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1.
Abstract— Amoeboid olivine aggregates (AOAs) are irregularly shaped, fine‐grained aggregates of olivine and Ca, Al‐rich minerals and are important primitive components of CR chondrites. The AOAs in CR chondrites contain FeNi metal, and some AOAs contain Mn‐rich forsterite with up to 0.7 MnO and Mn:Fe ratios greater than one. Additionally, AOAs in the CR chondrites do not contain secondary phases (nepheline and fayalitic olivine) that are found in AOAs in some CV chondrites. The AOAs in CR chondrites record a complex petrogenetic history that included nebular gas‐solid condensation, reaction of minerals with the nebular gas, small degrees of melting, and sintering of the assemblage. A condensation origin for the Mn‐rich forsterite is proposed. The Mn‐rich forsterite found in IDPs, unequilibrated ordinary chondrite matrix, and AOAs in CR chondrites may have had a similar origin. A type A calcium, aluminum‐rich inclusion (CAI) with an AOA attached to its Wark‐Lovering rim is also described. This discovery reveals a temporal relationship between AOAs and type A inclusions. Additionally, a thin layer of forsterite is present as part of the Wark‐Lovering rim, revealing the crystallization of olivine at the end stages of Wark‐Lovering rim formation. The Ca, Al‐rich nodules in the AOAs may be petrogenetically related to the Ca, Al‐rich minerals in Wark‐Lovering rims on type A CAIs. AOAs are chondrite components that condensed during the final stage of Wark‐Lovering rim formation but, in general, were temporally, spatially, or kinetically isolated from reacting with the nebula vapor during condensation of the lower temperature minerals that were commonly present as chondrule precursors.  相似文献   

2.
Two compound calcium‐aluminum‐rich inclusions (CAIs), 3N from the oxidized CV chondrite Northwest Africa (NWA) 3118 and 33E from the reduced CV chondrite Efremovka, contain ultrarefractory (UR) inclusions. 3N is a forsterite‐bearing type B (FoB) CAI that encloses UR inclusion 3N‐24 composed of Zr,Sc,Y‐rich oxides, Y‐rich perovskite, and Zr,Sc‐rich Al,Ti‐diopside. 33E contains a fluffy type A (FTA) CAI and UR CAI 33E‐1, surrounded by Wark‐Lovering rim layers of spinel, Al‐diopside, and forsterite, and a common forsterite‐rich accretionary rim. 33E‐1 is composed of Zr,Sc,Y‐rich oxides, Y‐rich perovskite, Zr,Sc,Y‐rich pyroxenes (Al,Ti‐diopside, Sc‐rich pyroxene), and gehlenite. 3N‐24’s UR oxides and Zr,Sc‐rich Al,Ti‐diopsides are 16O‐poor (Δ17O approximately ?2‰ to ?5‰). Spinel in 3N‐24 and spinel and Al‐diopside in the FoB CAI are 16O‐rich (Δ17O approximately ?23 ± 2‰). 33E‐1’s UR oxides and Zr,Sc‐rich Al,Ti‐diopsides are 16O‐depleted (Δ17O approximately ?2‰ to ?5‰) vs. Al,Ti‐diopside of the FTA CAI and spinel (Δ17O approximately ?23 ± 2‰), and Wark‐Lovering rim Al,Ti‐diopside (Δ17O approximately ?7‰ to ?19‰). We infer that the inclusions experienced multistage formation in nebular regions with different oxygen‐isotope compositions. 3N‐24 and 33E‐1’s precursors formed by evaporation/condensation above 1600 °C. 3N and 33E’s precursors formed by condensation and melting (3N only) at significantly lower temperatures. 3N‐24 and 3N’s precursors aggregated into a compound object and experienced partial melting and thermal annealing. 33E‐1 and 33E avoided melting prior to and after aggregation. They acquired Wark‐Lovering and common forsterite‐rich accretionary rims, probably by condensation, followed by thermal annealing. We suggest 3N‐24 and 33E‐1 originated in a 16O‐rich gaseous reservoir and subsequently experienced isotope exchange in a 16O‐poor gaseous reservoir. Mechanism and timing of oxygen‐isotope exchange remain unclear.  相似文献   

3.
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 (Fa1–8), 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.  相似文献   

