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1.
Lithic fragments in LL-group chondrites commonly have poikilitic textures, in part or in whole, where mainly olivine is enclosed by orthopyroxene. Partially poikilitic fragments also have grano-blastic areas and anhedral olivine larger than the olivine enclosed by pyroxene. In analogy to lunar poikilitic rocks and lithic fragments, poikilitic lithic fragments in LL-group chondrites, i.e., meteorites which are highly brecciated due to repeated impacts, are also interpreted as being related to impact events on meteorite parent bodies where melting and reheating of protolith occurred. Compositional characteristics of minerals in certain fragments, such as highly-unequilibrated clinopyroxene (CaO, 14.5 to 17.3 wt %; Al2O3, 6.7%) and relatively high CaO (0.70 to 2.5 wt %) in orthopyroxene in a Ngawi fragment, seem to indicate a melt origin. However, as in the lunar case, it is difficult to decide whether the meteoritic poikilitic textures resulted from complete or partial melting or largely by solid-state recrystallization, although the large olivines that may be relict crystals appear to indicate that at least partial melting was involved. In all probability, all three processes are responsible for the poikilitic textures in chondrites, since temperature regimes produced by impact processes are likely to range widely. These interpretations may also apply to the poikilitic-textured Shaw chondrite, L-group, which may owe its poikilitic texture to impact partial-melting processes while in the parent body regolith.  相似文献   

2.
Chromites from Middle Ordovician fossil L chondrites and from matrix and shock‐melt veins in Catherwood, Tenham, and Coorara L chondrites were studied using Raman spectroscopy and TEM. Raman spectra of chromites from fossil L chondrites showed similarities with chromites from matrix and shock‐melt veins in the studied L chondrite falls and finds. Chromites from shock‐melt veins of L chondrites show polycrystallinity, while the chromite grains in fossil L chondrites are single crystals. In addition, chromites from shock‐melt veins in the studied L chondrites have high densities of planar fractures within the subgrains and many subgrains show intergrowths of chromite and xieite. Matrix chromite of Tenham has similar dislocation densities and planar fractures as a chromite from the fossil meteorite Golvsten 001 and higher dislocation densities than in chromite from the fossil meteorite Sextummen 003. Using this observation and knowing that the matrix of Tenham experienced 20–22 GPa and <1000° C, an upper limit for the P,T conditions of chromite from Golvsten 001 and Sextummen 003 can be estimated to be 20–22 GPa and 1000° C (shock stage S3–S6) and 20 GPa and 1000° C (S3–S5), respectively, and we conclude that the studied fossil meteorite chromites are from matrix.  相似文献   

3.
Abstract— Bencubbin is an unclassified meteorite breccia which consists mainly of host silicate (～40 vol.%) and host metal (～60%) components. Rare (< 1%) ordinary chondrite clasts and a dark xenolith (formerly called a carbonaceous chondrite clast) are also found. A petrologic study of the host silicates shows that they have textures, modes, mineralogy and bulk compositions that are essentially the same as that of barred olivine (BO) chondrules, and they are considered to be BO chondritic material. Bulk compositions of individual host silicate clasts are identical and differ only in their textures which are a continuum from coarsely barred, to finely barred, to feathery microcrystalline; these result from differing cooling rates. The host silicates differ from average BO chondrules only in being angular clasts rather than fluid droplet-shaped objects, and in being larger in size (up to 1 cm) than most chondrules; but large angular to droplet-shaped chondrules occur in many chondrites. Bencubbin host metallic FeNi clasts have a positive Ni-Co trend, which coincides with that of a calculated equilibrium nebular condensation path. This appears to indicate a chondritic, rather than impact, origin for this component as well. The rare ordinary chondrite clast and dark xenolith also contain FeNi metal with compositions similar to that of the host metal. Two scenarios are offered for the origin of the Bencubbin breccia. One is that the Bencubbin components are chondritic and were produced in the solar nebula. Later brecciation, reaggregation and minor melting of the chondritic material resulted in it becoming a monomict chondritic breccia. The alternative scenario is that the Bencubbin components formed as a result of major impact melting on a chondritic parent body; the silicate fragments were formed from an impact-induced lava flow and are analogous to the spinifex-textured rocks characteristic of terrestrial komatiites. Both scenarios have difficulties, but the petrologic, chemical and isotopic data are more consistent with Bencubbin being a brecciated chondrite. Bencubbin has a number of important chemical and isotopic characteristics in common with the major components in the CR (Renazzo-type) chondrites and the unique ALH85085 chondrite, which suggests that their major components may be related. These include: (1) Mafic silicates that are similarly Mg-rich and formed in similar reducing environments. (2) Similarly low volatiles; TiO2, Al2O3 and Cr2O3 contents are also similar. (3) Similar metallic FeNi compositions that sharply differ from those in other chondrites. (4) Remarkable enrichments in 15N. (5) Similar oxygen isotopic compositions that lie on the same mixing line. Thus, the major components of the Bencubbin breccia are highly similar to those of the ALH85085 and CR chondrites and they may have all formed in the same isotopic reservoir, under similar conditions, in the CR region of the solar nebula.  相似文献   

