chondrites, but most have similarities to those in highly primitive chondrites, such as CH, CR, and Acfer 094. We classified the CM samples into three categories based on metal composition and sulfide texture. Fe‐Ni metal in category A is kamacite to martensite. Category B is characterized by pyrrhotite grains always containing blebs or lamellae of pentlandite. Opaque mineral assemblages of category C are typically kamacite, Ni‐Co‐rich metal, and pyrrhotite. These categories are closely related to the degree of secondary heating and are not related to degree of the aqueous alteration. The characteristic features of the opaque minerals can be explained by secondary heating processes after aqueous alteration. Category A CM chondrites are unheated, whereas those in category B experienced small degrees of secondary heating. CMs in category C were subjected to the most severe secondary heating process. Thus, opaque minerals can provide constraints on the thermal history for CM chondrites.     

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.     

On unfractionated solar noble gases in the H3–6 meteorite Acfer 111

Abstract Solar noble gases He, Ne, Ar and Kr implanted in the H3–6 meteorite regolith breccia Acfer 111 agree in their elemental composition with that in present-day solar wind and, except for a 25% deficit of ⁴He, also with adopted solar abundances. The presence of such unfractionated solar gases makes Acfer 111 unique (until now). Closed system stepped etching releases noble gases that can be explained as mixtures of two distinct types of He, Ne, and Kr of isotopic compositions as they have been derived previously from meteorites and lunar samples that contain heavily fractionated solar gases. Since the same putative end members, ascribed to the solar wind (SW) and supra-thermal solar energetic particles (SEP), are also present in Acfer 111, we argue that these end members represent two truly independent components. We discount the possibility that one isotopic composition derived from the other by diffusion of the gases within, or upon their release from, their host phases. The isotopic signatures of noble gases in Acfer 111 agree with those in a lunar ilmenite of young antiquity ?100 Ma) but are in disagreement with the noble gases in lunar ilmenite 79035 of 1–2 Ga antiquity. Systematic changes are discussed of the nuclide abundance ratios as etching proceeds; they are ascribed to differences in trapping efficiency and in penetration depth of the different noble gas ion species upon their implantation.