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1.
Abstract– We investigate the hypothesis that many chondrules are frozen droplets of spray from impact plumes launched when thin‐shelled, largely molten planetesimals collided at low speed during accretion. This scenario, here dubbed “splashing,” stems from evidence that such planetesimals, intensely heated by 26Al, were abundant in the protoplanetary disk when chondrules were being formed approximately 2 Myr after calcium‐aluminum‐rich inclusions (CAIs), and that chondrites, far from sampling the earliest planetesimals, are made from material that accreted later, when 26Al could no longer induce melting. We show how “splashing” is reconcilable with many features of chondrules, including their ages, chemistry, peak temperatures, abundances, sizes, cooling rates, indented shapes, “relict” grains, igneous rims, and metal blebs, and is also reconcilable with features that challenge the conventional view that chondrules are flash‐melted dust‐clumps, particularly the high concentrations of Na and FeO in chondrules, but also including chondrule diversity, large phenocrysts, macrochondrules, scarcity of dust‐clumps, and heating. We speculate that type I (FeO‐poor) chondrules come from planetesimals that accreted early in the reduced, partially condensed, hot inner nebula, and that type II (FeO‐rich) chondrules come from planetesimals that accreted in a later, or more distal, cool nebular setting where incorporation of water‐ice with high Δ17O aided oxidation during heating. We propose that multiple collisions and repeated re‐accretion of chondrules and other debris within restricted annular zones gave each chondrite group its distinctive properties, and led to so‐called “complementarity” and metal depletion in chondrites. We suggest that differentiated meteorites are numerically rare compared with chondrites because their initially plentiful molten parent bodies were mostly destroyed during chondrule formation.  相似文献   

2.
Abstract— The outer portions of many type I chondrules (Fa and Fs <5 mol%) in CR chondrites (except Renazzo and Al Rais) consist of silica‐rich igneous rims (SIRs). The host chondrules are often layered and have a porphyritic core surrounded by a coarse‐grained igneous rim rich in low‐Ca pyroxene. The SIRs are sulfide‐free and consist of igneously‐zoned low‐Ca and high‐Ca pyroxenes, glassy mesostasis, Fe, Ni‐metal nodules, and a nearly pure SiO2 phase. The high‐Ca pyroxenes in these rims are enriched in Cr (up to 3.5 wt% Cr2O3) and Mn (up to 4.4 wt% MnO) and depleted in Al and Ti relative to those in the host chondrules, and contain detectable Na (up to 0.2 wt% Na2O). Mesostases show systematic compositional variations: Si, Na, K, and Mn contents increase, whereas Ca, Mg, Al, and Cr contents decrease from chondrule core, through pyroxene‐rich igneous rim (PIR), and to SIR; FeO content remains nearly constant. Glass melt inclusions in olivine phenocrysts in the chondrule cores have high Ca and Al, and low Si, with Na, K, and Mn contents that are below electron microprobe detection limits. Fe, Ni‐metal grains in SIRs are depleted in Ni and Co relative to those in the host chondrules. The presence of sulfide‐free, SIRs around sulfide‐free type I chondrules in CR chondrites may indicate that these chondrules formed at high (>800 K) ambient nebular temperatures and escaped remelting at lower ambient temperatures. We suggest that these rims formed either by gas‐solid condensation of silica‐normative materials onto chondrule surfaces and subsequent incomplete melting, or by direct SiO(gas) condensation into chondrule melts. In either case, the condensation occurred from a fractionated, nebular gas enriched in Si, Na, K, Mn, and Cr relative to Mg. The fractionation of these lithophile elements could be due to isolation (in the chondrules) of the higher temperature condensates from reaction with the nebular gas or to evaporation‐recondensation of these elements during chondrule formation. These mechanisms and the observed increase in pyroxene/olivine ratio toward the peripheries of most type I chondrules in CR, CV, and ordinary chondrites may explain the origin of olivine‐rich and pyroxene‐rich chondrules in general.  相似文献   