4.
Abstract— Recipes are presented for synthesizing various type A and type B Ca‐Al‐rich inclusions (CAIs), refractory volatilization residues, and the minerals forsterite and melilite that are required for experiments. These experiments (described in other works) aim to make two determinations: 1) the conditions under which the surfaces of CAIs were either “flash‐heated” or “volatilized subsolidus” to form a temporary ultra‐refractory residue, and 2) the conditions under which the residue was then metasomatized to form the mineral layers making up Wark‐Lovering (WL) rims on CAIs.  相似文献   

5.
Abstract— Primary minerals in calcium‐aluminum‐rich inclusions (CAIs), Al‐rich and ferromagnesian chondrules in each chondrite group have δ18O values that typically range from ?50 to +5%0. Neglecting effects due to minor mass fractionations, the oxygen isotopic data for each chondrite group and for micrometeorites define lines on the three‐isotope plot with slopes of 1.01 ± 0.06 and intercepts of ?2 ± 1. This suggests that the same kind of nebular process produced the 16O variations among chondrules and CAIs in all groups. Chemical and isotopic properties of some CAIs and chondrules strongly suggest that they formed from solar nebula condensates. This is incompatible with the existing two‐component model for oxygen isotopes in which chondrules and CAIs were derived from heated and melted 16O‐rich presolar dust that exchanged oxygen with 16O‐poor nebular gas. Some FUN CAIs (inclusions with isotope anomalies due to fractionation and unknown nuclear effects) have chemical and isotopic compositions indicating they are evaporative residues of presolar material, which is incompatible with 16O fractionation during mass‐independent gas phase reactions in the solar nebula. There is only one plausible reason why solar nebula condensates and evaporative residues of presolar materials are both enriched in 16O. Condensation must have occurred in a nebular region where the oxygen was largely derived from evaporated 16O‐rich dust. A simple model suggests that dust was enriched (or gas was depleted) relative to cosmic proportions by factors of ～10 to >50 prior to condensation for most CAIs and factors of 1–5 for chondrule precursor material. We infer that dust‐gas fractionation prior to evaporation and condensation was more important in establishing the oxygen isotopic composition of CAIs and chondrules than any subsequent exchange with nebular gases. Dust‐gas fractionation may have occurred near the inner edge of the disk where nebular gases accreted into the protosun and Shu and colleagues suggest that CAIs formed.  相似文献   

6.
Abstract— Fassaite is a major component of Ca‐Al‐rich inclusions (CAIs) of Types B and C that crystallized from liquids. In contrast, this mineral is rarely reported in Type A inclusions and has been much less studied. In this paper, we report highly Ti‐, Al‐enriched fassaite that occurs as rims on perovskite in two compact Type A inclusions from the Ningqiang meteorite. In addition, one of the inclusions contains an euhedral grain of Sc‐fassaite (16.4 wt% Sc2O3) isolated in melilite. The occurrence and mineral chemistry of the fassaite rims can be explained by a reaction of pre‐existing perovskite with CAI melts. Hence, such rims may serve as an indicator for partial melting of Type A inclusions. The Sc‐fassaite is probably a relict grain. A third spherical CAI contains several euhedral grains of V‐fassaite (4.8–5.4 wt% V2O3) enclosed in a melilite fragment. The high V content of fassaite cannot be related to any Fremdlinge, magnetite, or metallic Fe‐Ni, because these phases are absent in the inclusion. In the same CAI, other fassaites intergrow with spinel and minor perovskite, filling voids inside of the melilite and space adjacent to the Wark‐Lovering rim. The fassaite intergrown with spinel is almost V‐free. The coexistence of two types of fassaite suggests that this CAI has not been completely melted.  相似文献   

