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 203 毫秒
1.
We report newly measured noble gas isotopic concentrations of He, Ne, and Ar for 21 samples from the 10 ureilites, DaG 084, DaG 319, DaG 340, Dho 132, HaH 126, JaH 422, JaH 424, Kenna, NWA 5928, and RaS 247, including the results of both single and stepwise heating extractions. Cosmic ray exposure (CRE) ages calculated using model calculations that fully account for all shielding depths and a wide range of preatmospheric radii, and are tailored to ureilite chemistry, range from 3.7 Ma for Dho 132 to 36.3 Ma for one of several measured Kenna samples. In a Ne‐three‐isotope plot, the data for DaG 340 and JaH 422 plot below the Necos/Neureilite mixing envelope, possibly indicating the presence of Ne produced from solar cosmic rays. In combination with literature data and correcting for pairing, we established a fully consistent database containing 100 samples from 40 different ureilites. The CRE age histogram shows a trend of decreasing meteorite number with increasing CRE age. We speculate that the parent body of the known ureilites is moving closer to a resonance and/or that there is a loss mechanism that acts on ureilites independent of their size. In addition, there is a slight indication for a peak in the range 30 Ma, which might indicate a larger impact on the ureilite daughter body. Finally, we confirm earlier results that the majority of the studied ureilites have relatively small preatmospheric radii less or equal ~20 cm.  相似文献   

2.
The Almahata Sitta strewn field is dominated by ureilites, but contains a large fraction of chondritic fragments of various types. We analyzed stable isotopes of He, Ne, Ar, Kr, and Xe, and the cosmogenic radionuclides 10Be, 26Al, and 36Cl in six chondritic Almahata Sitta fragments (EL6 breccia, EL6, EL3‐5, CB, LL4/5, R‐like). The cosmic‐ray exposure (CRE) ages of five of the six samples have an average of 19.2 ± 3.3 Ma, close to the average of 19.5 ± 2.5 Ma for four ureilites. The cosmogenic radionuclide concentrations in the chondrites indicate a preatmospheric size consistent with Almahata Sitta. This corroborates that Almahata Sitta chondrite samples were part of the same asteroid as the ureilites. However, MS‐179 has a lower CRE age of 11.0 ± 1.4 Ma. Further analysis of short‐lived radionuclides in fragment MS‐179 showed that it fell around the same time, and from an object of similar size as Almahata Sitta, making it almost certain that MS‐179 is an Almahata Sitta fragment. Instead, its low CRE age could be due to gas loss, chemical heterogeneity that may have led to an erroneous 21Ne production‐rate, or, perhaps most likely, MS‐179 could represent the true 4π exposure age of Almahata Sitta (or an upper limit thereof), while all other samples analyzed so far experienced exposure on the parent body of similar lengths. Finally, MS‐179 had an extraordinarily high activity of neutron‐capture 36Cl, ~600 dpm kg?1, the highest activity observed in any meteorite to date, related to a high abundance of the Cl‐bearing mineral lawrencite.  相似文献   

3.
We report light noble gas (He, Ne, and Ar) concentrations and isotopic ratios in 11 achondrites, Tafassasset (unclassified primitive achondrite), Northwest Africa (NWA) 12934 (angrite), NWA 12573 (brachinite), Jiddat al Harasis (JaH) 809 (ureilite), NWA 11562 (ungrouped achondrite), four lodranites (NWA 11901, NWA 7474, NWA 6685, and NWA 6484), NWA 2871 (acapulcoite), and Sahara 02029 (winonaite), most of which have not been previously studied for noble gases. We discuss their noble gas isotopic composition, determine their cosmogenic nuclide content, and systematically calculate their cosmic ray exposure (CRE) and gas retention ages. In addition, we estimate their preatmospheric radii and preatmospheric masses based on the shielding parameter (22Ne/21Ne)cos. None of the studied meteorites shows evidence of contribution from solar cosmic rays (SCRs). JaH 809 and NWA 12934 show evidence of 3He diffusive losses of >90% and 40%, respectively. The winonaite Sahara 02029 has lost most of its noble gases, either during or before analysis. The average CRE age of Tafassasset of ~49 Ma is lower than that reported by Patzer et al. (2003), but is consistent with it within the uncertainties; this confirms that Tafassasset and CR chondrites are not source paired, CR chondrites having CRE ages from 1 to 25 Ma (Herzog & Caffee, 2014). The ureilite JaH 809 has a CRE age of ~5.4 Ma, which falls into the typical range of exposure ages for ureilites; the angrite NWA 12934 has a CRE age of ~49 Ma, which is within the main range of exposure ages reported for angrites (0.2–56 Ma). We calculate a CRE age of ~2.4 Ma for the brachinite NWA 12573, which falls into a possible “cluster” in the brachinite CRE age histogram around ~3 Ma. Three lodranites (NWA 11901, NWA 7474, and NWA 6685) have CRE ages higher than the average CRE ages of lodranites measured so far, NWA 11901 and NWA 6685 having CRE ages far higher than the CRE age already reported by Li et al. (2019) on NWA 8118. The measured 40K-40Ar gas retention ages fit well into established systematics. The gas retention age of Tafassasset is consistent, within respective uncertainties, with that previously calculated by Patzer et al. (2003). Our study indicates that Tafassasset originates from a meteoroid with a preatmospheric radius of ~20 cm, however discordant with the radius of ~85 cm inferred in a previous study (Patzer et al., 2003).  相似文献   

