首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 109 毫秒
1.
Abstract— Magmatic inclusions occur in type II ureilite clasts (olivine‐orthopyroxene‐augite assemblages with essentially no carbon) and in a large isolated plagioclase clast in the Dar al Gani (DaG) 319 polymict ureilite. Type I ureilite clasts (olivine‐pigeonite assemblages with carbon), as well as other lithic and mineral clasts in this meteorite, are described in Ikeda et al.(2000). The magmatic inclusions in the type II ureilite clasts consist mainly of magnesian augite and glass. They metastably crystallized euhedral pyroxenes, resulting in feldspar component‐enriched glass. On the other hand, the magmatic inclusions in the large plagioclase clast consist mainly of pyroxene and plagioclase, with a mesostasis. They crystallized with a composition along the cotectic line between the pyroxene and plagioclase liquidus fields. DaG 319 also contains felsic lithic clasts that represent various types of igneous lithologies. These are the rare components not found in the common monomict ureilites. Porphyritic felsic clasts, the main type, contain phenocrysts of plagioclase and pyroxene, and their groundmass consists mainly of plagioclase, pyroxene, and minor phosphate, ilmenite, chromite, and/or glass. Crystallization of these porphyritic clasts took place along the cotectic line between the pyroxene and plagioclase fields. Pilotaxitic felsic clasts crystallized plagioclase laths and minor interstitial pyroxene under metastable conditions, and the mesostasis is extremely enriched in plagioclase component in spite of the ubiquitous crystallization of plagioclase laths in the clasts. We suggest that there are two crystallization trends, pyroxene‐metal and pyroxene‐plagioclase trends, for the magmatic inclusions and felsic lithic clasts in DaG 319. The pyroxene‐metal crystallization trend corresponds to the magmatic inclusions in the type II ureilite clasts and the pilotaxitic felsic clasts, where crystallization took place under reducing and metastable conditions, suppressing precipitation of plagioclase. The pyroxene‐plagioclase crystallization trend corresponds to the magmatic inclusions in the isolated plagioclase clast and the porphyritic felsic clasts. This trend developed under oxidizing conditions in magma chambers within the ureilite parent body. The felsic clasts may have formed mainly from albite component‐rich silicate melts produced by fractional partial melting of chondritic precursors. The common monomict ureilites, type I ureilites, may have formed by the fractional partial melting of alkali‐bearing chondritic precursors. However, type II ureilites may have formed as cumulates from a basaltic melt.  相似文献   

2.
Abstract— For most elements, polymict ureilite EET83309 shows no significant compositional difference from other ureilites, including ordinary (“monomict”) ureilites. Polymict ureilites appear to be mixtures of a wide variety of ordinary ureilites, with little dilution by “foreign” extra-ureilitic materials. Thus, they apparently were mixed (i.e., the ureilites in general formed) on a very small number of parent bodies. In one respect, polymict ureilites do stand out. Along with the only other polymict ureilite that has been analyzed for REE (Nilpena), EET83309 has much higher concentrations of light-middle REE than most ordinary ureilites. Despite these relative enrichments in LREE, polymict ureilites are nearly devoid of basaltic (Al-rich) material. A basaltic component should have formed along with (and presumably above) the ultramafic ureilites, in any closed-system differentiation of an originally chondritic asteroid. This scarcity of complementary basaltic materials may be an important clue to ureilite origins. We suggest that ureilites originated as paracumulates (mushy, cumulate-like, partial melt residues) deep within a primordially-heated asteroid or asteroids. While still largely molten, the asteroid was severely disrupted, and most of its external basaltic portion was permanently blown away, by impact of a large, C-rich projectile. This partially-disruptive impact tended to permeate the paracumulates with C-rich, noble-gas-rich, and 16O-rich magma derived mainly from shock-melting of the projectile. After reaccumulation and cooling, the resultant mixtures of cumulus mafic silicates with essentially “foreign” C-matrix became “monomict” ureilites. Further small impacts produced polymict ureilites as components of a newly-developed, basalt-poor megaregolith. The consistently moderate pyroxene/olivine ratios of the ureilites are as expected for partial melt residues, but not for cumulate (sensu stricto) rocks. The final projectile/target mixing ratio tended to be greatest among the more magnesian and pyroxene-rich portions of the paracumulate, because these portions were lowest in density, and thus concentrated toward the upper surface of the paracumulate layer. As a result, ureilites show correlations among C, Δ17O, and silicate-core mg. This model appears to reconcile many paradoxical aspects of ureilite composition (primitive, near-chondritic, except depleted in basalt, diverse Δ17O) and petrography (igneous, cumulate-like).  相似文献   

