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1.
南极格罗夫山陨石GRV 020043是一块特殊的球粒陨石,虽与普通球粒陨石有着相似的矿物组合,但矿物成分超出普通球粒陨石范围.主要矿物组合及其模式含量(vol%)是:斜方辉石40、橄榄石24、透辉石8、斜长石10、 Fe-Ni合金14、陨硫铁4 vol%、及少量铬铁矿和磷灰石.主要组成矿物成分均一,如斜方辉石(Fs10...  相似文献   

2.
Acapulcoites are modeled as having formed by shock melting CR-like carbonaceous chondrite precursors; the degree of melting of some acapulcoites was low enough to allow the preservation of 3-6 vol % relict chondrules. Shock effects in acapulcoites include veins of metallic Fe-Ni and troilite, polycrystalline kamacite, fine-grained metal-troilite assemblages, metallic Cu, and irregularly shaped troilite grains within metallic Fe-Ni. While at elevated temperatures, acapulcoites experienced appreciable reduction. Because graphite is present in some acapulcoites and lodranites, it seems likely that carbon was the principal reducing agent. Reduction is responsible for the low contents of olivine Fa (4-14 mol %) and low-Ca pyroxene Fs (3-13 mol %) in the acapulcoites, the observation that, in more than two-thirds of the acapulcoites, the Fa value is lower than the Fs value (in contrast to the case for equilibrated ordinary chondrites), the low FeO/MnO ratios in acapulcoite olivine (16-18, compared to 32-38 in equilibrated H chondrites), the relatively high modal orthopyroxene/olivine ratios (e.g., 1.7 in Monument Draw compared to 0.74 in H chondrites), and reverse zoning in some mafic silicate grains. Lodranites formed in a similar manner to acapulcoites but suffered more extensive heating, loss of plagioclase, and loss of an Fe-Ni-S melt.Acapulcoites and lodranites experienced moderate post-shock annealing, presumably resulting from burial beneath material of low thermal diffusivity. The annealing process repaired damaged olivine crystal lattices, lending acapulcoites and lodranites the appearance of unshocked (i.e., shock-stage S1) rocks. Any high-pressure phases that may have formed during initial shock reverted to their low-pressure polymorphs during annealing. Some samples were subsequently shocked again; several acapulcoites reached shock-stage S2 levels, ALH 84190 reached S3, and the lodranite MAC 88177 reached S5.  相似文献   

3.
Pampa del Infierno, an L6 chondrite, displays strong evidence of impact metamorphism. Rare chondrules and two types of dark-colored clasts occur in a light-colored matrix. Granular clasts are similar in mineralogy and chemistry to the host meteorite, but display shock metamorphic features, produced mainly by deformation, such as mosaicism, undulatory extinction, and fracturing. Partial melting in the granular clasts is manifested by the presence of selvages of mafic glass with troilite-iron eutectic intergrowths around remnants of low-Ca pyroxene and plagioclase glass with skeletal poikilitic inclusions of olivine. Clasts with spinifex texture are believed to have crystallized from a supercooled, impact-generated, ultramafic melt of the host chondrite or a chondritic source of similar composition. The light-colored matrix mainly displays evidence of shock metamorphism under subsolidus conditions as manifested by kinking and deformation twinning in pyroxene; high-pressure phase transitions of olivine and low-Ca pyroxene to ringwoodite and majorite, respectively; and lineation that still preserves the deformation features in the different mineral phases. Pertinent shock-wave data used to interpret the metamorphic history of the Pampa del Infierno chondrite suggest metamorphism by impact at a minimum peak pressure greater than 300 kbar.  相似文献   

4.
Petrographic study of 124 chondrules in the Hallingeberg (L-3) chondrite and electron probe microanalyses of olivine and low-Ca pyroxene in 96 of them reveal patterns of variation like those encountered previously in Sharps (H-3). Chondrule mineralogy, mineral composition, and the incidence of shock-related textures vary systematically with chondrule type. This fact and evidence of recrystallization in at least a fourth of the chondrules studied indicate that the pre-accretion histories of chondrules included complex and overlapping episodes of magmatic crystallization, burial, metamorphism and exhumation, in which impact shock was heavily involved. Data for Hallingeberg and Sharps suggest that orthopyroxene accompanies or replaces clinoenstatite in some chondrules and that its presence is due, in part at least, to pre-accretion recrystallization. A comparison of modes for chondrules in Sharps and Hallingeberg shows the former to contain more olivine, on the average, than the latter. It appears that the mean compositions of chondrules in H- and L-group chondrites reflect bulk chemical differences between the two groups, and that chondrule formation followed the siderophile fractionation which differentiated H-, L- and LL-group ordinary chondrites.  相似文献   