4.
Abstract— The thermal and shock histories of ureilites can be divided into four periods: 1) formation, 2) initial shock, 3) post‐shock annealing, and 4) post‐annealing shock. Period 1 occurred ?4.55 Ga ago when ureilites formed by melting chondritic material. Impact events during period 2 caused silicate darkening, undulose to mosaic extinction in olivines, and the formation of diamond, lonsdaleite, and chaoite from indigenous carbonaceous material. Alkali‐rich fine‐grained silicates may have been introduced by impact injection into ureilites during this period. About 57% of the ureilites were unchanged after period 2. During period 3 events, impact‐induced annealing caused previously mosaicized olivine grains to become aggregates of small unstrained crystals. Some ureilites experienced reduction as FeO at the edges of olivine grains reacted with C from the matrix. Annealing may also be responsible for coarsening of graphite in a few ureilites, forming euhedral‐appearing, idioblastic crystals. Orthopyroxene in Meteorite Hills (MET) 78008 may have formed from pigeonite by annealing during this period. The Rb‐Sr internal isochron age of ?4.0 Ga for MET 78008 probably dates the annealing event. At this late date, impacts are the only viable heat source. About 36% of ureilites experienced period 3 events, but remained unchanged afterwards. During period 4, ?7% of the ureilites were shocked again, as is evident in the polymict breccia, Elephant Moraine (EET) 83309. This rock contains annealed mosaicized olivine aggregates composed of small individual olivine crystals that exhibit undulose extinction. Ureilites may have formed by impact‐melting chondritic material on a primitive body with heterogeneous O isotopes. Plagioclase was preferentially lost from the system due to its low impedance to shock compression. Brief melting and rapid burial minimized the escape of planetary‐type noble gases from the ureilitic melts. Incomplete separation of metal from silicates during impact melting left ureilites with relatively high concentrations of trace siderophile elements.  相似文献   

5.
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   

6.
Abstract– New analyses of mafic silicates from 14 ureilite meteorites further constrain a strong correlation ( Singletary and Grove 2003 ) between olivine‐core Fo ratio and the temperature of equilibration (TE) recorded by the composition of pigeonite. This correlation may be compared with relationships implied by various postulated combinations of Fo and pressure P in models for ureilite genesis by a putative process of anatectic (depth‐linked, P‐controlled) smelting. In such models, any combination of Fo and P together fixes the temperature of smelting. Agreement between the observed correlation and these models is poor. The anatectic smelting model also carries implausible implications for the depth range at which ureilites of a given composition (Fo) form. Actual ureilites (and polymict ureilite clasts: Downes et al. 2008 ) show a distribution strongly skewed toward the low‐Fo end of the compositional range, with approximately 58% in the range Fo76–81. In contrast, the P‐controlled smelting model implies that the Fo76–81 region is a small fraction of the volume of the parent body: not more than 3.2%, in a model consistent with the Fo‐TE observations; and even ignoring the Fo‐TE evidence not more than 11% (percentages cited require optimal assumptions concerning the size of the parent body). This region also must occur deep within the body, where no straightforward model would imply a strong bias in the impact‐driven sampling process. The ureilites did not derive preponderantly from one atypical “largest offspring” disruption survivor, because cooling history evidence shows that after the disruption (whose efficiency was increased by gas jetting), all of the known ureilites cooled in bodies that were tiny (mass of order 10?9) in comparison with the precursor body. The Ca/Al ratio of the ureilite starting matter cannot be 2.5 times chondritic, as has been suggested, unless the part of the body from which ureilites come is at most 50% of the whole body. Published variants of the anatectic, P‐controlled smelting model have the ureilites coming from a region that is >50 vol% of their parent body; and to invoke a larger body would have the drawback of implying that the Fo76–81 spike represents an even smaller fraction of the parent body’s interior. The ureilites’ moderate depletions in incompatible elements are difficult to reconcile with a fractional fusion model. It is not plausible that melt formed grossly out of equilibrium with the medium‐sized ureilite crystals. The alternative to pressure‐controlled smelting, i.e., a model of gasless or near‐gasless anatexis, has very different implications for the size and evolution of the original parent body. To yield internal pressures prohibitive of smelting in even the shallowest and most ferroan portion of its anatectic mantle, the body would have to be larger than roughly 690 km in diameter. A 400 km body would have approximately 12 vol% of the interior (or 13 vol% of the interior apart from the thermal “skin” that never undergoes anatexis) prone, if both extremely shallow and extremely ferroan, to mild smelting. Gasless anatexis also implies that this large parent body was compositionally, at least in terms of mg, grossly heterogeneous before anatexis, probably (in view of the oxygen isotopic diversity) as a result of mixed accretion.  相似文献   