7.
Abstract— Fine‐grained, spinel‐rich inclusions in the reduced CV chondrites Efremovka and Leoville consist of spinel, melilite, anorthite, Al‐diopside, and minor hibonite and perovskite; forsterite is very rare. Several CAIs are surrounded by forsterite‐rich accretionary rims. In contrast to heavily altered fine‐grained CAIs in the oxidized CV chondrite Allende, those in the reduced CVs experienced very little alteration (secondary nepheline and sodalite are rare). The Efremovka and Leoville fine‐grained CAIs are 16O‐enriched and, like their Allende counterparts, generally have volatility fractionated group II rare earth element patterns. Three out of 13 fine‐grained CAIs we studied are structurally uniform and consist of small concentrically zoned nodules having spinel ± hibonite ± perovskite cores surrounded by layers of melilite and Al‐diopside. Other fine‐grained CAIs show an overall structural zonation defined by modal mineralogy differences between the inclusion cores and mantles. The cores are melilite‐free and consist of tiny spinel ± hibonite ± perovskite grains surrounded by layers of anorthite and Al‐diopside. The mantles are calcium‐enriched, magnesium‐depleted and coarsergrained relative to the cores; they generally contain abundant melilite but have less spinel and anorthite than the cores. The bulk compositions of fine‐grained CAIs generally show significant fractionation of Al from Ca and Ti, with Ca and Ti being depleted relative to Al; they are similar to those of coarsegrained, type C igneous CAIs, and thus are reasonable candidate precursors for the latter. The finegrained CAIs originally formed as aggregates of spinel‐perovskite‐melilite ± hibonite gas‐solid condensates from a reservoir that was 16O‐enriched but depleted in the most refractory REEs. These aggregates later experienced low‐temperature gas‐solid nebular reactions with gaseous SiO and Mg to form Al‐diopside and ±anorthite. The zoned structures of many of the fine‐grained inclusions may be the result of subsequent reheating that resulted in the evaporative loss of SiO and Mg and the formation of melilite. The inferred multi‐stage formation history of fine‐grained inclusions in Efremovka and Leoville is consistent with a complex formation history of coarse‐grained CAIs in CV chondrites.  相似文献   

8.
Abstract— The metal‐rich chondrites Hammadah al Hamra (HH) 237 and Queen Alexandra Range (QUE) 94411, paired with QUE 94627, contain relatively rare (<1 vol%) calcium‐aluminum‐rich inclusions (CAIs) and Al‐diopside‐rich chondrules. Forty CAIs and CAI fragments and seven Al‐diopside‐rich chondrules were identified in HH 237 and QUE 94411/94627. The CAIs, ～50–400 μm in apparent diameter, include (a) 22 (56%) pyroxene‐spinel ± melilite (+forsterite rim), (b) 11 (28%) forsterite‐bearing, pyroxene‐spinel ± melilite ± anorthite (+forsterite rim) (c) 2 (5%) grossite‐rich (+spinel‐melilite‐pyroxene rim), (d) 2 (5%) hibonite‐melilite (+spinel‐pyroxene ± forsterite rim), (e) 1 (2%) hibonite‐bearing, spinel‐perovskite (+melilite‐pyroxene rim), (f) 1 (2%) spinel‐melilite‐pyroxene‐anorthite, and (g) 1 (2%) amoeboid olivine aggregate. Each type of CAI is known to exist in other chondrite groups, but the high abundance of pyroxene‐spinel ± melilite CAIs with igneous textures and surrounded by a forsterite rim are unique features of HH 237 and QUE 94411/94627. Additionally, oxygen isotopes consistently show relatively heavy compositions with Δ17O ranging from ?6%0 to ?10%0 (1σ = 1.3%0) for all analyzed CAI minerals (grossite, hibonite, melilite, pyroxene, spinel). This suggests that the CAIs formed in a reservoir isotopically distinct from the reservoir(s) where “normal”, 16O‐rich (Δ17O < ?20%0) CAIs in most other chondritic meteorites formed. The Al‐diopside‐rich chondrules, which have previously been observed in CH chondrites and the unique carbonaceous chondrite Adelaide, contain Al‐diopside grains enclosing oriented inclusions of forsterite, and interstitial anorthitic mesostasis and Al‐rich, Ca‐poor pyroxene, occasionally enclosing spinel and forsterite. These chondrules are mineralogically similar to the Al‐rich barred‐olivine chondrules in HH 237 and QUE 94411/94627, but have lower Cr concentrations than the latter, indicating that they may have formed during the same chondrule‐forming event, but at slightly different ambient nebular temperatures. Aluminum‐diopside grains from two Al‐diopside‐rich chondrules have O‐isotopic compositions (Δ17O ? ?7 ± 1.1 %0) similar to CAI minerals, suggesting that they formed from an isotopically similar reservoir. The oxygen‐isotopic composition of one Ca, Al‐poor cryptocrystalline chondrule in QUE 94411/94627 was analyzed and found to have Δ17O ? ?3 ± 1.4%0. The characteristics of the CAIs in HH 237 and QUE 94411/94627 are inconsistent with an impact origin of these metal‐rich meteorites. Instead they suggest that the components in CB chondrites are pristine products of large‐scale, high‐temperature processes in the solar nebula and should be considered bona fide chondrites.  相似文献   