5.
Bulk compositions of metallic Fe-Ni from two equilibrated ordinary chondrites, Jilin (H5) and Anlong (H5), and two unequilibrated ones, GRV 9919 (L3) and GRV 021603 (H3), were analyzed by inductively coupled plasma mass spectrometry (ICP-MS). The CI-, Co-normalized abundances of siderophile and chalcophile elements of metallic Fe-Ni from the unequilibrated ordinary chondrites correlate with 50% condensation temperatures (i.e., volatility) of the elements. The refractory siderophile elements (i.e., platinum group elements, Re), Au, Ni and Co show a flat pattern (1.01×CI Co-normalized), while moderate elements (As, Cu, Ag, Ga, Ge, Zn) decrease with volatility from 0.63×CI (Co-normalized, As) to 0.05×CI (Co-normalized, Zn). Cr and Mn show deficit relative to the trend, probably due to their main partition in silicates and sulfides (nonmagnetic). Metallic Fe-Ni from the equilibrated ordinary chondrites shows similar patterns, except for strong deficit of Cr, Mn, Ag and Zn. It is indicated that these elements were almost all partitioned into silicates and/or sulfides during thermal metamorphism. The similar deficit of Cr, Mn, Ag and Zn was also found in iron meteorites. Our analyses demonstrate similar behaviors of W and Mo as refractory siderophile elements during condensation of the solar nebula, except for slight depletion of Mo in the L3 and H5 chondrites. The Mo-depletion of metallic Fe-Ni from GRV 9919 (L3) relative to GRV 021603 (H3) could be due to a more oxidizing condition of the former than the latter in the solar nebula. In contrast, the Mo-depletion of the metallic Fe-Ni from the H5 chondrites may reflect partition of Mo from metal to silicates and/or sulfides during thermal metamorphism in the asteroidal body.  相似文献   

6.
The recently discovered metal-rich carbonaceous chondrite Isheyevo consists of Fe, Ni-metal grains, chondrules, heavily hydrated matrix lumps and rare refractory inclusions. It contains several lithologies with mineralogical characteristics intermediate between the CH and CB carbonaceous chondrites; the contacts between the lithologies are often gradual. Here we report the mineralogy and petrography of chondrules in the metal-rich (70 vol%) and metal-poor (20 vol%) lithologies. The chondrules show large variations in textures [cryptocrystalline, skeletal olivine, barred olivine, porphyritic olivine, porphyritic olivine-pyroxene, porphyritic pyroxene], mineralogy and bulk chemistry (magnesian, ferrous, aluminum-rich, silica-rich). The porphyritic magnesian (Type I) and ferrous (Type II) chondrules, as well as silica- and Al-rich plagioclase-bearing chondrules are texturally and mineralogically similar to those in other chondrite groups and probably formed by melting of mineralogically diverse precursor materials. We note, however, that in contrast to porphyritic chondrules in other chondrite groups, those in Isheyevo show little evidence for multiple melting events; e.g., relict grains are rare and igneous rims or independent compound chondrules have not been found. The magnesian cryptocrystalline and skeletal olivine chondrules are chemically and mineralogically similar to those in the CH and CB carbonaceous chondrites Hammadah al Hamra 237, Queen Alexandra Range 94411 (QUE94411) and MacAlpine Hills 02675 (MAC02675), possibly indicating a common origin from a vapor–melt plume produced by a giant impact between planetary embryos; the interchondrule metal grains, many of which are chemically zoned, probably formed during the same event. The magnesian cryptocrystalline chondrules have olivine–pyroxene normative compositions and are generally highly depleted in Ca, Al, Ti, Mn and Na; they occasionally occur inside chemically zoned Fe, Ni-metal grains. The skeletal olivine chondrules consist of skeletal forsteritic olivine grains overgrown by Al-rich (up to 20 wt% Al2O3) low-Ca and high-Ca pyroxene, and interstitial anorthite-rich mesostasis. Since chondrules with such characteristics are absent in ordinary, enstatite and other carbonaceous chondrite groups, the impact-related chondrule-forming mechanism could be unique for the CH and CB chondrites. We conclude that Isheyevo and probably other CH chondrites contain chondrules of several generations, which may have formed at different times, places and by different mechanisms, and subsequently accreted together with the heavily hydrated matrix lumps and refractory inclusions into a CH parent body. Short-lived isotope chronology, oxygen isotope and trace element studies of the Isheyevo chondrules can provide a possible test of this hypothesis.  相似文献   