4.
Queen Alexandra Range (QUE) meteorite 94204 is an anomalous enstatite meteorite whose petrogenesis has been ascribed to either partial melting or impact melting. We studied the meteorite pairs QUE 94204, 97289/97348, 99059/99122/99157/99158/99387, and Yamato (Y)‐793225; these were previously suggested to represent a new grouplet. We present new data for mineral abundances, mineral chemistries, and siderophile trace element compositions (of Fe,Ni metal) in these meteorites. We find that the texture and composition of Y‐793225 are related to EL6, and that this meteorite is unrelated to the QUEs. The mineralogy and siderophile element compositions of the QUEs are consistent with petrogenesis from an enstatite chondrite precursor. We caution that potential re‐equilibration during melting and recrystallization of enstatite chondrite melt‐rocks make it unreliable to use mineral chemistries to assign a specific parent body affinity (i.e., EH or EL). The QUEs have similar mineral chemistries among themselves, while slight variations in texture and modal abundances exist between them. They are dominated by inclusion‐bearing millimeter‐sized enstatite (average En99.1–99.5) with interstitial spaces filled predominantly by oligoclase feldspar (sometimes zoned), kamacite (Si approximately 2.4 wt%), troilite (≤2.4 wt% Ti), and cristobalite. Siderophile elements that partition compatibly between solid metal and liquid metal are not enriched like in partial melt residues Itqiy and Northwest Africa (NWA) 2526. We find that the modal compositions of the QUEs are broadly unfractionated with respect to enstatite chondrites. We conclude that a petrogenesis by impact melting, not partial melting, is most consistent with our observations.  相似文献   

5.
Ancient, SiO2‐rich achondrites have previously been proposed to have formed by disequilibrium partial melting of chondrites. Here, we test the alternative hypothesis that these achondrites formed by fractional crystallization of impact melts of Rumuruti (R) chondrites. We identified two new melt clasts in R chondrites, one in Pecora Escarpment (PCA) 91241 and one in LaPaz Icefield (LAP) 031275. We analyzed major, minor, and trace element concentrations, as well as oxygen isotopes, of these two clasts and a third one that had been previously recognized (Bischoff et al. 2011) as an impact melt in Dar al Gani (DaG) 013. The melt clast in PCA 91241 is an R chondrite impact melt closely resembling the one previously recognized in DaG 013. The melt clast in LAP 031275 has an L chondrite provenance. We show that SiO2‐rich melts could form from the mesostases of R chondrite impact melts. However, their CI‐normalized rare earth element patterns are flat, whereas those of ancient SiO2‐rich achondrites (Day et al. 2012; Srinivasan et al. 2018) and those of disequilibrium partial melts of chondrites (Feldstein et al. 2001) have positive Eu anomalies from preferential melting of plagioclase. Thus, we conclude that ancient SiO2‐rich achondrites were probably formed by disequilibrium partial melting (due to an internal heat source on their parent bodies), rather than from impact melts.  相似文献   

