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1.
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   

2.
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   

3.
Micrometeorites provide a large range of samples sourced from a wide variety of planetary materials, thereby providing a scope for expanding the known inventory of solar system materials. Here we report the micrometeorite AAS62‐34‐P117 having the assemblage of corundum, hibonite, unknown Al‐rich phases, FeNi metal blebs, sulfide, and phosphate embedded in Al‐rich silicate composition, and Pt‐group element nuggets dispersed throughout the micrometeorite. Here, we report the presence of corundum in micrometeorites as a major refractory phase with sizes greater than ~10 μm. The Al‐rich phases have Al2O3 ~50–70%, such high Al phases are not known from meteoritic components either in chondrules or refractory inclusions. In addition, the Ca content is extremely poor to relate it directly to known refractory inclusions, but is very high in Al. The presence of corundum in Al‐rich phases indicates the micrometeorite to be early condensate from solar nebula that later got incorporated into Si‐rich materials leading to a transformation that produced the unusual Al‐rich and Ca‐poor phases different from the average solar composition. The observed texture and mineralogy of the micrometeorite appears to have evolved in a nebular setting that has compositional reservoirs different from those of any known components of meteorites.  相似文献   

4.
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   

5.
Abstract— Here we report the petrography, mineralogy, and bulk compositions of Ca,Al‐rich inclusions (CAIs), amoeboid olivine aggregate (AOA), and Al‐rich chondrules (ARCs) in Sayh al Uhaymir (SaU) 290 CH chondrite. Eighty‐two CAIs (0.1% of the section surface area) were found. They are hibonite‐rich (9%), grossite‐rich (18%), melilite ± spinel‐rich (48%), fassaite ± spinel‐rich (15%), and fassaite‐anorthite‐rich (10%) refractory inclusions. Most CAIs are rounded in shape and small in size (average = 40 μm). They are more refractory than those of other groups of chondrites. CAIs in SaU 290 might have experienced higher peak heating temperatures, which could be due to the formation region closer to the center of protoplanetary disk or have formed earlier than those of other groups of chondrites. In SaU 290, refractory inclusions with a layered texture could have formed by gas‐solid condensation from the solar nebula and those with an igneous texture could have crystallized from melt droplets or experienced subsequent melting of pre‐existing condensates from the solar nebula. One refractory inclusion represents an evaporation product of pre‐existing refractory solid on the basis of its layered texture and melting temperature of constituting minerals. Only one AOA is observed (75 μm across). It consists of olivine, Al‐diopside, anorthite, and minor spinel with a layered texture. CAIs and AOA show no significant low‐temperature aqueous alteration. ARCs in SaU 290 consist of diopside, forsterite, anorthite, Al‐enstatite, spinel, and mesostasis or glass. They can be divided into diopside‐rich, Al‐enstatite‐rich, glass‐rich, and anorthite‐rich chondrules. Bulk compositions of most ARCs are consistent with a mixture origin of CAIs and ferromagnesian chondrules. Anorthite and Al‐enstatite do not coexist in a given ARC, implying a kinetic effect on their formation.  相似文献   

6.
A calcium‐aluminum‐rich inclusion 3N from the Northwest Africa (NWA) 3118 CV3 carbonaceous chondrite is a unique cm‐sized compound object, primarily a forsterite‐bearing type B (FoB) CAI, that encloses at least 26 smaller CAIs of different types, including compact type A (CTA), B, C, and an ultra‐refractory inclusion. Relative to typical type A and B CAIs found elsewhere, the bulk compositions of the types A and B CAIs within 3N more closely match the bulk compositions predicted by equilibrium condensation of a gas of solar composition. Being trapped within the FoB melt may have protected them from melt evaporation that affected most “stand‐alone” CAIs. 3N originated either as an aggregate of many smaller (mostly types A, B, C) CAIs plus accreted Fo‐bearing material (like an amoeboid olivine aggregate) which experienced partial melting of the whole, or else as a FoB melt droplet that collided with and trapped many smaller solid CAIs. In the former case, 3N recorded the earliest accretion of pebble‐sized bodies known. In the latter case, the presence of a large number of individual refractory inclusions within 3N suggests a very high local density of refractory solids in the immediate region of the host CAI during the brief time while it was melted. Collisions would have occurred on time scales of hours at most, assuming a melt solidification interval for the host CAI of 300–400 °C (maximum) and a cooling rate of ~10 °C/h.  相似文献   