3.
Abstract— The Elephant Moraine (EET) 96001 ureilite contains a remarkable diversity of feldspars, which occur as tiny (no more than 60 μm maximum dimension) grains within a few Fe,S‐rich (now weathered to mostly Fe oxide) veins. Molar S: Fe ratio in the veins averages 0.08 ± 0.02. The veins meander and feature large fluctuations in apparent width; they appear to have entered this monomict breccia by a gentle, percolative process, not by violent impact injection. The feldspars are accompanied by a diverse suite of K‐rich (and generally also Ti‐rich) feldspathic glasses, and also major proportions of silica and pyroxene, which is largely fassaitic. A rhönite‐like phase is also found, and, as inclusions in one of the fassaites, a Cr‐poor spinel‐like phase. The feldspars mostly feature remarkably high K/Na compared to feldspars of comparable An from polymict ureilites. The EET 96001 feldspathic component was probably once part of a thin basaltic crust on a ureilite asteroid. The spinel included in one of the fassaites formed at remarkably high f02 (apparent oxidation state of iron: ～41 atom% Fe3+), suggesting that the parent magma possibly assimilated near‐surface water (however, the Fe3+ was not directly measured, and has conceivably been affected by terrestrial weathering; also, there is no assurance that this fassaite originated together with the typical feldspar). We speculate that the feldspathic component was mixed into the dense, Fe,S‐rich vein material, and very soon thereafter the Fe,S‐rich vein material was emplaced adjacent to the EET 96001 host ureilite, at an advanced stage in a chaotic catastrophic disruption and partial reassembly process that affected all ureilites. The high‐K nature of the EET 96001 feldspathic component, including the feldspathic glasses, suggests that fractional fusion may not have been as common during ureilite anatexis as has been inferred from recent studies of clast assemblages in polymict ureilites.  相似文献   

4.
Abstract— This paper explores the possible origin of the light rare earth element (LREE) enrichments observed in some ureilites, a question that has both petrogenetic and chronologic implications for this group of achondritic meteorites. Rare earth element and other selected elemental abundances were measured in situ in 14 thin sections representing 11 different ureilites. The spatial microdistributions of REEs in C‐rich matrix areas of the three ureilites with the most striking V‐shaped whole‐rock REE patterns (Kenna, Goalpara, and Novo Urei) were investigated using the ion imaging capability of the ion microprobe. All olivines and clinopyroxenes measured have LREE‐depleted patterns with little variation in REE abundances, despite large differences in their major element compositions from ureilite to ureilite. Furthermore, we searched for but did not find any minor mineral phases that carry LREEs. The only exception is one Ti‐rich area (～20μm) in Lewis Cliff (LEW) 85400 with a major element composition similar to that of titanite; REE abundances in this area are high, ranging from La ? 400 × CI to Lu ? 40 × CI. In contrast, all ion microprobe analyses of C‐rich matrix in Kenna, Goalpara, and Novo Urei revealed large LREE enrichments. In addition, C‐rich matrix areas in the three polymict ureilites, Elephant Moraine (EET) 83309, EET 87720, and North Haig, which have less pronounced V‐shaped whole‐rock REE patterns, show smaller but distinct LREE‐enrichments. The C‐rich matrix in Antarctic ureilites tends to have much lower LREE concentrations than the matrix in non‐Antarctic ureilites. There is no obvious association of the LREEs with other major or minor elements in the C‐rich areas. Ion images further show that the LREE enrichments are homogeneously distributed on a microscale in most C‐rich matrix areas of Kenna, Goalpara, and Novo Urei. These observations suggest that the LREEs in ureilites most probably are absorbed on the surface of fine‐grained amorphous graphite in the C‐rich matrix. It is unlikely that the LREE enrichments are due to shock melts or are the products of metasomatism on the ureilite parent body. We favor LREE introduction by terrestrial contamination.  相似文献   