6.
Abstract– High pressure phases majorite, possibly majorite‐pyropess, wadsleyite, and coesite are present in the matrix and in barred olivine fragments in the Gujba CB chondrite. Grossular‐pyrope was also observed in some small inclusions. The CB chondrites are metal‐rich meteorites with characteristics that sharply distinguish them from other chondrite groups. All of the CB chondrites contain impact melt regions interstitial to their chondrules, fragments and metal and a major impact event (or events), on the CB chondrite parent body is clearly a significant stage in its history. We studied three areas interstitial to chondrules and metal in the Gujba CBa chondrite. From Raman spectra, the barred olivine fragments and matrix in these regions have various combinations of olivine and low‐Ca pyroxene, as well as majorite garnet (Mg4Si4O12), a phase that forms by high‐pressure transformation of low‐Ca pyroxene and wadsleyite, a high pressure product of olivine. Compositions of the majorite suggest both majorite and majorite‐pyrope solid solution may be present. The mineral assemblage of majorite and wadsleyite suggest minimum shock pressures and temperatures of ～19 GPa and ～2000 °C, respectively. The occurrences of high pressure phases are variable from one area to another, on the scale of millimeters or less, suggesting heterogeneous distribution of shock and/or back transformation to low pressure polymorphs throughout the meteorite. The high pressure phases record a high temperature–pressure impact event that is superimposed onto, and thus postdates formation of, the chondrules and other components in the CB chondrites. The barred chondrules and metal in the CB chondrites are primary materials formed prior to the impact event either generated in an earlier planetesimal scale impact event or in the nebula.  相似文献   

7.
The brecciation and shock classification of 2280 ordinary chondrites of the meteorite thin section collection at the Institut für Planetologie (Münster) has been determined. The shock degree of S3 is the most abundant shock stage for the H and LL chondrites (44% and 41%, respectively), while the L chondrites are on average more heavily shocked having more than 40% of rocks of shock stage S4. Among the H and LL chondrites, 40–50% are “unshocked” or “very weakly shocked.” Considering the petrologic types, in general, the shock degree is increasing with petrologic type. This is the case for all meteorite groups. The main criteria to define a rock as an S6 chondrite are the solid‐state recrystallization and staining of olivine and the melting of plagioclase often accompanied by the formation of high‐pressure phases like ringwoodite. These characteristics are typically restricted to local regions of a bulk chondrite in or near melt zones. In the past, the identification of high‐pressure minerals (e.g., ringwoodite) was often taken as an automatic and practical criterion for a S6 classification during chondrite bulk rock studies. The shock stage classification of many significantly shocked chondrites (>S3) revealed that most ringwoodite‐bearing rocks still contain more than 25% plagioclase (74%). Thus, these bulk chondrites do not even fulfill the S5 criterion (e.g., 75% of plagioclase has to be transformed into maskelynite) and have to be classified as S4. Studying chondrites on typically large thin sections (several cm2) and/or using samples from different areas of the meteorites, bulk chondrites of shock stage S6 should be extremely rare. In this respect, the paper will discuss the probability of the existence of bulk rocks of S6.  相似文献   

8.
On February 13, 2023, a huge fireball was visible over Western Europe (fireball event 2023 CX1). After the possible strewn field was calculated, the first of several recovered samples, with a mass of about 100 g, was discovered just 2 days after the fireball event on the ground of the village of Saint-Pierre-le-Viger. Meanwhile, more than 60 samples with a total mass of more than 1 kg were recovered and a piece of one of these is studied here. The fall occurred 220 years after the historic meteorite fall of L'Aigle on April 26, 1803, <120 km south. L'Aigle is the closest meteorite fall to Saint-Pierre-le-Viger and belongs to the same chondrite group. Both meteorites are breccias containing only clasts of high metamorphic degree (type 5 and type 6). Since only 20% of the L chondrites are breccias this coincidence is remarkable. As just mentioned, both samples studied from these rocks in this work are ordinary chondrite breccias and consist of equilibrated and recrystallized lithologies of petrologic type 6. The brecciated texture in L'Aigle, resulting in a remarkable light–dark structure, is more pronounced than the brecciated features in Saint-Pierre-le-Viger, from which also type 5 fragments have been reported. The compositions of low-Ca pyroxene and olivine grains in Saint-Pierre-le-Viger (Fs21.2 and Fa23.4, respectively) clearly require an L-group classification. L'Aigle was classified as an L6 breccia in the past, and this has now been confirmed by new data on low-Ca pyroxene and olivine (Fs20.7 and Fa23.8, respectively). Saint-Pierre-le-Viger contains local thin shock veins, and both meteorites are moderately shocked. Most olivines in the studied samples have planar fractures, but the estimated abundance of mosaicized olivines of 30%–40% among the large grains require a S4 shock classification. Oxygen isotope and bulk chemical data of Saint-Pierre-le-Viger certainly support the L chondrite classification. Bulk spectral data of Saint-Pierre-le-Viger are dominated by silicate minerals, that is, Fe-bearing low-Ca pyroxene, olivine, and plagioclase. Isotopic, chemical, and spectral data of the L'Aigle meteorite are shown for comparison and are very similar, providing additional circumstantial evidence of Saint-Pierre-le-Viger's L chondritic nature.  相似文献   