7.
Abstract— Polymict ureilites contain various mineral and lithic clasts not observed in monomict ureilites, including plagioclase, enstatite, feldspathic melt clasts and dark inclusions. This paper investigates the microdistributions and petrogenetic implications of rare earth elements (REEs) in three polymict ureilites (Elephant Moraine (EET) 83309, EET 87720 and North Haig), focusing particularly on the mineral and lithic clasts not found in monomict ureilites. As in monomict ureilites, olivine and pyroxene are the major heavy (H)REE carriers in polymict ureilites. They have light (L)REE‐depleted patterns with little variation in REE abundances, despite large differences in major element compositions. The textural and REE characteristics of feldspathic melt clasts in the three polymict ureilites indicate that they are most likely shocked melt that sampled the basaltic components associated with ureilites on their parent body. Simple REE modeling shows that the most common melt clasts in polymict ureilites can be produced by 20–30% partial melting of chondritic material, leaving behind a ureilitic residue. The plagioclase clasts, as well as some of the high‐Ca pyroxene grains, probably represent plagioclase‐pyroxene rock types on the ureilite parent body. However, the variety of REE patterns in both plagioclase and melt clasts cannot be the result of a single igneous differentiation event. Multiple processes, probably including shock melting and different sources, are required to account for all the REE characteristics observed in lithic and mineral clasts. The C‐rich matrix in polymict ureilites is LREE‐enriched, like that in monomict ureilites. The occurrence of Ce anomalies in C‐rich matrix, dark inclusions and the presence of the hydration product, iddingsite, imply significant terrestrial weathering. A search for 26Mg excesses, from the radioactive decay of 26Al, in the polymict ureilite EET 83309 was negative.  相似文献   

8.
Abstract— We report data for Ag, Au, Bi, Cd, Co, Cs, Ga, In, Rb, Sb, Se, Te, Tl, U and Zn in 15 different Antarctic ureilites. Contents of these mainly volatile trace elements in Antarctic ureilites are roughly comparable to those in the four known falls. Trends exhibited by these data apparently reflect geochemical fractionations in parental magma(s), which were closed to loss of vapor. Subsequent events (e.g., shock and crystallization) do not seem to have affected contents of these elements.  相似文献   

9.
Cosmic ray exposure (CRE) ages of CM chondrites have been found to have multiple peaks (as many as four), in stark contrast to other groups of chondrites (Nishiizumi and Caffee 2012; Herzog and Caffee 2014). In this study, we sought correlations between the CRE ages and petrography of CM chondrites, and we conclude that the degree of aqueous alteration does appear to vary with the CRE ages—the CMs displaying the most aqueous alteration all have relatively short exposure ages. However, some CMs with low degrees of alteration also have short exposure ages—thus, this apparent correlation is not exclusive. We also found a definite inverse relation between the number of distinctive lithologies in a CM and its exposure age, which could indicate different responses of homogeneous and heterogeneous meteoroids to the space environment between their onset of exposure (exhumation and ejection from the parent body) and arrival at Earth. Breccias have more internal surfaces of lithologic discontinuity, possibly resulting in weaker meteoroids that disintegrate more readily than their more homogeneous counterparts. Our results suggest that CM chondrite regoliths consist of numerous genomict lithologies in a breccia with millimeter‐ to decimeter‐scale clasts, with varying degree of heating/metamorphism.  相似文献   