3.
To better understand the formation conditions of ferromagnesian chondrules from the Renazzo‐like carbonaceous (CR) chondrites, a systematic study of 210 chondrules from 15 CR chondrites was conducted. The texture and composition of silicate and opaque minerals from each observed FeO‐rich (type II) chondrule, and a representative number of FeO‐poor (type I) chondrules, were studied to build a substantial and self‐consistent data set. The average abundances and standard deviations of Cr2O3 in FeO‐rich olivine phenocrysts are consistent with previous work that the CR chondrites are among the least thermally altered samples from the early solar system. Type II chondrules from the CR chondrites formed under highly variable conditions (e.g., precursor composition, redox conditions, cooling rate), with each chondrule recording a distinct igneous history. The opaque minerals within type II chondrules are consistent with formation during chondrule melting and cooling, starting as S‐ and Ni‐rich liquids at 988–1350 °C, then cooling to form monosulfide solid solution (mss) that crystallized around olivine/pyroxene phenocrysts. During cooling, Fe,Ni‐metal crystallized from the S‐ and Ni‐rich liquid, and upon further cooling mss decomposed into pentlandite and pyrrhotite, with pentlandite exsolving from mss at 400–600 °C. The composition, texture, and inferred formation temperature of pentlandite within chondrules studied here is inconsistent with formation via aqueous alteration. However, some opaque minerals (Fe,Ni‐metal versus magnetite and panethite) present in type II chondrules are a proxy for the degree of whole‐rock aqueous alteration. The texture and composition of sulfide‐bearing opaque minerals in Graves Nunataks 06100 and Grosvenor Mountains 03116 suggest that they are the most thermally altered CR chondrites.  相似文献   

4.
Abstract— Isheyevo is a metal‐rich carbonaceous chondrite that contains several lithologies with different abundances of Fe,Ni metal (7–90 vol%). The metal‐rich lithologies with 50–60 vol% of Fe,Ni metal are dominant. The metal‐rich and metal‐poor lithologies are most similar to the CBb and CH carbonaceous chondrites, respectively, providing a potential link between these chondrite groups. All lithologies experienced shock metamorphism of shock stage S4. All consist of similar components—Fe,Ni metal, chondrules, refractory inclusions (Ca, Al‐rich inclusions [CAIs] and amoeboid olivine aggregates [AOAs]), and heavily hydrated lithic clasts—but show differences in their modal abundances, chondrule sizes, and proportions of porphyritic versus non‐porphyritic chondrules. Bulk chemical and oxygen isotopic compositions are in the range of CH and CB chondrites. Bulk nitrogen isotopic composition is highly enriched in 15N (δ15N = 1122‰). The magnetic fraction is very similar to the bulk sample in terms of both nitrogen release pattern and isotopic profile; the non‐magnetic fraction contains significantly less heavy N. Carbon released at high temperatures shows a relatively heavy isotope signature. Similarly to CBb chondrites, ～20% of Fe,Ni‐metal grains in Isheyevo are chemically zoned. Similarly to CH chondrites, some metal grains are Ni‐rich (>20 wt% Ni). In contrast to CBb and CH chondrites, most metal grains are thermally decomposed into Ni‐rich and Ni‐poor phases. Similar to CH chondrites, chondrules have porphyritic and non‐porphyritic textures and ferromagnesian (type I and II), silica‐rich, and aluminum‐rich bulk compositions. Some of the layered ferromagnesian chondrules are surrounded by ferrous olivine or phyllosilicate rims. Phyllosilicates in chondrule rims are compositionally distinct from those in the hydrated lithic clasts. Similarly to CH chondrites, CAIs are dominated by the hibonite‐, grossite‐, and melilite‐rich types; AOAs are very rare. We infer that Isheyevo is a complex mixture of materials formed by different processes and under different physico‐chemical conditions. Chondrules and refractory inclusions of two populations, metal grains, and heavily hydrated clasts accreted together into the Isheyevo parent asteroid in a region of the protoplanetary disk depleted in fine‐grained dust. Such a scenario is consistent with the presence of solar wind—implanted noble gases in Isheyevo and with its comparatively old K‐Ar age. We cannot exclude that the K‐Ar system was affected by a later collisional event. The cosmic‐ray exposure (CRE) age of Isheyevo determined by cosmogenic 38Ar is ～34 Ma, similar to that of the Bencubbin (CBa) meteorite.  相似文献   