9.
The Grier(b), New Mexico meteorite, a single mass of 929.4 grams, was found in 1969. This brecciated chondrite can be classified as an L-group from the bulk chemical analysis, ～ 8 wt % metal with an estimated total iron content of 25 wt %, and the constant olivine (Fa25.5) and orthopyroxene (Fs23) compositions. The main portion of the meteorite fits the criteria for an L5 (grey to intermediate hypersthene) chondrite. A conspicuous, large (several cm3) dense fragment, texturally an L6–7 chondrite, contains practically no metal or chondrules. However, there is little variation in the bulk silicate and individual phase compositions between the fragment and the matrix. In spite of this, it seems unlikely that the fragment was created in situ because metal and sulfide are not found in the fragment-matrix contact zone; thus the formation of olivines and pyroxenes in both parts, as well as the “draining” of metal from the fragment, occurred prior to accretion with little, if any, subsequent thermal metamorphism.  相似文献   

10.
The Kramer Creek, Colorado, chondrite was found in 1966 and identified as a meteorite in 1972. Bulk chemical analysis, particularly the total iron content (20.36%) and the ratio of Fetotal/SiO2 (0.52), as well as the compositions of olivine (Fa21.7) and orthopyroxene (Fs18.3) place the meteorite into the L-group of chondrites. The well-defined chondritic texture of the meteorite, the presence of igneous glass in the chondrules and of low-Ca clinopyroxene, as well as the slight variations in FeO contents of olivine (2.4% MD) and orthopyroxene (5.6% MD) indicate that the chondrite belongs to the type 4 petrologic class.  相似文献   

11.
High‐precision oxygen three‐isotope ratios were measured for four mineral phases (olivine, low‐Ca and high‐Ca pyroxene, and plagioclase) in equilibrated ordinary chondrites (EOCs) using a secondary ion mass spectrometer. Eleven EOCs were studied that cover all groups (H, L, LL) and petrologic types (4, 5, 6), including S1–S4 shock stages, as well as unbrecciated and brecciated meteorites. SIMS analyses of multiple minerals were made in close proximity (mostly <100 μm) from several areas in each meteorite thin section, to evaluate isotope exchange among minerals. Oxygen isotope ratios in each mineral become more homogenized as petrologic type increases with the notable exception of brecciated samples. In type 4 chondrites, oxygen isotope ratios of olivine and low‐Ca pyroxene are heterogeneous in both δ18O and Δ17O, showing similar systematics to those in type 3 chondrites. In type 5 and 6 chondrites, oxygen isotope ratios of the four mineral phases plot along mass‐dependent fractionation lines that are consistent with the bulk average Δ17O of each chondrite group. The δ18O of three minerals, low‐Ca and high‐Ca pyroxene and plagioclase, are consistent with equilibrium fractionation at temperatures of 700–1000 °C. In most cases the δ18O values of olivine are higher than those expected from pyroxene and plagioclase, suggesting partial retention of premetamorphic values due to slower oxygen isotope diffusion in olivine than pyroxene during thermal metamorphism in ordinary chondrite parent bodies.  相似文献   

12.
The Loop meteorite was found in 1962 in Gaines County, Texas, at a location very close to that where the Ashmore chondrite was found in 1969. The two specimens were assumed to be fragments of the same meteorite. The Loop meteorite is a type L6 chondrite composed of olivine (Fo75.4Fa24.6), orthopyroxene (En77.6Wo1.5Fs20.9), clinopyroxene (En47.5Wo45.1Fs7.4), plagioclase (Ab84.3Or5.5An10.2), Fe-Ni metal, troilite, and chromite. Fe-Ni metal is represented by kamacite (5.8-6.4 wt % Ni, 0.88-1.00 wt % Co), taenite (30.0–52.9 wt % Ni, 0.16-0.34 wt % Co), and plessite (16.8–28.5 wt % Ni, 0.38-0.54 wt % Co). Native copper occurs as rare inclusions in Fe-Ni metal. Both chondrules and matrix have similar mineral compositions. The mineral chemistry of the Loop meteorite is quite different from that of the Ashmore, which was classified as an H5 chondrite by Bryan and Kullerud (1975). Therefore, the Ashmore and Loop meteorites are two different chondrites, even though they were recovered from the same geographic location.  相似文献   