7.
Abstract— –The CH/CB‐like chondrite Isheyevo consists of metal‐rich (70–90 vol% Fe,Ni‐metal) and metal‐poor (7–20 vol% Fe,Ni‐metal) lithologies which differ in size and relative abundance of Fe,Ni‐metal and chondrules, as well as proportions of porphyritic versus non‐porphyritic chondrules. Here, we describe the mineralogy and petrography of Ca,Al‐rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs) in these lithologies. Based on mineralogy, refractory inclusions can be divided into hibonite‐rich (39%), grossite‐rich (16%), melilite‐rich (19%), spinel‐rich (14%), pyroxene‐anorthite‐rich (8%), fine‐grained spinel‐rich CAIs (1%), and AOAs (4%). There are no systematic differences in the inclusion types or their relative abundances between the lithologies. About 55% of the Isheyevo CAIs are very refractory (hibonite‐rich and grossite‐rich) objects, 20–240 μm in size, which appear to have crystallized from rapidly cooling melts. These inclusions are texturally and mineralogically similar to the majority of CAIs in CH and CB chondrites. They are distinctly different from CAIs in other carbonaceous chondrite groups dominated by the spinel‐pyroxene ± melilite CAIs and AOAs. The remaining 45% of inclusions are less refractory objects (melilite‐, spinel‐ and pyroxene‐rich CAIs and AOAs), 40–300 μm in size, which are texturally and mineralogically similar to those in other chondrite groups. Both types of CAIs are found as relict objects inside porphyritic chondrules indicating recycling during chondrule formation. We infer that there are at least two populations of CAIs in Isheyevo which appear to have experienced different thermal histories. All of the Isheyevo CAIs apparently formed at an early stage, prior to chondrule formation and prior to a hypothesized planetary impact that produced magnesian cryptocrystalline and skeletal chondrules and metal grains in CB, and possibly CH chondrites. However, some of the CAIs appear to have undergone melting during chondrule formation and possibly during a major impact event. We suggest that Isheyevo, as well as CH and CB chondrites, consist of variable proportions of materials produced by different processes in different settings: 1) by evaporation, condensation, and melting of dust in the protoplanetary disk (porphyritic chondrules and refractory inclusions), 2) by melting, evaporation and condensation in an impact generated plume (magnesian cryptocrystalline and skeletal chondrules and metal grains; some igneous CAIs could have been melted during this event), and 3) by aqueous alteration of pre‐existing planetesimals (heavily hydrated lithic clasts). The Isheyevo lithologies formed by size sorting of similar components during accretion in the Isheyevo parent body; they do not represent fragments of CH and CB chondrites.  相似文献   

8.
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.  相似文献   

9.
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   

10.
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.  相似文献   

11.
CK chondrites are the only group of carbonaceous chondrites with petrologic types ranging from 3 to 6. Although CKs are described as calcium‐aluminum‐rich inclusion (CAI)‐poor objects, the abundance of CAIs in the 18 CK3–6 we analyzed ranges from zero to approximately 16.4%. During thermal metamorphism, some of the fine‐grained CAIs recrystallized as irregular assemblages of plagioclase + Ca‐rich pyroxene ± olivine ± Ca‐poor pyroxene ± magnetite. Coarse‐grained CAIs display zoned spinel, fassaite destabilization, and secondary grossular and spinel. Secondary anorthite, grossular, Ca‐rich pyroxene, and spinel derive from the destabilization of melilite, which is lacking in all CAIs investigated. The Al‐Mg isotopic systematics measured in fine‐ and coarse‐grained CAIs from Tanezrouft (Tnz) 057 was affected by Mg redistribution. The partial equilibration of Al‐Mg isotopic signatures obtained in the core of a coarse‐grained CAI (CG1‐CAI) in Tnz 057 may indicate a lower peak temperature for Mg diffusion of approximately 540–580 °C, while grossular present in the core of this CAI indicates a higher temperature of around 800 °C for the metamorphic event on the parent body of Tnz 057. Excluding metamorphic features, the similarity in nature and abundance of CAIs in CK and CV chondrites confirms that CVs and CKs form a continuous metamorphic series from type 3 to 6.  相似文献   

12.
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   

13.
Abstract— In situ io n microprobe analyses of spinel in refractory calcium‐aluminium‐rich inclusions (CAIs) from type 3 EH chondrites yield 16O‐rich compositions (δ 18O and δ 17O about‐40‰). Spinel and feldspar in a CAI from an EL3 chondrite have significantly heavier isotopic compositions (δ 18O and δ 17O about ?5‰). A regression through the data results in a line with slope 1.0 on a three‐isotope plot, similar to isotopic results from unaltered minerals in CAIs from carbonaceous chondrites. The existence of CAIs with 16O‐rich and 16O‐poor compositions in carbonaceous as well as enstatite chondrites indicates that CAIs formed in at least two temporally or spatially distinct oxygen reservoirs. General similarities in oxygen isotopic compositions of CAIs from enstatite, carbonaceous, and ordinary chondrites indicate a common nebular mechanism or locale for the production of most CAIs.  相似文献   