7.
Northwest Africa (NWA) 12379 is a new metal-rich chondrite with unique characteristics distinguishing it from all previously described meteorites. It contains high Fe,Ni-metal content (∼ 70 vol.%) and completely lacks interchondrule matrix; these characteristics are typical only for metal-rich carbonaceous (CH and CB) and G chondrites. However, chondrule sizes (60 to 1200 μm; mean = 370 μm), their predominantly porphyritic textures, nearly equilibrated chemical compositions of chondrule olivines (Fa18.1–28.3, average Fa24.9±3.2, PMD = 12.8; Cr2O3 = 0.03 ± 0.02 wt.%; FeO/MnO = 53.2 ± 6.5 (wt.-ratio); n = 28), less equilibrated compositions of low-Ca pyroxenes (Fs3.2–18.7Wo0.2–4.5; average Fs14.7±3.7Wo1.4±1.3; n = 20), oxygen-isotope compositions of chondrule olivine phenocrysts (Δ17O ∼ 0.2–1.4‰, average ∼ 0.8‰), and the presence of coarse-grained Ti-bearing chromite, Cl-apatite, and merrillite, all indicate affinity of NWA 12379 to unequilibrated (type 3.8) ordinary chondrites (OCs). Like most OCs, NWA 12379 experienced fluid-assisted thermal metamorphism that resulted in formation of secondary ferroan olivine (Fa27) that replaces low-Ca pyroxene grains in chondrules and in inclusions in Fe,Ni-metal grains. Δ17O of the ferroan olivine (∼ 4‰) is similar to those of aqueously-formed fayalite in type 3 OCs, but its δ18O is significantly higher (15–19‰, average = 17‰ vs. 3―12‰, average = 8‰, respectively). We suggest classifying NWA 12379 as the ungrouped metal-rich chondrite with affinities of its non-metal fraction to unequilibrated OCs and speculate that it may have formed by a collision between an OC-like body and a metal-rich body and subsequently experienced fluid-assisted thermal metamorphism. Trace siderophile element abundances and isotopic compositions (e.g., Mo, Ni, Fe) of the NWA 12379 metal could help to constrain its origin.  相似文献   

8.
A set of troilite-silicate-metal (TSM) inclusions and chondrule rims in the Bishunpur (LL3.1) chondrite provide information regarding impact brecciation of small bodies in the early solar system. The TSM inclusions and chondrule rims consist of numerous angular to subrounded silicate grains that are individually enclosed by fine networks of troilite. FeNi metal also occurs in the troilite matrix. The silicates include olivine (Fo55-98), low-Ca pyroxene (En78-98), and high-Ca pyroxene (En48-68Wo11-32). Al- and Si-rich glass coexists with the silicates. Relatively coarse silicate grains are apparently fragments of chondrules typical of petrologic type-3 chondrites. Troilite fills all available cracks and pores in the silicate grains. Some of the TSM inclusions and rims are themselves surrounded by fine-grained silicate-rich rims (FGR).The TSM inclusions and rims texturally resemble the troilite-rich regions in the Smyer H-chondrite breccia. They probably formed by shock-induced mobilization of troilite during an impact event on a primitive asteroidal body. Because silicates in the TSM inclusions and rims have highly unequilibrated compositions, their precursor was presumably type-3 chondritic material like Bishunpur itself. The TSM inclusions and the chondrules with the TSM rims were fragmented and dispersed after the impact-induced compaction, then reaccreted onto the Bishunpur parent body. FGR probably formed around the TSM inclusions and rims, as well as around some chondrules, during the reaccumulation process. Components of most type-2 and 3 chondrites probably experienced similar processing, i.e., dispersal of unconsolidated materials and subsequent reaccumulation.  相似文献   

9.
<正>GRV 020175 is an Antarctic mesosiderite,containing about 43 vol%silicates and 57 vol% metal.Metal occurs in a variety of textures from irregular large masses,to veins penetrating silicates, and to matrix fine grains.The metallic portion contains kamacite,troilite and minor taenite.Terrestrial weathering is evident as partial replacement of the metal and troilite veins by Fe oxides.Silicate phases exhibit a porphyritic texture with pyroxene,plagioclase,minor silica and rare olivine phenocrysts embedded in a fine-grained groundmass.The matrix is ophitic and consists mainly of pyroxene and plagioclase grains.Some orthopyroxene phenocrysts occur as euhedral crystals with chemical zoning from a magnesian core to a ferroan overgrowth;others are characterized by many fine inclusions of plagioclase composition.Pigeonite has almost inverted to its orthopyroxene host with augite lamellae, enclosed by more magnesian rims.Olivine occurs as subhedral crystals,surrounded by a necklace of tiny chromite grains(about 2-3μm).Plagioclase has a heterogeneous composition without zoning. Pyroxene geothermometry of GRV 020175 gives a peak metamorphic temperature(～1000℃) and a closure temperature(～875℃).Molar Fe/Mn ratios(19-32) of pyroxenes are consistent with mesosiderite pyroxenes(16-35) and most plagioclase compositions(An_(87.5_96.6)) are within the range of mesosiderite plagioclase grains(An_(88-95)).Olivine composition(Fo_(53.8)) is only slightly lower than the range of olivine compositions in mesosiderites(Fo_(55-90)).All petrographic characteristics and chemical compositions of GRV 020175 are consistent with those of mesosiderite and based on its matrix texture and relatively abundant plagioclase,it can be further classified as a type 3A mesosiderite.Mineralogical, penological,and geochemical studies of GRV 020175 imply a complex formation history starting as rapid crystallization from a magma in a lava flow on the surface or as a shallow intrusion.Following primary igneous crystallization,the silicate underwent varying degrees of reheating.It was reheated to 1000℃,followed by rapid cooling to 875℃.Subsequently,metal mixed with silicate,during or after which,reduction of silicates occurred;the reducing agent is likely to have been sulfur.After redox reaction,the sample underwent thermal metamorphism,which produced the corona on the olivine, rims on the inverted pigeonite phenocrysts and overgrowths on the orthopyroxene phenocrysts,and homogenized matrix pyroxenes.Nevertheless,metamorphism was not extensive enough to completely reequilibrate the GRV 020175 materials.  相似文献   