13.
Abstract— Fusion crusts develop on all meteorites during their passage through the atmosphere but have been little studied. We have characterized the textures and compositions of the fusion crusts of 73 stony meteorites to identify the nature of meteorite ablation spheres (MAS) and constrain the processes operating during the entry heating. Most chondrite fusion crusts are porphyritic and are dominated by olivine, glass, and accessory magnetite; whereas those of the achondrites are mainly glassy. Chondrite fusion crusts contain sulphide droplets with high-Ni contents (>55 wt%). The partially melted substrate of ordinary chondrites (underlying the outer melted crusts) are dominated by silicate glass and composite metal, sulphide, and Cr-bearing Fe-oxide droplets that form as coexisting immiscible liquids. Enstatite chondrite substrates contain Cr- and Mn- bearing sulphides. The substrates of the carbonaceous chondrites comprise a sulphide-enriched layer of matrix. The compositions of melted crusts are similar to those of the bulk meteorite. However, differences from whole rock suggest that three main processes control their chemical evolution: (1) the loss and reaction of immiscible Fe-rich liquids, (2) mixing between substrate partial melts and bulk melts of the melted crust, and (3) the loss of volatile components by evaporation and degassing. Data from fusion crusts suggest that MAS produced at low altitude have compositions within the range of those of silicate-dominated cosmic spherules that are formed by the melting dust particles. Meteorite ablation spheres produced at high altitude probably have compositions very different from bulk meteorite and will resemble cosmic spherules derived from coarse-grained precursors.  相似文献   

14.
Abstract— On July 21, 2002, a meteorite fall occurred over the Thuathe plateau of western Lesotho. The well‐defined strewn field covers an area of 1.9 times 7.4 km. Many of the recovered specimens display a brecciated texture with leucocratic, angular to subrounded clasts in a somewhat darker groundmass. Mineralogical and chemical data, as well as oxygen isotopic analysis, indicate that Thuathe is an H4/5, S2/3 meteorite, with local H3 or H6 character. A number of anomalous features include somewhat high Co contents of kamacite and taenite relative to normal H‐group chondrites. Oxygen isotopic data plot at the edge of the normal H chondrite data field. Variable contents of metallic mineral phases and troilite result in a heterogeneous bulk composition (e.g., with regard to Si, Fe, and Mg), resulting in a spread of major element ratios that is not consistent with previously accepted H‐group composition. Trace element abundances are generally consistent with H chondritic composition, and Kr and Xe isotopic data agree with an H4 classification for this meteorite. Noble gas analysis gave U, Th‐4He gas retention and K‐Ar ages typical for H chondrites; no major thermal event affected this material since ～3.7 Ga. The exposure age for Thuathe is 5 Ma, somewhat lower than for other H chondrites. Cosmogenic nuclide analysis indicates a pre‐atmospheric radius of this meteorite between 35 and 40 cm. In the absence of evidence for solar gases, we classify Thuathe as a fragmental breccia. Numerous narrow, black veins cut across samples of Thuathe and are the result of a brittle deformation event that also caused local melting, especially in portions rich in sulfide. The formation of these veinlets is not the result of locally enhanced shock pressures (i.e., of shock melting) but rather of shearing under brittle conditions with local, friction‐related temperature excursions causing melting mostly of Fe‐sulfide and FeNi‐metal but also, locally, of silicate minerals. Frictional temperature excursions must have attained values in excess of 1500 °C to permit complete melting of forsteritic olivine.  相似文献   

15.
Abstract— Itqiy is a unique coarse‐grained, metal‐rich enstatite meteorite that was found in the Western Sahara and consists of two rocks together weighing 4.72 kg, which are both completely coated with fusion crust. We report results from our electron microprobe and instrumental neutron activation analysis techniques. Itqiy consists of subhedral, equigranular, millimeter‐sized enstatite, ?25 vol% of millimeter‐sized kamacite and a few tiny intergrowths of sulfides and kamacite. Relic chondrules are absent. Pyroxene (Fs0.2) is chemically similar to enstatite in EL chondrites, but the metal is closer in composition to that in EH chondrites. Sulfides resemble those in E chondrites but their compositions are distinct from those in both EL and EH chondrites. Itqiy clearly formed under very reducing conditions, but it does not appear to have formed from EH or EL chondrites. Two thermal events can be distinguished. Silicate compositions including rare earth element abundances indicate loss of partial melt and slow cooling. Heterogeneous sulfides indicate a subsequent reheating and quenching event, which may have been due to shock as many enstatite grains show shock stage S3 features.  相似文献   