14.
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.  相似文献   

15.
Transmission electron microscope examination of more than 250 fragments, >1 μm from comet Wild 2 and a giant cluster interplanetary dust particle (GCP) of probable cometary origin has revealed four new calcium‐aluminum‐rich inclusions (CAIs), an amoeboid olivine aggregate (AOA), and an additional AOA or Al‐rich chondrule (ARC) object. All of the CAIs have concentric mineral structures and are composed of spinel + anorthite cores surrounded by Al,Ti clinopyroxenes and are similar to two previous CAIs discovered in Wild 2. All of the cometary refractory objects are of moderate refractory character. The mineral assemblages, textures, and bulk compositions of the comet CAIs are similar to nodules in fine‐grained, spinel‐rich inclusions (FGIs) found in primitive chondrites and like the nodules may be nebular condensates that were altered via solid–gas reactions in the solar nebula. Oxygen isotopes collected on one Wild 2 CAI also match FGIs. The lack of the most refractory inclusions in the comet samples may reflect the higher abundances of small moderately refractory CAI nodules that were produced in the nebula and the small sample sizes collected. In the comet samples, approximately 2–3% of all fragments larger than 1 μm, by number, are CAIs and nearly 50% of all bulbous Stardust tracks contain at least one CAI. We estimate that ~0.5 volume % of Wild 2 material and ~1 volume % of GCP is in the form of CAIs. ARCs and AOAs account for <1% of the Wild 2 and GCP grains by number.  相似文献   

16.
Bulk major element composition, petrography, mineralogy, and oxygen isotope compositions of twenty Al‐rich chondrules (ARCs) from five CV3 chondrites (Northwest Africa [NWA] 989, NWA 2086, NWA 2140, NWA 2697, NWA 3118) and the Ningqiang carbonaceous chondrite were studied and compared with those of ferromagnesian chondrules and refractory inclusions. Most ARCs are marginally Al‐richer than ferromagnesian chondrules with bulk Al2O3 of 10–15 wt%. ARCs are texturally similar to ferromagnesian chondrules, composed primarily of olivine, pyroxene, plagioclase, spinel, Al‐rich glass, and metallic phases. Minerals in ARCs have intermediate compositions. Low‐Ca pyroxene (Fs0.6–8.8Wo0.7–9.3) has much higher Al2O3 and TiO2 contents (up to 12.5 and 2.3 wt%, respectively) than that in ferromagnesian chondrules. High‐Ca pyroxene (Fs0.3–2.0Wo33–54) contains less Al2O3 and TiO2 than that in Ca,Al‐rich inclusions (CAIs). Plagioclase (An77–99Ab1–23) is much more sodic than that in CAIs. Spinel is enriched in moderately volatile element Cr (up to 6.7 wt%) compared to that in CAIs. Al‐rich enstatite coexists with anorthite and spinel in a glass‐free chondrule, implying that the formation of Al‐enstatite was not due to kinetic reasons but is likely due to the high Al2O3/CaO ratio (7.4) of the bulk chondrule. Three ARCs contain relict CAIs. Oxygen isotope compositions of ARCs are also intermediate between those of ferromagnesian chondrules and CAIs. They vary from ?39.4‰ to 13.9‰ in δ18O and yield a best fit line (slope = 0.88) close to the carbonaceous chondrite anhydrous mineral (CCAM) line. Chondrules with 5–10 wt% bulk Al2O3 have a slightly more narrow range in δ18O (?32.5 to 5.9‰) along the CCAM line. Except for the ARCs with relict phases, however, most ARCs have oxygen isotope compositions (>?20‰ in δ18O) similar to those of typical ferromagnesian chondrules. ARCs are genetically related to both ferromagnesian chondrules and CAIs, but the relationship between ARCs and ferromagnesian chondrules is closer. Most ARCs were formed during flash heating and rapid cooling processes like normal chondrules, only from chemically evolved precursors. ARCs extremely enriched in Al and those with relict phases could have had a hybrid origin (Krot et al. 2002) which incorporated refractory inclusions as part of the precursors in addition to ferromagnesian materials. The occurrence of melilite in ARCs indicates that melilite‐rich CAIs might be present in the precursor materials of ARCs. The absence of melilite in most ARCs is possibly due to high‐temperature interactions between a chondrule melt and the solar nebula.  相似文献   