9.
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.  相似文献   

10.
Chondrites consist of three major components: refractory inclusions (Ca,Al‐rich inclusions [CAIs] and amoeboid olivine aggregates), chondrules, and matrix. Here, I summarize recent results on the mineralogy, petrology, oxygen, and aluminum‐magnesium isotope systematics of the chondritic components (mainly CAIs in carbonaceous chondrites) and their significance for understanding processes in the protoplanetary disk (PPD) and on chondrite parent asteroids. CAIs are the oldest solids originated in the solar system: their U‐corrected Pb‐Pb absolute age of 4567.3 ± 0.16 Ma is considered to represent time 0 of its evolution. CAIs formed by evaporation, condensation, and aggregation in a gas of approximately solar composition in a hot (ambient temperature >1300 K) disk region exposed to irradiation by solar energetic particles, probably near the protoSun; subsequently, some CAIs were melted in and outside their formation region during transient heating events of still unknown nature. In unmetamorphosed, type 2–3.0 chondrites, CAIs show large variations in the initial 26Al/27Al ratios, from <5 × 10–6 to ~5.25 × 10–5. These variations and the inferred low initial abundance of 60Fe in the PPD suggest late injection of 26Al by a wind from a nearby Wolf–Rayet star into the protosolar molecular cloud core prior to or during its collapse. Although there are multiple generations of CAIs characterized by distinct mineralogies, textures, and isotopic (O, Mg, Ca, Ti, Mo, etc.) compositions, the 26Al heterogeneity in the CAI‐forming region(s) precludes determining the duration of CAIs formation using 26Al‐26Mg systematics. The existence of multiple generations of CAIs and the observed differences in CAI abundances in carbonaceous and noncarbonaceous chondrites may indicate that CAIs were episodically formed and ejected by a disk wind from near the Sun to the outer solar system and then spiraled inward due to gas drag. In type 2–3.0 chondrites, most CAIs surrounded by Wark–Lovering rims have uniform Δ17O (= δ17O?0.52 × δ18O) of ~ ?24‰; however, there is a large range of Δ17O (from ~?40 to ~ ?5‰) among them, suggesting the coexistence of 16O‐rich (low Δ17O) and 16O‐poor (high Δ17O) gaseous reservoirs at the earliest stages of the PPD evolution. The observed variations in Δ17O of CAIs may be explained if three major O‐bearing species in the solar system (CO, H2O, and silicate dust) had different O‐isotope compositions, with H2O and possibly silicate dust being 16O‐depleted relative to both the Genesis solar wind Δ17O of ?28.4 ± 3.6‰ and even more 16O‐enriched CO. Oxygen isotopic compositions of CO and H2O could have resulted from CO self‐shielding in the protosolar molecular cloud (PMC) and the outer PPD. The nature of 16O‐depleted dust at the earliest stages of PPD evolution remains unclear: it could have either been inherited from the PMC or the initially 16O‐rich (solar‐like) MC dust experienced O‐isotope exchange during thermal processing in the PPD. To understand the chemical and isotopic composition of the protosolar MC material and the degree of its thermal processing in PPD, samples of the primordial silicates and ices, which may have survived in the outer solar system, are required. In metamorphosed CO3 and CV3 chondrites, most CAIs exhibit O‐isotope heterogeneity that often appears to be mineralogically controlled: anorthite, melilite, grossite, krotite, perovskite, and Zr‐ and Sc‐rich oxides and silicates are 16O‐depleted relative to corundum, hibonite, spinel, Al,Ti‐diopside, forsterite, and enstatite. In texturally fine‐grained CAIs with grain sizes of ~10–20 μm, this O‐isotope heterogeneity is most likely due to O‐isotope exchange with 16O‐poor (Δ17O ~0‰) aqueous fluids on the CO and CV chondrite parent asteroids. In CO3.1 and CV3.1 chondrites, this process did not affect Al‐Mg isotope systematics of CAIs. In some coarse‐grained igneous CV CAIs, O‐isotope heterogeneity of anorthite, melilite, and igneously zoned Al,Ti‐diopside appears to be consistent with their crystallization from melts of isotopically evolving O‐isotope compositions. These CAIs could have recorded O‐isotope exchange during incomplete melting in nebular gaseous reservoir(s) with different O‐isotope compositions and during aqueous fluid–rock interaction on the CV asteroid.  相似文献   