10.
Silicates are found in many group IAB irons; in some cases as abundant angular cm-sized inclusions and in other cases as smaller fragments or single grains in troilite or graphite nodules. The mineralogy of the silicates is chondritic—olivine, pyroxene, albitic plagioclase—as is the bulk composition. The degree of oxidation of the olivine and pyroxene is intermediate between E and H chondrites (Fa 1–8, Fs 4–9). IAB inclusions have ages of about 4.5 Gyr, I129-Xe129 formation intervals in the ranges of chondrites and contain planetary-type rare gases.Samples of San Cristobal, Campo del Cielo, Mundrabilla and Woodbine were examined by microprobe and bulk inclusions from Campo del Cielo, Copiapo, Landes and Woodbine were analyzed by instrumental and radiochemical neutron activation analysis. Nonvolatile lithophilic and siderophih'c elements in Copiapo, Landes and Woodbine have approximately chondritic abundances. The chondritic level of lithophiles indicates the inclusions have not undergone igneous differentiation while the chondritic levels of siderophiles is evidence the metal is native to the inclusions and not matrix metal injected into the silicates. The two Campo del Cielo inclusions analyzed have roughly chondritic abundances of lithophiles but have fractionated rare earth patterns and widely varying amounts and abundances (relative to Ni) of siderophiles. These inclusions appear to have experienced some partial melting. Siderophile ratios for the inclusions have some differences when compared to matrix metal. One Campo del Cielo inclusion contains kamacite (5.5% Ni) with over 1000 μg Ge.Three-isotope O analyses by Clayton and coworkers of parts of the same or neighboring inclusions to those analyzed chemically place the inclusions slightly below the terrestrial fractionation line of clayton et al. (1976) and rule out the possibility of the inclusions being trapped fragments of one of the ordinary chondrite groups.The IAB silicates formed probably in a similar manner as chondrite groups but in a different region of the nebula and they record the O2 fugacity and O isotopic composition of that location. They later became trapped in the metal-rich matrix probably as the result of collisions producing the breccialike texture. The relationship of the silicates to the kamacite-taenite structure of the metal requires that the metal-silicate mix have been heated to over 1000 K for an extended period.Two anomalous stony meteorites, Winona and Mt. Morris (Wis), are similar to IAB inclusions in mineralogy, bulk composition, FeO(FeO + Mg) ratio of the silicates, and chromite composition and are possibly related to the IAB silicates. Winona also has an age of 4.6 Gyr and contains planetary-type rare gases. Microprobe data are reported for the major minerals of these anomalous meteorites. Although attempts to detect IAB levels of Ge in the metal phases were not successful, the weight of the evidence favors a relationship between these meteorites and IAB  相似文献   

11.
The Bencubbin meteorite is a polymict breccia consisting of a host fraction of ~60% metal and ~40% ferromagnesian silicates and a selection of carbonaceous, ordinary and ‘enstatite’ chondritic clasts. Concentrations of 27 elements were determined by neutron activation in replicate samples of the host silicates and the ordinary and carbonaceous chondritic clasts; 12 elements were determined in the host metal. Compositional data for the ordinary chondrite clast indicate a classification of LL4 ± 1. Refractory element data for the carbonaceous chondrite clast indicate that it belongs to the CI-CM-CO clan; its volatile element abundances are intermediate between those of CM and CO chondrites. Abundances of nonvolatile elements in the silicate host are similar to those in the carbonaceous chondrite clast and in CM chondrites; the rare earths are unfractionated. We conclude that it is not achondritic as previously designated, but chondritic and that it is probably related to the CI-CM-CO clan; its volatile abundances are lower than those in CO chondrites. Oxygen isotope data are consistent with these classifications. Host metal in Bencubbin and in the closely related Weatherford meteorite has low abundances of moderately volatile siderophiles; among iron meteorite groups its nearest relative is group IIIF.We suggest that Bencubbin and Weatherford formed as a result of an impact event on a carbonaceous chondrite regolith. The impact generated an ‘instant magma’ that trapped and surrounded regolithic clasts to form the polymict breccia. The parent of this ‘magma’ was probably the regolith itself, perhaps mainly consisting of the so-called ‘enstatite’ chondrite materials. Accretion of such a variety of materials to a small parent body was probably only possible in the asteroid belt.  相似文献   