5.
The meteorite Mount DeWitt (DEW) 12007 is a polymict regolith breccia mainly consisting of glassy impact‐melt breccia particles, gabbroic clasts, feldspathic clasts, impact and volcanic glass beads, basaltic clasts, and mingled breccia clasts embedded in a matrix dominated by fine‐grained crystals; vesicular glassy veins and rare agglutinates are also present. Main minerals are plagioclase (typically An>85) and clinopyroxene (pigeonites and augites, sometimes interspersed). The presence of tranquillityite, coupled with the petrophysical data, the O‐isotope data (Δ17O = ?0.075), and the FeOtot/MnO ratios in olivine (91), pyroxene (65), and bulk rock (77) indicate a lunar origin for DEW 12007. Impactites consist of Al‐rich impact‐melt splashes and plagioclase‐rich meta‐melt clasts. The volcanic products belong to the very low titanium (VLT) or low titanium (LT) suites; an unusual subophitic fragment could be cryptomare‐related. Gabbroic clasts could represent part of a shallow intrusion within a volcanic complex with prevailing VLT affinity. DEW 12007 has a mingled bulk composition with relatively high incompatible element abundances and shows a high crustal diversity comprising clasts from the Moon's major terranes and rare lithologies. First‐order petrographic and chemical features suggest that DEW 12007 could be launch‐paired with other meteorites including Y 793274/981031, QUE 94281, EET 87521/96008, and NWA 4884.  相似文献   

6.
Abstract— Sayh al Uhaymir (SaU) 300 comprises a microcrystalline igneous matrix (grain size <10 μm), dominated by plagioclase, pyroxene, and olivine. Pyroxene geothermometry indicates that the matrix crystallized at ?1100 °C. The matrix encloses mineral and lithic clasts that record the effects of variable levels of shock. Mineral clasts include plagioclase, low‐ and high‐Ca pyroxene, pigeonite, and olivine. Minor amounts of ilmenite, FeNi metal, chromite, and a silica phase are also present. A variety of lithic clast types are observed, including glassy impact melts, impact‐melt breccias, and metamorphosed impact melts. One clast of granulitic breccia was also noted. A lunar origin for SaU 300 is supported by the composition of the plagioclase (average An95), the high Cr content in olivine, the lack of hydrous phases, and the Fe/Mn ratio of mafic minerals. Both matrix and clasts have been locally overprinted by shock veins and melt pockets. SaU 300 has previously been described as an anorthositic regolith breccia with basaltic components and a granulitic matrix, but we here interpret it to be a polymict crystalline impact‐melt breccia with an olivine‐rich anorthositic norite bulk composition. The varying shock states of the mineral and lithic clasts suggest that they were shocked to between 5–28 GPa (shock stages S1–S2) by impact events in target rocks prior to their inclusion in the matrix. Formation of the igneous matrix requires a minimum shock pressure of 60 GPa (shock stage >S4). The association of maskelynite with melt pockets and shock veins indicates a subsequent, local 28–45 GPa (shock stage S2–S3) excursion, which was probably responsible for lofting the sample from the lunar surface. Subsequent fracturing is attributed to atmospheric entry and probable breakup of the parent meteor.  相似文献   

7.
Abstract— The lunar meteorite Dhofar 081, found as a single fragment of 174 g in the Dhofar region of Oman, is a shocked feldspathic fragmental highland breccia dominated by anorthosite‐rich lithic and mineral clasts embedded into a fine‐grained mostly shock melted clastic matrix. Major mineral phases in the bulk rock are Ca‐rich plagioclase (An96.5–99.5), pyroxene (FS21.9–46.2Wo3.0–41.4), and olivine (Fa29.3–47.8); accessory phases include Fe‐Ni metal, ilmenite, and Ti‐Cr‐rich spinel. Dhofar 081 contains subordinate crystalline fragments of large anorthosites, intersertal impact‐melt rocks, microporphyritic impact‐melt breccias, dark fine‐grained impact‐melt breccias, large cataclastic feldspars, and irregularly shaped brown glass clasts. Mafic components are rare and no genuine regolith components were found in the sections studied. Minerals in Dhofar 081 show homogeneously distributed shock features: intergranular recrystallization, strong fracturing and mosaicism in feldspar as well as a high density of mostly irregular fractures in pyroxene and olivine. Localized impact melting caused by one or several impacts led to a strong lithification. Based on these effects an equilibration shock pressure of about 15–20 GPa is estimated for the strongest shock event in Dhofar 081. Devitrification of the “glassy” material in the rock indicates thermal annealing after shock melting suggesting that the 15–20 GPa shock event predated the ejection event. According to the concentrations of implanted solar noble gases Dhofar 081 represents a polymict clastic breccia deposit with possibly a minor regolith component. A similar noble gas record of Dhofar 081 and MacAlpine Hills 88104/05 suggests the possibility of a source crater pairing of both meteorites. As indicated by noble gas measurements pairing of Dhofar 081 with the other lunar meteorites found in Oman, Dhofar 025 and Dhofar 026, is unlikely.  相似文献   