11.
Abstract— Anorthite‐rich chondrules in CR and CH carbonaceous chondrites consist of magnesian low‐Ca pyroxene and forsterite phenocrysts, FeNi‐metal nodules, interstitial anorthite, Al‐Ti‐Cr‐rich low‐Ca and high‐Ca pyroxenes, and crystalline mesostasis composed of silica, anorthite and high‐Ca pyroxene. Three anorthite‐rich chondrules contain relic calcium‐aluminum‐rich inclusions (CAIs) composed of anorthite, spinel, ±Al‐diopside, and ± forsterite. A few chondrules contain regions which are texturally and mineralogically similar to magnesian (type I) chondrules and consist of forsterite, low‐Ca pyroxene and abundant FeNi‐metal nodules. Anorthite‐rich chondrules in CR and CH chondrites are mineralogically similar to those in CV and CO carbonaceous chondrites, but contain no secondary nepheline, sodalite or ferrosilite. Relatively high abundances of moderately‐volatile elements such as Cr, Mn and Si in the anorthite‐rich chondrules suggest that these chondrules could not have been produced by volatilization of the ferromagnesian chondrule precursors or by melting of the refractory materials only. We infer instead that anorthite‐rich chondrules in carbonaceous chondrites formed by melting of the reduced chondrule precursors (olivine, pyroxenes, FeNi‐metal) mixed with the refractory materials, including relic CAIs, composed of anorthite, spinel, high‐Ca pyroxene and forsterite. The observed mineralogical and textural similarities of the anorthite‐rich chondrules in several carbonaceous chondrite groups (CV, CO, CH, CR) may indicate that these chondrules formed in the region(s) intermediate between the regions where CAIs and ferromagnesian chondrules originated. This may explain the relative enrichment of anorthite‐rich chondrules in 16O compared to typical ferromagnesian chondrules (Russell et al., 2000).  相似文献   

12.
Abstract— Wark‐Lovering rims of six calcium‐aluminum‐rich inclusions (CAIs) representing the main CAI types and groups in Allende, Efremovka and Vigarano were microsurgically separated and analysed by neutron activation analysis (NAA). All the rims have similar ～4x enrichments, relative to the interiors, of highly refractory lithophile and siderophile elements. The NAA results are confirmed by ion microprobe and scanning electron microscope (SEM) analyses of rim perovskites and rim metal grains. Less refractory Eu, Yb, V, Sr, Ca and Ni are less enriched in the rims. The refractory element patterns in the rims parallel the patterns in the outer parts of the CAIs. In particular, the rims on type B1 CAIs have the igneously fractionated rare earth element (REE) pattern of the melilite mantle below the rim and not the REE pattern of the bulk CAI, proving that the refractory elements in the rims were derived from the outer mantle and were not condensates onto the CAIs. The refractory elements were enriched in an Al2O3‐rich residue <50 μm thick after the most volatile ～80% of the outermost 200 μm of each CAI had been volatilized, including much Mg, Si and Ca. Some volatilization occurred below the rim, and created refractory partial melts that crystallized hibonite and gehlenitic melilite. The required “flash heating” probably exceeded 2000 °C, but for only a few seconds, in order to melt only the outer CAI and to unselectively volatilize slow‐diffusing O isotopes which show no mass fractionation in the rim. The volatilization did, however, produce “heavy” mass‐fractionated Mg in rims. In some CAIs this was later obscured when “normal” Mg diffused in from accreted olivine grains at relatively high temperature (not the lower temperature meteorite metamorphism) and created the ～50 μm set of monomineralic rim layers of pyroxene, melilite and spinel.  相似文献   