12.
In the Piancaldoli LL3 chondrite, we found a mm-sized clast containing ~100 chondrules 0.2–64 μm in apparent diameter (much smaller than any previously reported) that are all of the same textural type (radial pyroxene; FS1–17). This clast, like other type 3 chondrites, has a fine-grained Ferich opaque silicate matrix, sharply defined chondrules, abundant low-Ca clinopyroxene and minor troilite and Si- and Cr-bearing metallic Fe,Ni. However, the very high modal matrix abundance (63 ± 8 vol. %), unique characteristics of the chondrules, and absence of microscopically-observable olivine indicate that the clast is a new kind of type 3 chondrite. Most chondrules have FeO-rich edges, and chondrule size is inversely correlated with chondrule-core FeO concentration (the first reported correlation of chondrule size and composition). Chondrules acquired Fe by diffusion from Fe-rich matrix material during mild metamorphism, possibly before final consolidation of the rock. Microchondrules (those chondrules ? 100 μm in diameter) are also abundant in another new kind of type 3 chondrite clast in the Rio Negro L chondrite regolith breccia. In other type 3 chondrite groups, microchondrule abundance appears to be anticorrelated with mean chondrule size, viz. 0.02–0.04 vol. % in H and CO chondrites and ?0.006 vol. % in L, LL, and CV chondrites.Microchondrules probably formed by the same process that formed normal-sized droplet chondrules: melting of pre-existing dustballs. Because most compound chondrules in the clast and other type 3 chondrites formed by collisions between chondrules of the same textural type, we suggest that dust grains were mineralogically sorted in the nebula before aggregating into dustballs. The sizes of compound chondrules and chondrule craters, which resulted from collisions of similarly-sized chondrules while they were plastic, indicate that size-sorting (of dustballs) occurred before chondrule formation, probably by aerodynamic processes in the nebula. We predict that other kinds of type 3 chondrites exist which contain chondrule abundances, size-ranges and proportions of textural types different from known chondrite groups.  相似文献   

13.
陨石矿物种类的研究进展和矿物表   总被引：3，自引：0，他引：3  
早在约 2 0 0年前 ,科学家就在铁陨石中鉴定出了两种陨石矿物——陨硫铁和金属铁—镍。到了 1 9世纪 80年代 ,陨石矿物的数量增加到 1 6种。 2 0世纪 60年代以来 ,随着显微镜的广泛应用 ,以及许多新的测试技术如 X射线衍射、电子探针、扫描电镜和透射电镜的应用 ,使更多的陨石矿物能被发现。1 967年 ( Mason列出 60种陨石矿物 )至 1 987年( Yudin和 Kolomenskiy列出了 2 0 0种陨石矿物 )的2 0年间陨石矿物种数增加到原来的 3倍多。1 991年Ulyanov完成了陨石、星际尘粒和玻璃陨石共 350种矿物的列表。Rubin〔1〕在前人工作的基础上 ,去掉…  相似文献   

14.
Based on their mineralogy and petrography, ∼200 refractory inclusions studied in the unique carbonaceous chondrite, Acfer 094, can be divided into corundum-rich (0.5%), hibonite-rich (1.1%), grossite-rich (8.5%), compact and fluffy Type A (spinel-melilite-rich, 50.3%), pyroxene-anorthite-rich (7.4%), and Type C (pyroxene-anorthite-rich with igneous textures, 1.6%) Ca,Al-rich inclusions (CAIs), pyroxene-hibonite spherules (0.5%), and amoeboid olivine aggregates (AOAs, 30.2%). Melilite in some CAIs is replaced by spinel and Al-diopside and/or by anorthite, whereas spinel-pyroxene assemblages in CAIs and AOAs appear to be replaced by anorthite. Forsterite grains in several AOAs are replaced by low-Ca pyroxene. None of the CAIs or AOAs show evidence for Fe-alkali metasomatic or aqueous alteration. The mineralogy, textures, and bulk chemistry of most Acfer 094 refractory inclusions are consistent with their origin by gas-solid condensation and may reflect continuous interaction with SiO and Mg of the cooling nebula gas. It appears that only a few CAIs experienced subsequent melting. The Al-rich chondrules (ARCs; >10 wt% bulk Al2O3) consist of forsteritic olivine and low-Ca pyroxene phenocrysts, pigeonite, augite, anorthitic plagioclase, ± spinel, FeNi-metal, and crystalline mesostasis composed of plagioclase, augite and a silica phase. Most ARCs are spherical and mineralogically uniform, but some are irregular in shape and heterogeneous in mineralogy, with distinct ferromagnesian and aluminous domains. The ferromagnesian domains tend to form chondrule mantles, and are dominated by low-Ca pyroxene and forsteritic olivine, anorthitic mesostasis, and Fe,Ni-metal nodules. The aluminous domains are dominated by anorthite, high-Ca pyroxene and spinel, occasionally with inclusions of perovskite; have no or little FeNi-metal; and tend to form cores of the heterogeneous chondrules. The cores are enriched in bulk Ca and Al, and apparently formed from melting of CAI-like precursor material that did not mix completely with adjacent ferromagnesian melt. The inferred presence of CAI-like material among precursors for Al-rich chondrules is in apparent conflict with lack of evidence for melting of CAIs that occur outside chondrules, suggesting that these CAIs were largely absent from chondrule-forming region(s) at the time of chondrule formation. This may imply that there are several populations of CAIs in Acfer 094 and that mixing of “normal” CAIs that occur outside chondrules and chondrules that accreted into the Acfer 094 parent asteroid took place after chondrule formation. Alternatively, there may have been an overlap in the CAI- and chondrule-forming regions, where the least refractory CAIs were mixed with Fe-Mg chondrule precursors. This hypothesis is difficult to reconcile with the lack of evidence of melting of AOAs which represent aggregates of the least refractory CAIs and forsterite grains.  相似文献   