8.
Northwest Africa 7533, a polymict Martian breccia, consists of fine‐grained clast‐laden melt particles and microcrystalline matrix. While both melt and matrix contain medium‐grained noritic‐monzonitic material and crystal clasts, the matrix also contains lithic clasts with zoned pigeonite and augite plus two feldspars, microbasaltic clasts, vitrophyric and microcrystalline spherules, and shards. The clast‐laden melt rocks contain clump‐like aggregates of orthopyroxene surrounded by aureoles of plagioclase. Some shards of vesicular melt rocks resemble the pyroxene‐plagioclase clump‐aureole structures. Submicron size matrix grains show some triple junctions, but most are irregular with high intergranular porosity. The noritic‐monzonitic rocks contain exsolved pyroxenes and perthitic intergrowths, and cooled more slowly than rocks with zoned‐pyroxene or fine grain size. Noritic material contains orthopyroxene or inverted pigeonite, augite, calcic to intermediate plagioclase, and chromite to Cr‐bearing magnetite; monzonitic clasts contain augite, sodic plagioclase, K feldspar, Ti‐bearing magnetite, ilmenite, chlorapatite, and zircon. These feldspathic rocks show similarities to some rocks at Gale Crater like Black Trout, Mara, and Jake M. The most magnesian orthopyroxene clasts are close to ALH 84001 orthopyroxene in composition. All these materials are enriched in siderophile elements, indicating impact melting and incorporation of a projectile component, except for Ni‐poor pyroxene clasts which are from pristine rocks. Clast‐laden melt rocks, spherules, shards, and siderophile element contents indicate formation of NWA 7533 as a regolith breccia. The zircons, mainly derived from monzonitic (melt) rocks, crystallized at 4.43 ± 0.03 Ga (Humayun et al. 2013 ) and a 147Sm‐143Nd isochron for NWA 7034 yielding 4.42 ± 0.07 Ga (Nyquist et al. 2016 ) defines the crystallization age of all its igneous portions. The zircon from the monzonitic rocks has a higher Δ17O than other Martian meteorites explained in part by assimilation of regolith materials enriched during surface alteration (Nemchin et al. 2014 ). This record of protolith interaction with atmosphere‐hydrosphere during regolith formation before melting demonstrates a thin atmosphere, a wet early surface environment on Mars, and an evolved crust likely to have contaminated younger extrusive rocks. The latest events recorded when the breccia was on Mars are resetting of apatite, much feldspar and some zircons at 1.35–1.4 Ga (Bellucci et al. 2015 ), and formation of Ni‐bearing pyrite veins during or shortly after this disturbance (Lorand et al. 2015 ).  相似文献   

9.
We report on the petrography and mineralogy of three types of silicate veinlets in the brecciated eucrite Northwest Africa (NWA) 1109. These include Fe‐rich olivine, Mg‐rich olivine, and pyroxene veinlets. The Fe‐rich olivine veinlets mainly infill fractures in pyroxene and also occur along grain boundaries between pyroxene and plagioclase crystals, in both nonequilibrated and equilibrated lithic clasts. The host pyroxene of Fe‐rich olivine veinlets shows large chemical variations between and within grains. The Fe‐rich olivine veinlets also contain fine‐grained Fe3+‐bearing chromite, highly calcic plagioclase, merrillite, apatite, and troilite. Based on texture and mineral chemistry, we argue that the formation of Fe‐rich olivine was related to fluid deposition at relatively high temperatures. However, the source of Fe‐rich olivine in the veinlets remains unclear. Magnesium‐rich olivine veinlets were found in three diogenitic lithic clasts. In one of these, the Mg‐rich olivine veinlets only occur in one of the fine‐grained interstitial regions and extend into fractures within surrounding coarse‐grained orthopyroxene. Based on the texture of the interstitial materials, we suggest that the Mg‐rich olivine veinlets formed by shock‐induced localized melting and recrystallization. Pyroxene veinlets were only observed in one clast where they infill fractures within large plagioclase grains and are associated with fine‐grained pyroxene surrounding coarse‐grained pyroxene. The large chemical variations in pyroxene and the fracture‐filling texture indicate that the pyroxene veinlets might also have formed by shock‐induced localized melting and rapid crystallization. Our study demonstrates that silicate veinlets formed by a range of different surface processes on the surface of Vesta.  相似文献   