13.
Palisade bodies, mineral assemblages with spinel shells, in coarse‐grained Ca‐, Al‐rich inclusions (CAIs) have been considered either as exotic “mini‐CAIs” captured by their host inclusions (Wark and Lovering 1982 ) or as in situ crystallization products of a bubble‐rich melt (Simon and Grossman 1997 ). In order to clarify their origins, we conducted a comprehensive study of palisade bodies in an Allende Type B CAI (BBA‐7), using electron backscatter diffraction (EBSD), micro‐computed tomography (Micro‐CT), electron probe microanalysis (EPMA), and secondary ion mass spectrometry (SIMS). New observations support the in situ crystallization mechanism: early/residual melt infiltrated into spinel‐shelled bubbles and crystallized inside. Evidence includes (1) continuous crystallography of anorthite from the interior of the palisade body to the surrounding host; (2) partial consolidation of two individual palisade bodies revealed by micro‐CT; (3) a palisade body was entirely enclosed in a large anorthite crystal, and the anorthite within the palisade body shows the same crystallographic orientation as the anorthite host; and (4) identical chemical and oxygen isotopic compositions of the constituent minerals between the palisade bodies and the surrounding host. Oxygen isotopic compositions of the major minerals in BBA‐7 are bimodal‐distributed. Spinel and fassaite are uniformly 16O‐rich with ?17O = ?23.3 ± 1.5‰ (2SD), and melilite and anorthite are homogeneously 16O‐poor with ?17O = ?3.2 ± 0.7‰ (2SD). The latter ?17O value overlaps with that of the Allende matrix (?17O ~ ?2.87‰) (Clayton and Mayeda 1999 ), which could be explained by secondary alteration with a 16O‐poor fluid in the parent body. The mobility of fluid could be facilitated by the high porosity (1.56–2.56 vol%) and connectivity (~0.17–0.55 vol%) of this inclusion.  相似文献   

14.
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.  相似文献   

15.
Abstract— Like calcium‐aluminum‐rich inclusions (CAIs) from carbonaceous and ordinary chondrites, enstatite chondrite CAIs are composed of refractory minerals such as spinel, perovskite, Al, Ti‐diopside, melilite, hibonite, and anorthitic plagioclase, which may be partially to completely surrounded by halos of Na‐(±Cl)‐rich minerals. Porous, aggregate, and compact textures of the refractory cores in enstatite chondrite CAIs and rare Wark—Lovering rims are also similar to CAIs from other chondrite groups. However, the small size (<100μm), low abundance (<1% by mode in thin section), occurrence of only spinel or hibonite‐rich types, and presence of primary Ti‐(±V)‐oxides, and secondary geikelite and Ti, Fe‐sulfides distinguish the assemblage of enstatite chondrite CAIs from other groups. The primary mineral assemblage in enstatite chondrite CAIs is devoid of indicators (e.g., oldhamite, osbornite) of low O fugacities. Thus, high‐temperature processing of the CAIs did not occur under the reducing conditions characteristic of enstatite chondrites, implying that either (1) the CAIs are foreign to enstatite‐chondrite‐forming regions or (2) O fugacities fluctuated within the enstatite‐chondrite‐forming region. In contrast, secondary geikelite and Ti‐Fe‐sulfide, which replace perovskite, indicate that alteration of perovskite occurred under reducing conditions distinct from CAIs in the other chondrite groups. We have not ascertained whether the reduced alteration of enstatite chondrite CAIs occurred in a nebular or parent‐body setting. We conclude that each chondrite group is correlated with a unique assemblage of CAIs, indicating spatial or temporal variations in physical conditions during production or dispersal of CAIs.  相似文献   

16.
Abstract— Plagioclase‐rich chondrules (PRCs) in the reduced CV chondrites Efremovka, Leoville, Vigarano and Grosvenor Mountains (GRO) 94329 consist of magnesian low‐Ca pyroxene, Al‐Ti‐Cr‐rich pigeonite and augite, forsterite, anorthitic plagioclase, FeNi‐metal‐sulfide nodules, and crystalline mesostasis composed of silica, anorthitic plagioclase and Al‐Ti‐Cr‐rich augite. The silica grains in the mesostases of the CV PRCs are typically replaced by hedenbergitic pyroxenes, whereas anorthitic plagioclase is replaced by feldspathoids (nepheline and minor sodalite). Some of the PRCs contain regions that are texturally and mineralogically similar to type I chondrules and consist of forsterite, low‐Ca pyroxene and abundant FeNi‐metal nodules. Several PRCs are surrounded by igneous rims or form independent compound objects. Twelve PRCs contain relic calcium‐aluminum‐rich inclusions (CAIs) composed of anorthite, spinel, high‐Ca pyroxene, ± forsterite, and ± Al‐rich low‐Ca pyroxene. Anorthite of these CAIs is generally more heavily replaced by feldspathoids than anorthitic plagioclase of the host chondrules. This suggests that either the alteration predated formation of the PRCs or that anorthite of the relic CAIs was more susceptible to the alteration than anorthitic plagioclase of the host chondrules. These observations and the presence of igneous rims around PRCs and independent compound PRCs suggest that the CV PRCs may have had a complex, multistage formation history compared to a more simple formation history of the CR PRCs. Relatively high abundances of moderately‐volatile elements such as Cr, Mn and Si in the PRCs suggests that these chondrules could not have been produced by volatilization of ferromagnesian chondrule precursors or by melting of refractory materials only. We infer instead that PRCs in carbonaceous chondrites formed by melting of the reduced chondrule precursors (magnesian olivine and pyroxene, FeNi‐metal) mixed with refractory materials (relic CAIs) composed of anorthite, spinel, high‐Ca pyroxene, and forsterite. The mineralogical, chemical and textural similarities of the PRCs in several carbonaceous chondrite groups (CV, CO, CH, CR) and common presence of relic CAIs in these chondrules suggest that PRCs may have formed in the region(s) intermediate between the regions where CAIs and ferromagnesian chondrules originated.  相似文献   