15.
Since 1994, the Rumuruti (R) chondrites have been recognized as a new, well-established chondrite group differing from carbonaceous, ordinary, and enstatite chondrites. The first R chondrite, Carlisle Lakes, was found in Australia in 1977. Meanwhile, the number has increased to 107 (December, 2010). This group is named after the Rumuruti meteorite, the first and so far the only R chondrite fall. Most of the R chondrites are breccias containing a variety of different clasts embedded in a clastic matrix. Some textural and mineralogical characteristics can be summarized as follows: (a) the chondrule abundance in large fragments and in unbrecciated rocks is ∼35–50 vol%; (b) Ca,Al-rich inclusions are rare; (c) the olivine abundance is typically 65–78 vol%; (d) the mean chondrule diameter is ∼400 μm; (e) in unequilibrated R chondrites, low-Ca pyroxene is dominating, whereas in equilibrated R chondrites it is Ca-rich pyroxene; (f) the typical olivine in a metamorphosed lithology is ∼Fa38–40; (g) matrix olivine in unequilibrated, type 3 fragments and rocks has much higher Fa (∼45–60 mol%) compared to matrix olivines in type 4–6 lithologies (∼Fa38–41); (h) spinels have a high TiO2 of ∼5 wt%; (i) abundant different noble metal-bearing phases (metals, sulfides, tellurides, arsenides) occur. The exception is the metamorphosed, type 5/6 R chondrite La Paz Icefield 04840 which contains hornblende, phlogopite, and Ca-poor pyroxene, the latter phase typically occurring in low-grade metamorphosed R chondrites only.In bulk composition, R chondrites have some affinity to ordinary chondrites: (a) the absence of significant depletions in Mn and Na in R chondrites and ordinary chondrites is an important feature to distinguish these groups from carbonaceous chondrites; (b) total Fe (∼24 wt%) of R chondrites is between those of H and L chondrites (27.1 and 21.6 wt%, respectively); (c) the average CI/Mg-normalized lithophile element abundances are ∼0.95 × CI, which is lower than those for carbonaceous chondrites (≥1.0 × CI) and slightly higher than those for ordinary chondrites (∼0.9 × CI); (d) trace element concentrations such as Zn (∼150 ppm) and Se (∼15 ppm) are much higher than in ordinary chondrites; (e) the whole rock Δ17O of ∼2.7 for R chondrites is the highest among all meteorite groups, and the mean oxygen isotope composition is δ17O = 5.36 ± 0.43, δ18O = 5.07 ± 0.86, Δ17O = +2.72 ± 0.31; (f) noble gas cosmic ray exposure ages of R chondrites range between ∼0.1 and ∼70 Ma. More than half of the R chondrites analyzed for noble gases contain implanted solar wind and, thus, are regolith breccias. The 43 R chondrites from Northern Africa analyzed so far for noble gases seem to represent at least 16 falls. Although the data base is still scarce, the data hint at a major collision event on the R chondrite parent body between 15 and 25 Ma ago.  相似文献   

16.
An Fe isotope study of ordinary chondrites   总被引：3，自引：0，他引：3  
The Fe isotope composition of ordinary chondrites and their constituent chondrules, metal and sulphide grains have been systematically investigated. Bulk chondrites fall within a restricted isotopic range of <0.2‰ δ56Fe, and chondrules define a larger range of >1‰ (−0.84‰ to 0.21‰ relative to the IRMM-14 Fe standard). Fe isotope compositions do not vary systematically with the very large differences in total Fe concentration, or oxidation state, of the H, L, and LL chondrite classes. Similarly, the Fe isotope compositions of chondrules do not appear to be determined by the H, L or LL classification of their host chondrite. This may support an origin of the three ordinary chondrite groups from variable accretion of identical Fe-bearing precursors.A close relationship between isotopic composition and redistribution of Fe during metamorphism on ordinary chondrite parent bodies was identified; the largest variations in chondrule compositions were found in chondrites of the lowest petrologic types. The clear link between element redistribution and isotopic composition has implications for many other non-traditional isotope systems (e.g. Mg, Si, Ca, Cr). Isotopic compositions of chondrules may also be determined by their melting history; porphyritic chondrules exhibit a wide range in isotope compositions whereas barred olivine and radial pyroxene chondrules are generally isotopically heavier than the ordinary chondrite mean. Very large chondrules preserve the greatest heterogeneity of Fe isotopes.The mean Fe isotope composition of bulk ordinary chondrites was found to be −0.06‰ (±0.12‰ 2 SD); this is isotopically lighter than the terrestrial mean composition and all other published non-chondritic meteorite suites e.g. lunar and Martian samples, eucrites, pallasites, and irons. Ordinary chondrites, though the most common meteorites found on Earth today, were not the sole building blocks of the terrestrial planets.  相似文献   