10.
This study characterizes carbon and nitrogen abundances and isotopic compositions in ureilitic fragments of Almahata Sitta. Ureilites are carbon‐rich (containing up to 7 wt% C) and were formed early in solar system history, thus the origin of carbon in ureilites has significance for the origin of solar system carbon. These samples were collected soon after they fell, so they are among the freshest ureilite samples available and were analyzed using stepped combustion mass spectrometry. They contained 1.2–2.3 wt% carbon; most showed the major carbon release at temperatures of 600–700 °C with peak values of δ13C from ?7.3 to +0.4‰, similar to literature values for unbrecciated (“monomict”) ureilites. They also contained a minor low temperature (≤500 °C) component (δ13C = ca ?25‰). Bulk nitrogen contents (9.4–27 ppm) resemble those of unbrecciated ureilites, with major releases mostly occurring at 600–750 °C. A significant lower temperature release of nitrogen occurred in all samples. Main release δ15N values of ?53 to ?94‰ fall within the range reported for diamond separates and acid residues from ureilites, and identify an isotopically primordial nitrogen component. However, they differ from common polymict ureilites which are more nitrogen‐rich and isotopically heavier. Thus, although the parent asteroid 2008TC3 was undoubtedly a polymict ureilite breccia, this cannot be deduced from an isotopic study of individual ureilite fragments. The combined main release δ13C and δ15N values do not overlap the fields for carbonaceous or enstatite chondrites, suggesting that carbon in ureilites was not derived from these sources.  相似文献   

11.
We report on the petrography and mineralogy of five Yamato polymict eucrites to better constrain the formation and alteration of crustal material on differentiated asteroids. Each sample consists of different lithic clasts that altogether form four dominant textures and therefore appear to originate from closely related petrological areas within Vesta′s crust. The textures range from subophitic to brecciated, porphyritic, and quench‐textured, that differ from section to section. Comparison with literature data for these samples is therefore difficult, which stresses that polymict eucrites are extremely complex in their petrography and investigation of only one thick section may not be representative for the host rock. We also show that sample Y‐793548 consists of more than one lithic unit and must therefore be classified as polymict instead of monomict. The variety and nature of lithic textures in the investigated Yamato meteorites indicate shock events, intense post‐magmatic thermal annealing, and secondary alteration. These postmagmatic features occur in different intensities, varying from clast to clast or among coexisting mineral fragments on a small, local scale. Several clasts within the eucrites studied have been modified by late‐stage alteration processes that caused deposition of Fe‐rich olivine and Fe enrichment along cracks crosscutting pyroxene crystals. However, formation of these secondary phases seems to be independent of the degree of thermal metamorphism observed within every type of clast, which would support a late‐stage metasomatism model for their formation.  相似文献   

12.
The Allan Hills 76005 polymict eucrite pairing group consists of 15 paired masses recovered during six different field seasons in the Transantarctic Mountains. Although this group has been well studied in general, most of the meteorites contain a significant portion of dark clasts that have not been well characterized. The Dawn mission to Vesta discovered dark materials that provide insight into its evolution. The ALH dark clasts are thus of great interest to understanding the evolution of Vesta. Here, 45 different dark clasts from 15 different thin sections from the pairing group are characterized in detail to better understand their nature and origin. Five different textural types of dark clasts are recognized among this group—skeletal, vitrophyric, pilotaxitic, fan spherulitic, and troilite‐silica‐plagioclase‐rich clasts with aphyric or blobby textures. Mineralogy of the clasts is dominated by plagioclase and pyroxene, with minor troilite, silica, ilmenite, chromite, and rare Fe‐Ni metal. All of the textures can be produced by rapid cooling rates on the order of 60–2500°C h?1. Bulk compositions of the clasts are demonstrably eucritic, and not chondritic, howarditic, or diogenitic. The combination of mineralogy, composition, and textures strongly suggests that the dark clasts are eucritic impact melts. Several craters on Vesta have associated orange deposits that have been proposed as impact melt breccias. The ALH pairing group may thus represent material that originated near Oppia or Octavia craters.  相似文献   