16.
Pecora Escarpment 91002: A member of the new Rumuruti (R) chondrite group   总被引：1，自引：0，他引：1  
Abstract— Pecora Escarpment (PCA)91002 is a light/dark-structured chondrite breccia related to Carlisle Lakes and Rumuruti; the meteorite contains ～10–20 vol% equilibrated (type ?5 and ?6) clasts within a clastic groundmass, much of which was metamorphosed to type-3.8 levels. The olivine compositional distribution forms a tight cluster that peaks at Fa38–40; by contrast, low-Ca pyroxene compositions are highly variable. Opaque phases identified in PCA91002 and its paired specimen, PCA91241, include pyrrhotite, pentlandite, pyrite, chromite, ilmenite, metallic Cu and magnetite. The majority of the rock is of shock stage S3-S4; there are numerous sulfide-rich shock veins and 50-μm plagioclase melt pockets. Instrumental neutron activation analysis shows that, unlike Carlisle Lakes and ALH85151, PCA91002 exhibits no Ca enrichment or Au depletion; because PCA91002 is relatively unweathered, it seems probable that the Ca and Au fractionations in Carlisle Lakes and ALH85151 were caused by terrestrial alteration. The Rumuruti-like (formerly Carlisle-Lakes-like) chondrites now include eight separate meteorites. Their geochemical and petrographic similarities suggest that they constitute a distinct chondrite group characterized by unfractionated refractory lithophile abundances (0.95 ± 0.05x CI), high bulk Δ17O, a low chondrule/groundmass modal abundance ratio, mean chondrule diameters in the 400 ± 100 μm range, abundant NiO-bearing ferroan olivine, sodic plagioclase, titanian chromite, abundant pyrrhotite and pentlandite and negligible metallic Fe-Ni. We propose that this group be called R chondrites after Rumuruti, the only fall. The abundant NiO-bearing ferroan olivine grains, the occurrence of Cu-bearing sulfide, and the paucity of metallic Fe-Ni indicate that R chondrites are highly oxidized. It is unlikely that appreciable oxidation took place on the parent body because of the essential lack of plausible oxidizing agents (e.g., magnetite or hydrated silicates). Therefore, oxidation of R chondrite material must have occurred in the nebula. A few type-I porphyritic olivine chondrules containing olivine grains with cores of Fa3–4 composition occur in PCA91002; these chondrules probably formed initially as metallic-Fe-Ni-bearing objects at high nebular temperatures. As temperatures decreased and more metallic Fe was oxidized, these chondrules accreted small amounts of oxidized material and were remelted. The ferroan compositions of the >5-μm olivine grains in the R chondrites reflect equilibration with fine-grained FeO-rich matrix material during parent body metatnorphism.  相似文献   

17.
Abstract— The Cerro los Calvos meteorite is a single stone of 68.5 g found in the Nuevo Mercurio strewn field of Zacatecas, Mexico (24°20′N, 102°8′W). It is an unusual H4 chondrite. Its olivine (Fa12.5) and orthopyroxene (Fs 11.7, Wo 0.8) are reduced relative to typical H chondrites. The La Banderia meteorite of 54.3 g from the same vicinity is an LL5 chondrite of shock classification e.  相似文献   