17.
Relatively coarse-grained rims occur around all types of chondrules in type 3 carbonaceous and ordinary chondrites. Those in H-L-LL3 chondrites are composed primarily of olivine and low-Ca pyroxene; those in CV3 chondrites contain much less low-Ca pyroxene. Average grain sizes range from ～4 μm in H-L-LL3 chondrites to ～10 μm in CV3 chondrites. Such rims surround ～50%, ～10% and ≤ 1% of chondrules in CV3, H-L-LL3 and CO3 chondrites, respectively, but are rare (≤1%) around CV3 Ca,Al-rich inclusions. Rim thicknesses average ～150 μm in H-L-LL3 chondrites and ～400 μm in CV3 chondrites.The rims in H-L-LL3 chondrites are composed of material very similar to that which comprises darkzoned chondrules and recrysiallized matrix. Dark-zoned chondrules and coarse-grained rims probably formed in the solar nebula from clumps of opaque matrix material heated to sub-solidus to sub-liquidus temperatures during chondrule formation. Mechanisms capable of completely melting some material while only sintering other material require steep thermal gradients; suitable processes are lightning, reconnecting magnetic field lines and, possibly, aerodynamic drag heating.CV chondrites may have formed in a region where the chondrule formation mechanism was less efficient, probably at greater solar distances than the ordinary chondrites. The lesser efficiency of heating could be responsible for the greater abundance of coarse-grained rims around CV chondrules. Alternatively, CV chondrules may have suffered fewer particle collisions prior to agglomeration.  相似文献   

18.
在南极格罗夫山普通球粒陨石的风化等级划分中出现了和Wlotzka(1993)标准矛盾的现象。部分普通球粒陨石的金属和陨硫铁氧化不足20%,然而硅酸盐却发生了蚀变。如果考虑金属的氧化量,这种风化程度应为W1,如果考虑硅酸盐的蚀变,这种风化程度应为W5。对于存在如此大的差异本文给出了折衷的解决办法——对金属和硅酸盐同时进行风化等级划分。金属的风化等级划分为W_m0-W_m4五个,硅酸盐风化等级划分为W_s0-W_s3四个。依据新方案,GRV 021588、021636、021772和021957等4块无法用Wlotzka(1993)标准来确定风化等级的陨石的风化等级均为W_m1-W_s1。而陨石GRV 023312的风化等级为W_m3-W_s0,其相当于Wlotzka(1993)标准中的W3。  相似文献   