5.
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   

6.
Abstract— The matrices of all primitive chondrites contain presolar materials (circumstellar grains and interstellar organics) in roughly CI abundances, suggesting that all chondrites accreted matrix that is dominated by a CI‐like component. The matrix‐normalized abundances of the more volatile elements (condensation temperatures <750–800 K) in carbonaceous and ordinary chondrites are also at or slightly above CI levels. The modest excesses may be due to low levels of these elements in chondrules and associated metal. Subtraction of a CI‐like matrix component from a bulk ordinary chondrite composition closely matches the average composition of chondrules determined by instrumental neutron activation analysis (INAA) if some Fe‐metal is added to the chondrule composition. Measured matrix compositions are not CI‐like. Sampling bias and secondary redistribution of elements may have played a role, but the best explanation is that ?10–30% of refractory‐rich, volatile depleted material was added to matrix. If most of the more volatile elements are in a CI‐dominated matrix, the major and volatile element fractionations must be largely carried by chondrules. There is both direct and indirect evidence for evaporation during chondrule formation. Type IIA and type B chondrules could have formed from a mixture of CI material and material evaporated from type IA chondrules. The Mg‐Si‐Fe fractionations in the ordinary chondrites can be reproduced with the loss of type IA chondrule material and associated metal. The loss of evaporated material from the chondrules could explain the volatile element fractionations. Mechanisms for how these fractionations occurred are necessarily speculative, but two possibilities are briefly explored.  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 the results of our petrological and mineralogical study of Fe‐Ni metal in type 3 ordinary and CO chondrites, and the ungrouped carbonaceous chondrite Acfer 094. Fe‐Ni metal in ordinary and CO chondrites occurs in chondrule interiors, on chondrule surfaces, and as isolated grains in the matrix. Isolated Ni‐rich metal in chondrites of petrologic type lower than type 3.10 is enriched in Co relative to the kamacite in chondrules. However, Ni‐rich metal in type 3.15–3.9 chondrites always contains less Co than does kamacite. Fe‐Ni metal grains in chondrules in Semarkona typically show plessitic intergrowths consisting of submicrometer kamacite and Ni‐rich regions. Metal in other type 3 chondrites is composed of fine‐ to coarse‐grained aggregates of kamacite and Ni‐rich metal, resulting from metamorphism in the parent body. We found that the number density of Ni‐rich grains in metal (number of Ni‐rich grains per unit area of metal) in chondrules systematically decreases with increasing petrologic type. Thus, Fe‐Ni metal is a highly sensitive recorder of metamorphism in ordinary and carbonaceous chondrites, and can be used to distinguish petrologic type and identify the least thermally metamorphosed chondrites. Among the known ordinary and CO chondrites, Semarkona is the most primitive. The range of metamorphic temperatures were similar for type 3 ordinary and CO chondrites, despite them having different parent bodies. Most Fe‐Ni metal in Acfer 094 is martensite, and it preserves primary features. The degree of metamorphism is lower in Acfer 094, a true type 3.00 chondrite, than in Semarkona, which should be reclassified as type 3.01.  相似文献   

9.
Carbonaceous chondrites are classified into several groups. However, some are ungrouped. We studied one such ungrouped chondrite, Y‐82094, previously classified as a CO. In this chondrite, chondrules occupy 78 vol%, and the matrix is distinctly poor in abundance (11 vol%), compared with CO and other C chondrites. The average chondrule size is 0.33 mm, different from that in C chondrites. Although these features are similar to those in ordinary chondrites, Y‐82094 contains 3 vol% Ca‐Al‐rich inclusions and 5% amoeboid olivine aggregates (AOAs). Also, the bulk composition resembles that of CO chondrites, except for the volatile elements, which are highly depleted. The oxygen isotopic composition of Y‐82094 is within the range of CO and CV chondrites. Therefore, Y‐82094 is an ungrouped C chondrite, not similar to any other C chondrite previously reported. Thin FeO‐rich rims on AOA olivine and the mode of occurrence of Ni‐rich metal in the chondrules indicate that Y‐82094 is petrologic type 3.2. The extremely low abundance of type II chondrules and high abundance of Fe‐Ni metal in the chondrules suggest reducing condition during chondrule formation. The depletion of volatile elements indicates that the components formed under high‐temperature conditions, and accreted to the parent body of Y‐82094. Our study suggests a wider range of formation conditions than currently recorded by the major C chondrite groups. Additionally, Y‐82094 may represent a new, previously unsampled, asteroidal body.  相似文献   