13.
Ureilites are carbon‐rich ultramafic achondrites that have been heated above the silicate solidus, do not contain plagioclase, and represent the melting residues of an unknown planetesimal (i.e., the ureilite parent body, UPB). Melting residues identical to pigeonite‐olivine ureilites (representing 80% of ureilites) have been produced in batch melting experiments of chondritic materials not depleted in alkali elements relative to the Sun’s photosphere (e.g., CI, H, LL chondrites), but only in a relatively narrow range of temperature (1120 ºC–1180 ºC). However, many ureilites are thought to have formed at higher temperature (1200 ºC–1280 ºC). New experiments, described in this study, show that pigeonite can persist at higher temperature (up to 1280 ºC) when CI and LL chondrites are melted incrementally and while partial melts are progressively extracted. The melt productivity decreases dramatically after the exhaustion of plagioclase with only 5–9 wt% melt being generated between 1120 ºC and 1280 ºC. The relative proportion of pyroxene and olivine in experiments is compared to 12 ureilites, analyzed for this study, together with ureilites described in the literature to constrain the initial Mg/Si ratio of the UPB (0.98–1.05). Experiments are also used to develop a new thermometer based on the partitioning of Cr between olivine and low‐Ca pyroxene that is applicable to all ureilites. The equilibration temperature of ureilites increases with decreasing Al2O3 and Wo contents of pyroxene and decreasing bulk REE concentrations. The UPB melted incrementally, at different fO2, and did not cool significantly (0 ºC–30 ºC) prior to its disruption. It remained isotopically heterogenous, but the initial concentration of major elements (SiO2, MgO, CaO, Al2O3, alkali elements) was similar in the different mantle reservoirs.  相似文献   

14.
Abstract— Mineralogical information recovered from the howardite, eucrite, diogenite (HED) meteorites was employed to reconstruct the history of the parent body and relate it to 4 Vesta. These interpreted crustal evolution processes were then compared to the expected geological features on the surface of a likely proto-planet, 4 Vesta. The original crustal materials of the HED parent body were preserved as mineral grains and lithic clasts, but in many eucrites, Fe/Mg ratios in pyroxenes were homogenized by diffusion after crystallization. The crystallization trend of the protocrust has been deciphered by (1) examining monomict and crystalline samples and using their mineralogical and chemical information to formulate a sequence of crystallization and cooling trends; and by (2) reconstructing the original crust prior to cratering events from lithic clasts and mineral fragments in polymict breccias such as howardites and polymict eucrites. Mineral components are identical, both in the individual HED and in polymict breccias, and no remnants of primitive materials were preserved in the polymict breccias. A layered crust model reconstructed from such breccias consists of an upper crust with extrusive lava-like eucrites that have been brecciated and metamorphosed, diogenite mantle, and cumulate eucrites of varying thickness between them. This model can be used to explain the surface geological features of Vesta observed from the Hubble space telescope. A large crater with diogenitic orthopyroxene at the crater floor is consistent with the deepest diogenitic layer of the layered crust model; and an underlying olivine layer is expected from early crystallized olivine in the crystal fractionation model. The old terrain of eucritic surface materials of Vesta can be howardites, polymict eucrites, or regolith-like eucrites produced from eucrites extruded and impacted on the surface. Partial melting models of eucrites seem to be favored by the rare-earth element (REE) chemistry and experimental studies. Unfortunately, partial melting models have not demonstrated how the HED parent body is converted to a layered crust without producing any metamorphosed primitive material in the layered crust. The origin of cumulate eucrites with systematic variation of textures and chemistries of pyroxene can be explained by the layered crust model with excavation and mixing of trapped liquid. Discovery of basaltic materials with Na-rich plagioclase and augite in iron meteorites, which are the products of partial melting, suggests that eucrites may be unique to a body that underwent large-scale differentiation and metamorphism.  相似文献   

15.
A total of seven lithic clasts from the polymict eucrites ALHA76005 and ALHA77302 have been studied petrographically and analyzed with the electron microprobe. All clasts are composed predominantly of pyroxene and plagioclase, ± ilmenite, troilite, Fe-Ni metal, mesostasis, and silica. Pyroxene compositions in unequilibrated clasts and clast bulk compositions, calculated by modal recombination, indicate that the clasts originally crystallized under similar conditions and that they may be genetically related to each other by fractionation of pigeonite and plagioclase.  相似文献   