17.
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   

18.
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   

19.
Abstract— Textures, mineral assemblages, and Al‐Mg isotope systematics indicate a protracted, episodic secondary mineralization history for Allende Ca‐Al‐rich inclusions (CAIs). Detailed observations from one type B1 CAI, one B2, one compact type A (CTA), and one fluffy type A (FTA) indicate that these diverse types of CAIs are characterized by two distinct textural and mineralogic types of secondary mineralization: (1) grossular‐rich domains, concentrated along melilite grain boundaries in CAI interiors, and (2) feldspathoid‐bearing domains, confined mostly to CAI margins just interior to the Wark‐Lovering rim sequence. The Al‐Mg isotopic compositions of most secondary minerals in the type B1 CAI, and some secondary minerals in the other CAIs, show no resolvable excesses of 26Mg, whereas the primary CAI phases mostly yield correlated excesses of 26Mg with increasing Al/Mg corresponding to “canonical” initial 26Al/27Al ～ 4.5–5 × 10?5. These secondary minerals formed at least 3 Ma after the primary CAI minerals. All but two analyses of secondary minerals from the fluffy type‐A CAI define a correlated increase in 26Mg/24Mg with increasing Al/Mg, yielding (26Al/27Al)0 = (4.9 ± 2.8) × 10?6. The secondary minerals in this CAI formed 1.8–3.2 Ma after the primary CAI minerals. In both cases, the timing of secondary alteration is consistent with, but does not necessarily require, alteration in an asteroidal setting. One grossular from the type B2 CAI, and several grossular and secondary feldspar analyses from the compact type A CAI, have excesses of 26Mg consistent with initial 26Al/27Al ～ 4.5 × 10?5. Especially in the compact type A CAI, where 26Mg/24Mg in grossular correlates with increasing Al/Mg, these 26Mg excesses are almost certainly due to in situ decay of 26Al. They indicate a nebular setting for formation of the grossular. The preservation of these diverse isotopic patterns indicates that heating on the Allende parent body was not pervasive enough to reset isotopic systematics of fine‐grained secondary minerals. Secondary mineralization clearly was not restricted to a short time interval, and at least some alteration occurred coincident with CAI formation and melting events (chondrule formation) in the nebula. This observation supports the possibility that alteration followed by melting affected the compositional evolution of CAIs.  相似文献   

20.
Abstract— Petrographic, compositional, and isotopic characteristics were studied for three calcium‐aluminum‐rich inclusions (CAIs) and four plagioclase‐bearing chondrules (three of them Al‐rich) from the Axtell (CV3) chondrite. All seven objects have analogues in Allende (CV3) and other primitive chondrites, yet Axtell, like most other chondrites, contains a distinctive suite of CAIs and chondrules. In common with Allende CAIs, CAIs in Axtell exhibit initial 26Al/27Al ratios ((26Al/27Al)0) ranging from ～5 × 10?5 to <1.1 × 10?5, and plagioclase‐bearing chondrules have (26Al/27Al)0 ratios of ～3 × 10?6 and lower. One type‐A CAI has the characteristics of a FUN inclusion. The Al‐Mg data imply that the plagioclase‐bearing chondrules began to form >2 Ma after the first CAIs. As in other CV3 chondrites, some objects in Axtell show evidence of isotopic disturbance. Axtell has experienced only mild thermal metamorphism (<600 °C), probably not enough to disturb the Al‐Mg systematics. Its CAIs and chondrules have suffered extensive metasomatism, probably prior to final accretion. These data indicate that CAIs and chondrules in Axtell (and other meteorites) had an extended history of several million years before their incorporation into the Axtell parent body. These long time periods appear to require a mechanism in the early solar system to prevent CAIs and chondrules from falling into the Sun via gas drag for several million years before final accretion. We also examined the compositional relationships among the four plagioclase‐bearing chondrules (two with large anorthite laths and two barred‐olivine chondrules) and between the chondrules and CAIs. Three processes were examined: (1) igneous differentiation, (2) assimilation of a CAI by average nebular material, and (3) evaporation of volatile elements from average nebular material. We find no evidence that igneous differentiation played a role in producing the chondrule compositions, although the barred olivine compositions can be related by addition or subtraction of olivine. Methods (2) and (3) could have produced the composition of one chondrule, AXCH‐1471, but neither process explains the other compositions. Our study indicates that plagioclase‐bearing objects originated through a variety of processes.  相似文献   

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