17.
Abstract— Oxygen isotopes have been measured by ion microprobe in individual minerals (spinel, Al‐Ti‐diopside, melilite, and anorthite) within four relatively unaltered, fine‐grained, spinel‐rich Ca‐Al‐rich inclusions (CAIs) from the reduced CV chondrite Efremovka. Spinel is uniformly 16O‐rich (Δ17O ≤ ?20‰) in all four CAIs; Al‐Ti‐diopside is similarly 16O‐rich in all but one CAI, where it has smaller 16O excesses (‐15‰ ≤ Δ17O ≤ ?10‰). Anorthite and melilite vary widely in composition from 16O‐rich to 16O‐poor (‐22‰ ≤ Δ17O ≤ ?5‰). Two of the CAIs are known to have group II volatility‐fractionated rare‐earth‐element patterns, which is typical of this variety of CAI and which suggests formation by condensation. The association of such trace element patterns with 16O‐enrichment in these CAIs suggests that they formed by gas‐solid condensation from an 16O‐rich gas. They subsequently experienced thermal processing in an 16O‐poor reservoir, resulting in partial oxygen isotope exchange. Within each inclusion, oxygen isotope variations from mineral to mineral are consistent with solid‐state oxygen self‐diffusion at the grain‐to‐grain scale, but such a model is not consistent with isotopic variations at a larger scale in two of the CAIs. The spatial association of 16O depletions with both elevated Fe contents in spinel and the presence of nepheline suggests that late‐stage iron‐alkali metasomatism played some role in modifying the isotopic patterns in some CAIs. One of the CAIs is a compound object consisting of a coarse‐grained, melilite‐rich (type A) lithology joined to a fine‐grained, spinel‐rich one. Melilite and anorthite in the fine‐grained portion are mainly 16O‐rich, whereas melilite in the type A portion ranges from 16O‐rich to 16O‐poor, suggesting that oxygen isotope exchange predated the joining together of the two parts and that both 16O‐rich and 16O‐poor gaseous reservoirs existed simultaneously in the early solar nebula.  相似文献   

18.
Abstract— In situ SIMS oxygen isotope data were collected from a coarse‐grained type B1 Ca‐Al‐rich inclusion (CAI) and an adjacent fine‐grained CAI in the reduced CV3 Efremovka to evaluate the timing of isotopic alteration of these two objects. The coarse‐grained CAI (CGI‐10) is a sub‐spherical object composed of elongate, euhedral, normally‐zoned melilite crystals ranging up to several hundreds of Pm in length, coarse‐grained anorthite and Al, Ti‐diopside (fassaite), all with finegrained (～10 μm across) inclusions of spinel. Similar to many previously examined coarse‐grained CAIs from CV chondrites, spinel and fassaite are 16O‐rich and melilite is 16O‐poor, but in contrast to many previous results, anorthite is 16O‐rich. Isotopic composition does not vary with textural setting in the CAI: analyses of melilite from the core and mantle and analyses from a variety of major element compositions yield consistent 16O‐poor compositions. CGI‐10 originated in an 16O‐rich environment, and subsequent alteration resulted in complete isotopic exchange in melilite. The fine‐grained CAI (FGI‐12) also preserves evidence of a 1st‐generation origin in an 16O‐rich setting but underwent less severe isotopic alteration. FGI‐12 is composed of spinel ± melilite nodules linked by a mass of Al‐diopside and minor forsterite along the CAI rim. All minerals are very fine‐grained (<5 μm) with no apparent igneous textures or zoning. Spinel, Al‐diopside, and forsterite are 16O‐rich, while melilite is variably depleted in 16O (δ17,18O from ～‐40‰ to ?5‰). The contrast in isotopic distributions in CGI‐10 and FGI‐12 is opposite to the pattern that would result from simultaneous alteration: the object with finer‐grained melilite and a greater surface area/ volume has undergone less isotopic exchange than the coarser‐grained object. Thus, the two CAIs were altered in different settings. As the CAIs are adjacent to each other in the meteorite, isotopic exchange in CGI‐10 must have preceded incorporation of this CAI in the Efremovka parent body. This supports a nebular setting for isotopic alteration of the commonly observed 16O‐poor melilite in coarse‐grained CAIs from CV chondrites.  相似文献   