19.
Amoeboid olivine aggregates (AOAs) in primitive (unmetamorphosed and unaltered) carbonaceous chondrites are uniformly 16O-enriched (Δ17O ∼ −20‰) and consist of forsterite (Fa<2), FeNi-metal, and a refractory component (individual CAIs and fine-grained minerals interspersed with forsterite grains) composed of Al-diopside, anorthite, ±spinel, and exceptionally rare melilite (Åk<15); some CAIs in AOAs have compact, igneous textures. Melilite in AOAs is replaced by a fine-grained mixture of spinel, Al-diopside, and anorthite. Spinel is corroded by anorthite or by Al-diopside. In ∼10% of > 500 AOAs studied in the CR, CV, CM, CO, CH, CB, and ungrouped carbonaceous chondrites Acfer 094, Adelaide, and LEW85332, forsterite is replaced to a various degree by low-Ca pyroxene. There are three major textural occurrences of low-Ca pyroxene in AOAs: (i) thin (<10 μm) discontinuous layers around forsterite grains or along forsterite grain boundaries in AOA peripheries; (ii) haloes and subhedral grains around FeNi-metal nodules in AOA peripheries, and (iii) thick (up to 70 μm) continuous layers with abundant tiny inclusions of FeNi-metal grains around AOAs. AOAs with low-Ca pyroxene appear to have experienced melting of various degrees. In the most extensively melted AOA in the CV chondrite Leoville, only spinel grains are relict; forsterite, anorthite and Al-diopside were melted. This AOA has an igneous rim of low-Ca pyroxene with abundant FeNi-metal nodules and is texturally similar to Type I chondrules.Based on these observations and thermodynamic analysis, we conclude that AOAs are aggregates of relatively low temperature solar nebular condensates originated in 16O-rich gaseous reservoir(s), probably CAI-forming region(s). Some of the CAIs were melted before aggregation into AOAs. Many AOAs must have also experienced melting, but of a much smaller degree than chondrules. Before and possibly after aggregation, melilite and spinel reacted with the gaseous SiO and Mg to form Ca-Tschermakite (CaAl2SiO6)-diopside (CaMgSi2O6) solid solution and anorthite. Solid or incipiently melted olivine in some AOAs reacted with gaseous SiO in the CAI- or chondrule-forming regions to form low-Ca pyroxene: Mg2SiO4 + SiO(g) + H2O(g) = Mg2Si2O6 + H2(g). Some low-Ca pyroxenes in AOAs may have formed by oxidation of Si-bearing FeNi-metal: Mg2SiO4 + Si(in FeNi) + 2H2O(g) = Mg2Si2O6 + 2H2(g) and by direct gas-solid condensation: Mg(g) + SiO(g) +H2O(g) = Mg2Si2O6(s) + H2(g) from fractionated (Mg/Si ratio < solar) nebular gas.Although bulk compositions of AOAs are rather similar to those of Type I chondrules, on the projection from spinel onto the plane Ca2SiO4-Mg2SiO4-Al2O3, these objects plot on different sides of the anorthite-forsterite thermal divide, suggesting that Type I chondrules cannot be produced from AOAs by an igneous fractionation. Formation of low-Ca pyroxene by reaction of AOAs with gaseous SiO and by melting of silica-rich dust accreted around AOAs moves bulk compositions of the AOAs towards chondrules, and provide possible mechanisms of transformation of refractory materials into chondrules or chondrule precursors. The rare occurrences of low-Ca pyroxene in AOAs may indicate that either AOAs were isolated from the hot nebular gas before condensation of low-Ca pyroxene or that condensation of low-Ca pyroxene by reaction between forsterite and gaseous SiO was kinetically inhibited. If the latter is correct, then the common occurrences of pyroxene-rich Type I chondrules may require either direct condensation of low-Ca pyroxenes or SiO2 from fractionated nebular gas or condensation of gaseous SiO into chondrule melts.  相似文献   

20.
Acapulcoites (most ancient Hf-W ages are 4,563.1?±?0.8 Ma), lodranites (most ancient Hf-W ages are 4,562.6?±?0.9 Ma) and rocks transitional between them are ancient residues of different degrees of partial melting of a chondritic source lithology (e.g., as indicated by the occurrence of relict chondrules in 9 acapulcoites), although the precise chondrite type is unknown. Acapulcoites are relatively fine- grained (～150–230?μm) rocks with equigranular, achondritic textures and consist of olivine, orthopyroxene, Ca-rich clinopyroxene, plagioclase, metallic Fe,Ni, troilite, chromite and phosphates. Lodranites are coarser grained (540–700?μm), with similar equigranular, recrystallized textures, mineral compositions and contents, although some are significantly depleted in eutectic Fe,Ni-FeS and plagioclase- clinopyroxene partial melts. The acapulcoite-lodranite clan is most readily distinguished from other groups of primitive achondrites (e.g., winoanites/IAB irons) by oxygen isotopic compositions, although more than 50% of meteorites classified as acapulcoites currently lack supporting oxygen isotopic data. The heat source for melting of acapulcoites-lodranites was internal to the parent body, most likely 26Al, although some authors suggest it was shock melting. Acapulcoites experienced lower temperatures of ～980–1170?°C and lower degrees of partial melting (～1–4?vol.%) and lodranites higher temperatures of ～1150–1200?°C and higher degrees (～5?≥?10?vol.%) of partial melting. Hand-specimen and thin section observations indicate movement of Fe,Ni-FeS, basaltic, and phosphate melts in veins over micrometer to centimeter distances. Mineralogical, chemical and isotopic properties, Cosmic Ray Exposure (CRE) ages which cluster around 4–6 Ma and the occurrence of some meteorites consisting of both acapulcoite and lodranite material, indicate that these meteorites come from one parent body and were most likely ejected in one impact event. Whereas the precise parent asteroid of these meteorites is unknown, there is general agreement that it was an S-type object. There is nearly total agreement that the acapulcoite-lodranite parent body was <～100?km in radius and, based on the precise Pb–Pb age for Acapulco of 4555.9?±?0.6 Ma, combined with the Hf/W and U/Pb records and cooling rates deduced from mineralogical and other investigations, that the parent body was fragmented during its cooling which the U/Pb system dates at precisely 4556?±?1 Ma. Hf-W chronometry suggests that the parent body of the acapulcoites-lodranites and, in fact, the parent bodies of all “primitive achondrites” accreted slightly later than those of the differentiated achondrites and, thus, had lower contents of 26Al, the heat producing radionuclide largely responsible for heating of both primitive and differentiated achondrites. Thus, the acapulcoite-lodranite parent body never experienced the high degrees of melting responsible for the formation of the differentiated meteorites, but arrested its melting history at relatively low degrees of ～15?vol.%.  相似文献   

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