16.
Abstract— The petrogenesis of four lunar highlands meteorites, Dhofar 025 (Dho 025), Dhofar 081 (Dho 081), Dar al Gani 262 (DaG 262), and Dar al Gani 400 (DaG 400) were studied. For Dho 025, measured oxygen isotopic values and Fe‐Mn ratios for mafic minerals provide corroboratory evidence that it originated on the Moon. Similarly, Fe‐Mn ratios in the mafic minerals of Dho 081 indicate lunar origin. Lithologies in Dho 025 and Dho 081 include lithic clasts, granulites, and mineral fragments. A large number of lithic clasts have plagioclase AN# and coexisting mafic mineral Mg# that plot within the “gap” separating ferroan anorthosite suite (FAN) and high‐magnesium suite (HMS) rocks. This is consistent with whole rock Ti‐Sm ratios for Dho 025, Dho 081, and DaG 262, which are also intermediate compared to FAN and HMS lithologies. Although ion microprobe analyses performed on Dho 025, Dho 081, DaG 262, and DaG 400 clasts and minerals show far stronger FAN affinities than whole rock data suggest, most clasts indicate admixture of ≤12% HMS component based on geochemical modeling. In addition, coexisting plagioclase‐pyroxene REE concentration ratios in several clasts were compared to experimentally determined plagioclase‐pyroxene REE distribution coefficient ratios. Two Dho 025 clasts have concordant plagioclase‐pyroxene profiles, indicating that equilibrium between these minerals has been sustained despite shock metamorphism. One clast has an intermediate FAN‐HMS composition. These lunar meteorites appear to represent a type of highland terrain that differs substantially from the KREEP‐signatured impact breccias that dominate the lunar database. From remote sensing data, it is inferred that the lunar far side appears to have appropriate geochemical signatures and lithologies to be the source regions for these rocks; although, the near side cannot be completely excluded as a possibility. If these rocks are, indeed, from the far side, their geochemical characteristics may have far‐reaching implications for our current scientific understanding of the Moon.  相似文献   

17.
Abstract— Here we report the petrography, mineralogy, and trace element geochemistry of the Dhofar 1180 lunar meteorite. Dhofar 1180 is predominantly composed of fine‐grained matrix with abundant mineral fragments and a few lithic and glassy clasts. Lithic clasts show a variety of textures including cataclastic, gabbroic, granulitic, ophitic/subophitic, and microporphyritic. Both feldspathic and mafic lithic clasts are present. Most feldspathic lithic clasts have a strong affinity to ferroan anorthositic suite rocks and one to magnesian suite rocks. Mafic lithic clasts are moderately to extremely Fe‐rich. The Ti/[Ti+Cr]‐Fe/[Fe+Mg] compositional trend of pyroxenes in mafic lithic clasts is consistent with that of low‐Ti mare basalts. Glasses display a wide chemical variation from mafic to feldspathic. Some glasses are very similar to those from Apollo 16 soils. KREEP components are essentially absent in Dhofar 1180. One glassy clast is rich in K, REE and P, but its Mg/[Mg+Fe] is very low (0.25). It is probably a last‐stage differentiation product of mare basalt. Molar Fe/Mn ratios of both olivine and pyroxene are essentially consistent with a lunar origin. Dhofar 1180 has a LREE‐enriched (La 18 × CI, Sm 14 × CI) pattern with a small positive Eu anomaly (Eu 15 × CI). Th concentration is 0.7 ppm in Dhofar 1180. Petrography, mineralogy, and trace element geochemistry of Dhofar 1180 are different from those of other lunar meteorites, indicating that Dhofar 1180 represents a unique mingled lunar breccia derived from an area on the lunar nearside but far away from the center of the Imbrium Basin.  相似文献   

18.
Abstract— We present a petrographic and petrologic analysis of 21 olivine‐pigeonite ureilites, along with new experimental results on melt compositions predicted to be in equilibrium with ureilite compositions. We conclude that these ureilites are the residues of a partial melting/smelting event. Textural evidence preserved in olivine and pigeonite record the extent of primary smelting. In pigeonite cores, we observe fine trains of iron metal inclusions that formed by the reduction of olivine to pigeonite and metal during primary smelting. Olivine cores lack metal inclusions but the outer grain boundaries are variably reduced by a late‐stage reduction event. The modal proportion of pigeonite and percentage of olivine affected by late stage reduction are inversely related and provide an estimation of the degree of primary smelting during ureilite petrogenesis. In our sample suite, this correlation holds for 16 of the 21 samples examined. Olivine‐pigeonite‐liquid phase equilibrium constraints are used to obtain temperature estimates for the ureilite samples examined. Inferred smelting temperatures range from ～1150°C to just over 1300°C and span the range of estimates published for ureilites containing two or more pyroxenes. Temperature is also positively correlated with modal percent pigeonite. Smelting temperature is inversely correlated with smelting depth—the hottest olivine‐pigeonite ureilites coming from the shallowest depth in the ureilite parent body. The highest temperature samples also have oxygen isotopic signatures that fall toward the refractory inclusion‐rich end of the carbonaceous chondrite‐anhydrous mineral (CCAM) slope 1 mixing line. These temperature‐depth variations in the ureilite parent body could have been created by a heterogeneous distribution of heat producing elements, which would indicate that isotopic heterogeneities existed in the material from which the ureilite parent body was assembled.  相似文献   