19.
Abstract– The oxygen isotopic microdistributions within melilite measured using in situ secondary ion mass spectrometry correspond to the chemical zoning profiles in single melilite crystals of a fluffy type A Ca‐Al‐rich inclusions (CAIs) of reduced CV3 Vigarano meteorite. The melilite crystals show chemical reverse zoning within an individual single crystal from the åkermanite‐rich core to the åkermanite‐poor rim. The composition changes continuously with the crystal growth. The zoning structures suggest that the melilite grew in a hot nebular gas by condensation with decreasing pressure. The oxygen isotopic composition of melilite also changes continuously from 16O‐poor to 16O‐rich with the crystal growth. These observations suggest that the melilite condensation proceeded with change consistent with an astrophysical setting around the inner edge of a protoplanetary disk where both 16O‐rich solar coronal gas and 16O‐poor dense protoplanetary disk gas could coexist. Fluffy type A CAIs could have been formed around the inner edge of the protoplanetary disk surrounding the early sun.  相似文献   

20.
The petrologic and oxygen isotopic characteristics of calcium‐aluminum‐rich inclusions (CAIs) in CO chondrites were further constrained by studying CAIs from six primitive CO3.0‐3.1 chondrites, including two Antarctic meteorites (DOM 08006 and MIL 090010), three hot desert meteorites (NWA 10493, NWA 10498, and NWA 7892), and the Colony meteorite. The CAIs can be divided into hibonite‐bearing inclusions (spinel‐hibonite spherules, monomineralic grains, hibonite‐pyroxene microspherules, and irregular/nodular objects), grossite‐bearing inclusions (monomineralic grains, grossite‐melilite microspherules, and irregular/nodular objects), melilite‐rich inclusions (fluffy Type A, compact type A, monomineralic grains, and igneous fragments), spinel‐pyroxene inclusions (fluffy objects resembling fine‐grained spinel‐rich inclusions in CV chondrites and nodular/banded objects resembling those in CM chondrites), and pyroxene‐anorthite inclusions. They are typically small (98.4 ± 54.4 µm, 1SD) and comprise 1.54 ± 0.43 (1SD) area% of the host chondrites. Melilite in the hot desert and Colony meteorites was extensively replaced by a hydrated Ca‐Al‐silicate during terrestrial weathering and converted melilite‐rich inclusions into spinel‐pyroxene inclusions. The CAI populations of the weathered COs are very similar to those in CM chondrites, suggesting that complete replacement of melilite by terrestrial weathering, and possibly parent body aqueous alteration, would make the CO CAIs CM‐like, supporting the hypothesis that CO and CM chondrites derive from similar nebular materials. Within the CO3.0‐3.1 chondrites, asteroidal alteration significantly resets oxygen isotopic compositions of CAIs in CO3.1 chondrites (?17O: ?25 to ?2‰) but left those in CO3.0‐3.05 chondrites mostly unchanged (?17O: ?25 to ?20‰), further supporting the model whereby thermal metamorphism became evident in CO chondrites of petrologic type ≥3.1. The resistance of CAI minerals to oxygen isotope exchange during thermal metamorphism follows in the order: melilite + grossite < hibonite + anorthite < spinel + diopside + forsterite. Meanwhile, terrestrial weathering destroys melilite without changing the chemical and isotopic compositions of melilite and other CAI minerals.  相似文献   

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