10.
We investigated the matrix mineralogy in primitive EH3 chondrites Sahara 97072, ALH 84170, and LAR 06252 with transmission electron microscopy; measured the trace and major element compositions of Sahara 97072 matrix and ferromagnesian chondrules with laser‐ablation, inductively coupled, plasma mass spectrometry (LA‐ICPMS); and analyzed the bulk composition of Sahara 97072 with LA‐ICPMS, solution ICPMS, and inductively coupled plasma atomic emission spectroscopy. The fine‐grained matrix of EH3 chondrites is unlike that in other chondrite groups, consisting primarily of enstatite, cristobalite, troilite, and kamacite with a notable absence of olivine. Matrix and pyroxene‐rich chondrule compositions differ from one another and are distinct from the bulk meteorite. Refractory lithophile elements are enriched by a factor of 1.5–3 in chondrules relative to matrix, whereas the matrix is enriched in moderately volatile elements. The compositional relation between the chondrules and matrix is reminiscent of the difference between EH3 pyroxene‐rich chondrules and EH3 Si‐rich, highly sulfidized chondrules. Similar refractory element ratios between the matrix and the pyroxene‐rich chondrules suggest the fine‐grained material primarily consists of the shattered, sulfidized remains of the formerly pyroxene‐rich chondrules with the minor addition of metal clasts. The matrix, chondrule, and metal‐sulfide nodule compositions are probably complementary, suggesting all the components of the EH3 chondrites came from the same nebular reservoir.  相似文献   

11.
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   

12.
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   

13.
Abstract— We present the first detailed study of a population of texturally distinct chondrules previously described by Kurat (1969), Christophe Michel‐Lévy (1976), and Skinner et al. (1989) that are sharply depleted in alkalis and Al in their outer portions. These “bleached” chondrules, which are exclusively radial pyroxene and cryptocrystalline in texture, have porous outer zones where mesostasis has been lost. Bleached chondrules are present in all type 3 ordinary chondrites and are present in lower abundances in types 4–6. They are most abundant in the L and LL groups, apparently less common in H chondrites, and absent in enstatite chondrites. We used x‐ray mapping and traditional electron microprobe techniques to characterize bleached chondrules in a cross section of ordinary chondrites. We studied bleached chondrules from Semarkona by ion microprobe for trace elements and H isotopes, and by transmission electron microscopy. Chondrule bleaching was the result of low‐temperature alteration by aqueous fluids flowing through finegrained chondrite matrix prior to thermal metamorphism. During aqueous alteration, interstitial glass dissolved and was partially replaced by phyllosilicates, troilite was altered to pentlandite, but pyroxene was completely unaffected. Calcium‐rich zones formed at the inner margins of the bleached zones, either as the result of the early stages of metamorphism or because of fluid‐chondrule reaction. The mineralogy of bleached chondrules is extremely sensitive to thermal metamorphism in type 3 ordinary chondrites, and bleached zones provide a favorable location for the growth of metamorphic minerals in higher petrologic types. The ubiquitous presence of bleached chondrules in ordinary chondrites implies that they all experienced aqueous alteration early in their asteroidal histories, but there is no relationship between the degree of alteration and metamorphic grade. A correlation between the oxidation state of chondrite groups and their degree of aqueous alteration is consistent with the source of water being either accreted ices or water released during oxidation of organic matter. Ordinary chondrites were probably open systems after accretion, and aqueous fluids may have carried volatile elements with them during dehydration. Individual radial pyroxene and cryptocrystalline chondrules were certainly open systems in all chondrites that experienced aqueous alteration leading to bleaching.  相似文献   

14.
Abstract— We address the origin of “dusty,” metal-bearing relict olivine grains in chondrules. It has been suggested previously that these grains may be either primitive condensates or derived from a previous generation of chondrules. In this paper, we infer the original composition of dusty olivine grains, before they were reduced, and examine the possibility that they were derived from a previous generation of chondrules. Original compositions of dusty grains, including their estimated initial FeO contents and their minor element contents, match closely with compositions of olivines from chondrules in unequilibrated chondrites. In addition, the cores of some dusty grains are unaltered, and the compositions of these cores are also consistent with a chondrule origin. Therefore, we conclude that a derivation from a previous generation of chondrules is a plausible origin for these relicts. Although alternative origins, such as condensates or interstellar grains, cannot be ruled out on the basis of the available data, chondrules are an obvious source, and we suggest that this is the most likely interpretation. If this is the case, it is additional evidence for the importance of recycling of chondrule material in the chondrule-forming region.  相似文献   