18.
Abstract— A large (≥4.5 × 7 × 4 mm), igneous-textured clast in the Bovedy (L3) chondrite is notable for its high bulk SiO2 content (57.5 wt%). The clast consists of normally zoned orthopyroxene (83.8 vol%), tridymite (6.2 %), an intergrowth of feldspar (5.8 %) and sodic glass (3.1 %), pigeonite (1.0 %), and small amounts of chromite (0.2 %), augite, and Fe, Ni-metal; it is best described as a silica-rich orthopyroxenite. The oxygen-isotopic composition of the clast is similar, but not identical, to Bovedy and other ordinary chondrites. The clast has a superchondritic Si/Mg ratio, but has Mg/(Mg + Fe) and Fe/Mn ratios that are similar to ordinary chondrite silicate. The closest chemical analogues to the clast are radial-pyroxene chondrules, diogenites, pyroxene-silica objects in ordinary chondrites, and silicates in the IIE iron meteorite Weekeroo Station. The clast crystallized from a siliceous melt that cooled fast enough to prevent complete attainment of equilibrium but slow enough to allow nearly complete crystallization. The texture, form, size and composition of the clast suggest that it is an igneous differentiate from an asteroid or planetesimal that formed in the vicinity of ordinary chondrites. The melt probably cooled in the near-surface region of the parent object. It appears that in the source region of the clast, metallic and silicate partial melt were largely-to-completely lost during a relatively low degree of melting, and that during a higher degree of melting, olivine and low-Ca pyroxene separated from the remaining liquid, which ultimately solidified to form the clast. While these fractionation steps could not have all occurred at the same temperature, they could have been accomplished in a single melting episode, possibly as a result of heating by radionuclides or by electromagnetic induction. Fractionated magmas can also account for other Si-rich objects in chondrites.  相似文献   

19.
Abstract— The enstatite chondrite reckling peak (rkp) a80259 contains feldspathic glass, kamacite, troilite, and unusual sets of parallel fine‐grained enstatite prisms that formed by rapid cooling of shock melts. Metallic Fe,Ni and troilite occur as spherical inclusions in feldspathic glass, reflecting the immiscible Fe‐Ni‐S and feldspathic melts generated during the impact. The Fe‐Ni‐S and feldspathic liquids were injected into fractures in coarse‐grained enstatite and cooled rapidly, resulting in thin (≤ 10 μm) semicontinuous to discontinuous veins and inclusion trails in host enstatite. Whole‐rock melt veins characteristic of heavily shocked ordinary chondrites are conspicuously absent. Raman spectroscopy shows that the feldspathic material is a glass. Elevated MgO and SiO2 contents of the glass indicate that some enstatite and silica were incorporated in the feldspathic melt. Metallic Fe,Ni globules are enclosed by sulfide and exhibit Nienrichment along their margins characteristic of rapid crystallization from a Fe‐Ni‐S liquid. Metal enclosed by sulfide is higher in Si and P than metal in feldspathic glass and enstatite, possibly indicating lower O fugacities in metal/sulfide than in silicate domains. Fine‐grained, elongate enstatite prisms in troilite or feldspathic glass crystallized from local pyroxene melts that formed along precursor grain boundaries, but most of the enstatite in the target rock remained solid during the impact and occurs as deformed, coarsegrained crystals with lower CaO, Al2O3, and FeO than the fine‐grained enstatite. Reckling Peak A80259 represents an intermediate stage of shock melting between unmelted E chondrites and whole‐rock shock melts and melt breccias documented by previous workers. The shock petrogenesis of RKPA80259 reflects the extensive impact processing of the enstatite chondrite parent bodies relative to those of other chondrite types.  相似文献   

20.
Dhofar 1671 is a relatively new meteorite that previous studies suggest belongs to the Rumuruti chondrite class. Major and REE compositions are generally in agreement with average values of the R chondrites (RCs). Moderately volatile elements such as Se and Zn abundances are lower than the R chondrite values that are similar to those in ordinary chondrites (OCs). Porphyritic olivine pyroxene (POP), radial pyroxene (RP), and barred olivine (BO) chondrules are embedded in a proportionately equal volume of matrix, one of the characteristic features of RCs. Microprobe analyses demonstrate compositional zoning in chondrule and matrix olivines showing Fa‐poor interior and Fa‐rich outer zones. Precise oxygen isotope data for chondrules and matrix obtained by laser‐assisted fluorination show a genetic isotopic relationship between OCs and RCs. On the basis of our data, we propose a strong affinity between these groups and suggest that OC chondrule precursors could have interacted with a 17O‐rich matrix to form RC chondrules (i.e., ?17O shifts from ~1‰ to ~3‰). These interactions could have occurred at the same time as “exotic” clasts in brecciated samples formed such as NWA 10214 (LL3–6), Parnallee (LL3), PCA91241 (R3.8–6), and Dhofar 1671 (R3.6). We also infer that the source of the oxidation and 17O enrichment is the matrix, which may have been enriched in 17O‐rich water. The abundance of matrix in RCs relative to OCs, ensured that these rocks would be apparently more oxidized and appreciably 17O‐enriched. In situ analysis of Dhofar 1671 is recommended to further strengthen the link between OCs and RCs.  相似文献   

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