19.
Abstract— Northwest Africa (NWA) 1500 is an ultramafic meteorite dominated by coarse (?100–500 μm) olivine (95–96%), augite (2–3%), and chromite (0.6–1.6%) in an equilibrated texture. Plagioclase (0.7–1.8%) occurs as poikilitic grains (up to ?3 mm) in vein‐like areas that have concentrations of augite and minor orthopyroxene. Other phases are Cl‐apatite, metal, sulfide, and graphite. Olivine ranges from Fo 65–73, with a strong peak at Fo 68–69. Most grains are reversezoned, and also have ?10–30 μm reduction rims. In terms of its dominant mineralogy and texture, NWA 1500 resembles the majority of monomict ureilites. However, it is more ferroan than known ureilites (Fo ≥75) and other mineral compositional parameters are out of the ureilite range as well. Furthermore, neither apatite nor plagioclase have ever been observed, and chromite is rare in monomict ureilites. Nevertheless, this meteorite may be petrologically related to the rare augite‐bearing ureilites and represent a previously unsampled part of the ureilite parent body (UPB). The Mn/Mg ratio of its olivine and textural features of its pyroxenes are consistent with this interpretation. However, its petrogenesis differs from that of known augite‐bearing ureilites in that: 1) it formed under more oxidized conditions; 2) plagioclase appeared before orthopyroxene in its crystallization sequence; and 3) it equilibrated to significantly lower temperatures (800–1000 °C, from two‐pyroxene and olivine‐chromite thermometry). Formation under more oxidized conditions and the appearance of plagioclase before orthopyroxene could be explained if it formed at a greater depth on the UPB than previously sampled. However, its significantly different thermal history (compared to ureilites) may more plausibly be explained if it formed on a different parent body. This conclusion is consistent with its oxygen isotopic composition, which suggests that it is an ungrouped achondrite. Nevertheless, the parent body of NWA 1500 may have been compositionally and petrologically similar to the UPB, and may have had a similar differentiation history.  相似文献   

20.
The anomalous polymict ureilite Almahata Sitta (AhS) fell in 2008 when asteroid 2008 TC3 disintegrated over Sudan and formed a strewn field of disaggregated clasts of various ureilitic and chondritic types. We studied the petrology and oxygen isotope compositions of enstatite meteorite samples from the University of Khartoum (UoK) collection of AhS. In addition, we describe the first bona fide (3.5 mm-sized) clast of an enstatite chondrite (EC) in a typical polymict ureilite, Northwest Africa (NWA) 10657. We evaluate whether 2008 TC3 and typical polymict ureilites have a common origin, and examine implications for the history of enstatite meteorite asteroids in the solar system. Based on mineralogy, mineral compositions, and textures, the seven AhS EC clasts studied comprise one EHa3 (S151), one ELb3 (AhS 1002), two EHb4-5 (AhS 2012, AhS 26), two EHb5-6 or possibly impact melt rocks (AhS 609, AhS 41), and one ELb6-7 (AhS 17), while the EC clast in NWA 10657 is EHa3. Oxygen isotope compositions analyzed for five of these are similar to those of EC from non-UoK collections of AhS, and within the range of individual EC meteorites. There are no correlations of oxygen isotope composition with chemical group or subgroup. The EC clasts from the UoK collection show the same large range of types as those from non-UoK collections of AhS. The enstatite achondrite, AhS 60, is a unique type (not known as an individual meteorite) that has also been found among non-UoK AhS samples. EC are the most abundant non-ureilitic clasts in AhS but previously were thought to be absent in typical polymict ureilites, necessitating a distinct origin for AhS. The discovery of an EC in NWA 10657 changes this. We argue that the types of materials in AhS and typical polymict ureilites are essentially similar, indicating a common origin. We elaborate on a model in which AhS and typical polymict ureilites formed in the same regolith on a ureilitic daughter body. Most non-ureilitic clasts are remnants of impactors implanted at ~50–60 Myr after CAI. Differences in abundances can be explained by the stochastic nature of impactor addition. There is no significant difference between the chemical/petrologic types of EC in polymict ureilites and individual EC meteorites. This implies that fragments of the same populations of EC parent bodies were available as impactors at ~50–60 Myr after CAI and recently. This can be explained if materials excavated from various depths on EC bodies at ~50–60 Myr after CAI were reassembled into mixed layers, leaving relatively large bodies intact to survive 4 billion years. Polymict ureilites record a critical timestep in the collisional and dynamical evolution of the solar system, showing that asteroids that may have accreted at distant locations had migrated to within proximity of one another by 50–60 Myr after CAI, and providing constraints on the dynamical processes that could have caused such migrations.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号