15.
Abstract— We have studied the CB carbonaceous chondrites Queen Alexandra Range (QUE) 94411, Hammadah al Hamra (HH) 237, and Bencubbin with an emphasis on the petrographical and mineralogical effects of the shock processing that these meteorite assemblages have undergone. Iron‐nickel metal and chondrule silicates are the main components in these meteorites. These high‐temperature components are held together by shock melts consisting of droplets of dendritically intergrown Fe,Ni‐metal/sulfide embedded in silicate glass, which is substantially more FeO‐rich (30–40 wt%) than the chondrule silicates (FeO <5 wt%). Fine‐grained matrix material, which is a major component in most other chondrite classes, is extremely scarce in QUE 94411 and HH 237, and has not been observed in Bencubbin. This material occurs as rare, hydrated matrix lumps with major and minor element abundances roughly similar to the ferrous silicate shock melts (and CI). We infer that hydrated, fine‐grained material, compositionally similar to these matrix lumps, was originally present between the Fe,Ni‐metal grains and chondrules, but was preferentially shock melted. Other shock‐related features in QUE 94411, HH 237, and Bencubbin include an alignment and occasionally strong plastic deformation of metal and chondrule fragments. The existence of chemically zoned and metastable Fe,Ni‐metal condensates in direct contact with shock melts indicates that the shock did not substantially increase the average temperature of the rock. Because porphyritic olivine‐pyroxene chondrules are absent in QUE 94411, HH 237, and Bencubbin, it is difficult to determine the precise shock stage of these meteorites, but the shock was probably relatively light (S2–S3), consistent with a bulk temperature increase of the assemblages of less than ?300 °C. The apparently similar shock processing of Bencubbin, Weatherford, Gujba (CBa) and QUE 94411/HH 237 (CBb) supports the idea of a common asteroidal parent body for these meteorites.  相似文献   

16.
Abstract– Chondrule compositions suggest either ferroan precursors and evaporation, or magnesian precursors and condensation. Type I chondrule precursors include granoblastic olivine aggregates (planetary or nebular) and fine‐grained (dustball) precursors. In carbonaceous chondrites, type I chondrule precursors were S‐free, while type II chondrules have higher Fe/Mn than in ordinary chondrites. Many type II chondrules contain diverse forsteritic relicts, consistent with polymict dustball precursors. The relationship between finer and coarser grained type I chondrules in ordinary chondrites suggests more evaporation from more highly melted chondrules. Fe metal in type I, and Na and S in type II chondrules indicate high partial pressures in ambient gas, as they are rapidly evaporated at canonical conditions. The occurrence of metal, sulfide, or low‐Ca pyroxene on chondrule rims suggests (re)condensation. In Semarkona type II chondrules, Na‐rich olivine cores, Na‐poor melt inclusions, and Na‐rich mesostases suggest evaporation followed by recondensation. Type II chondrules have correlated FeO and MnO, consistent with condensation onto forsteritic precursors, but with different ratios in carbonaceous chondrites and ordinary chondrites, indicating different redox history. The high partial pressures of lithophile elements require large dense clouds, either clumps in the protoplanetary disk, impact plumes, or bow shocks around protoplanets. In ordinary chondrites, clusters of type I and type II chondrules indicate high number densities and their similar oxygen isotopic compositions suggest recycling together. In carbonaceous chondrites, the much less abundant type II chondrules were probably added late to batches of type I chondrules from different O isotopic reservoirs.  相似文献   

17.
Abstract— We measured the sizes and textural types of 719 intact chondrules and 1322 chondrule fragments in thin sections of Semarkona (LL3.0), Bishunpur (LL3.1), Krymka (LL3.1), Piancaldoli (LL3.4) and Lewis Cliff 88175 (LL3.8). The mean apparent diameter of chondrules in these LL3 chondrites is 0.80 φ units or 570 μm, much smaller than the previous rough estimate of ～900 μm. Chondrule fragments in the five LL3 chondrites have a mean apparent cross‐section of 1.60 φ units or 330 μm. The smallest fragments are isolated olivine and pyroxene grains; these are probably phenocrysts liberated from disrupted porphyritic chondrules. All five LL3 chondrites have fragment/ chondrule number ratios exceeding unity, suggesting that substantial numbers of the chondrules in these rocks were shattered. Most fragmentation probably occurred on the parent asteroid. Porphyritic chondrules (porphyritic olivine + porphyritic pyroxene + porphyritic olivine‐pyroxene) are more readily broken than droplet chondrules (barred olivine + radial pyroxene + cryptocrystalline). The porphyritic fragment/chondrule number ratio (2.0) appreciably exceeds that of droplet‐textured objects (0.9). Intact droplet chondrules have a larger mean size than intact porphyritic chondrules, implying that large porphyritic chondrules are fragmented preferentially. This is consistent with the relatively low percentage of porphyritic chondrules within the set of the largest chondrules (57%) compared to that within the set of the smallest chondrules (81%). Differences in mean size among chondrule textural types may be due mainly to parent‐body chondrule‐fragmentation events and not to chondrule‐formation processes in the solar nebula.  相似文献   

18.
Abstract— The CV (Vigarano‐type) chondrites are a petrologically diverse group of meteorites that are divided into the reduced and the Bali‐like and Allende‐like oxidized subgroups largely based on secondary mineralogy (Weisberg et al., 1997; Krot et al., 1998b). Some chondrules and calcium‐aluminum‐rich inclusions (CAIs) in the reduced CV chondrite Vigarano show alteration features similar to those in Allende: metal is oxidized to magnetite; low‐Ca pyroxene, forsterite, and magnetite are rimmed and veined by ferrous olivine (Fs40–50); and plagioclase mesostases and melilite are replaced by nepheline and sodalite (Sylvester et al., 1993; Kimura and Ikeda, 1996, 1997, 1998). Our petrographic observations indicate that Vigarano also contains individual chondrules, chondrule fragments, and lithic clasts of the Bali‐like oxidized CV materials. The largest lithic clast (about 1 times 2 cm in size) is composed of opaque matrix, type‐I chondrules (400–2000 μm in apparent diameter) surrounded by coarse‐grained and fine‐grained rims, and rare CAIs. The matrix‐chondrule ratio is about 1.1. Opaque nodules in chondrules in the clast consist of Cr‐poor and Cr‐rich magnetite, Ni‐ and Co‐rich metal, Ni‐poor and Ni‐rich sulfide; low‐Ni metal nodules occur only inside chondrule phenocrysts. Chromium‐poor magnetite is preferentially replaced by fayalite. Chondrule mesostases are replaced by phyllosilicates; low‐Ca pyroxene and olivine phenocrysts appear to be unaltered. Matrix in the clast consists of very fine‐grained (<1 μm) ferrous olivine, anhedral fayalite grains (Fa80–100), rounded objects of porous Ca‐Fe‐rich pyroxenes (Fs10–50Wo50), Ni‐poor sulfide, Ni‐ and Co‐rich metal, and phyllosilicates; magnetite is rare. On the basis of the presence of the Bali‐like lithified chondritic clast—in addition to individual chondrules and CAIs of both Bali‐like and Allende‐like materials—in the reduced CV chondrite Vigarano, we infer that (1) all three types of materials were mixed during regolith gardening on the CV asteroidal body, and (2) the reduced and oxidized CV materials may have originated from a single, heterogeneously altered asteroid.  相似文献   

19.
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20.
Abstract– We report trace element analyses from mineral phases in chondrules from carbonaceous chondrites (Vigarano, Renazzo, and Acfer 187), carried out by laser ablation inductively coupled plasma‐mass spectrometry. Results are similar in all three meteorites. Mesostasis has rare earth element (REE) concentrations of 10–20 × CI. Low‐Ca pyroxene has light REE (LREE) concentrations near 0.1 × CI and heavy REE (HREE) near 1 × CI, respectively. Olivine has HREE concentrations at 0.1–1 × CI and LREE around 10?2 × CI. The coarsest olivine crystals tend to have the most fractionated REE patterns, indicative of equilibrium partitioning. Low‐Ca pyroxene in the most pyroxene‐rich chondrules tends to have the lowest REE concentrations. Type I chondrules seem to have undergone a significant degree of batch crystallization (as opposed to fractional crystallization), which requires cooling rates slower than 1–100 K h?1. This would fill the gap between igneous calcium‐aluminum‐rich inclusions (CAIs) and type II chondrules. The anticorrelation between REE abundances and pyroxene mode may be understood as due to dilution by addition of silica to the chondrule melt, as in the gas‐melt interaction scenario of Libourel et al. (2006). The rapid cooling rate (of the order of 1000 K h?1) which seems recorded by low‐Ca pyroxene, contrasted with the more diverse record of olivine, may point to a nonlinear cooling history or suggest that formation of pyroxene‐rich chondrule margins was an event distinct from the crystallization of the interior.  相似文献   

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