10.
The lightly-shocked ureilite RC027 was found in Roosevelt County, New Mexico in 1984. In terms of petrography, texture, mineral compositions, bulk chemical composition, and oxygen isotopic composition it is a typical ureilite. It contains ～75% olivine (Fo 79.4) and 25% pigeonite (mg 81.3, Wo 8.0), with intergranular graphite and (Fe, Ni) metal. It also contains less than 1% of fine-grained, interstitial silicate material, which had not previously been recognized in any ureilite. This material is an assemblage of low-Ca pyroxene (Wo 3.5–9, mg 87–93), augite (Wo 24–36, mg 90–98), glass (typically ～95% SiO2, 4% Al2O3, 0.5% Na2O), and crystalline SiO2. This material has an igneous texture, indicating that it crystallized from an interstitial liquid. Low-Ca pyroxene compositions indicate that the interstitial liquid was not in equilibrium with core pigeonite and olivine and cannot have been either an evolved intercumulus liquid or a low-degree partial melt. It may contain a component of shock-melted olivine and pigeonite, although petrographic evidence indicates that it could not have been an in situ shock melt. One sample of RC027 has a V-shaped rare earth element pattern, typical of ureilites. Another is depleted in light rare earth elements (LREE), similar to acid-treated samples of ureilites, which suggests that LREE in ureilites are contained in an inhomogeneously-distributed phase. RC027 shows the strongest olivine preferred-orientation yet observed in a ureilite. Its fabric is characteristic of fabrics formed by tabular minerals in a fluid laminar flow regime and is unlike those formed by syntectonic recrystallization and plastic flow. The elemental and isotopic compositions of noble gases in RC027 are typical of previously analyzed ureilites. This result indicates that there is no correlation of noble gas content with degree of shock in ureilites, and thus suggests that the gases were present in the ureilite material before shock. Cosmogenic He and Ne contents indicate cosmic ray exposure ages of 1.7 and 1.9 Myr, respectively. Thus, RC027 is not paired with Kenna (a ureilite also found in Roosevelt County), which has an exposure age of ～33 Myr.  相似文献   

11.
We have examined the magnetic characteristics of representative ureilites, with a view to identify the magnetic effects of shock and to isolate a primary component of the natural remanent magnetization (NRM). As a group, the ureilites show remarkably uniform patterns of magnetic behavior, attesting to a common genesis and history. However, a clearly observed gradation in magnetic properties of the ureilites studied with shock level, parallels their classification based on petrologic and chemical fractionation shock-related trends.The ureilite meteorites possess a strong and directionally stable NRM. Laboratory thermal modelling of this presumably primordial NRM preserved in Goalpara and Kenna produced reliable paleointensity estimates of order 1 Oe, thus providing evidence for strong early, nebular magnetic fields. This paleofield strength is compatible with values obtained previously from carbonaceous chondrites and supports isotopic evidence for a contemporary origin of these two groups of meteorites in the same nebular region. The mechanism for recording nebular fields, manifestly different in carbonaceous chondrite vs. ureilite meteorites, is thus relatively unimportant: violent collisional shock in ureilites seems to have only partially altered an original magnetization, by preferential removal of its least stable portion.  相似文献   

12.
Meteorites are impact-derived fragments from ≈ 85 parent bodies. For seven of these bodies, the meteorites record evidence suggesting that they may have been catastrophically fragmented. We identify three types of catastrophic events: (a) impact and reassembly events > 4.4 Gy ago, involving molten or very hot parent bodies(> 1200°C); this affected the parent bodies of the ureilites, Shallowater, and the mesosiderites. In each case, the fragments cooled rapidly (≈ 1–1000°C day−1) and then reassembled, (b) Later impacts involving cold bodies which, in some cases, reassembled; this occurred on the H and L ordinary chondrite parent bodies. The L parent body probably suffered another catastrophic event about 500 My ago. (c) Recent impacts of cold, multi-kilometer-sized bodies that generated meter-sized meteoroids; this occurred on the parent bodies of the IIIAB irons (650 My ago), the IVA irons (400 My ago), and the H ordinary chondrite (7 My ago).  相似文献   

13.
This work is the first detailed study of carbon phases in the ureilite Almahata Sitta (sample #7). We present microRaman data for diamond and graphite in Almahata Sitta, seven unbrecciated ureilites, and two brecciated ureilites. Diamond in Almahata Sitta was found to be distinct from that in unbrecciated and brecciated ureilites, although diamond in unbrecciated and brecciated ureilites is indistinguishable. Almahata Sitta diamond shows a peak center range of 1318.5–1330.2 cm?1 and a full width at half maximum (FWHM) range of 6.6–17.4 cm?1, representing a shock pressure of at least 60 kbar. The actual peak shock pressure may be higher than this due to postshock annealing, if shock synthesis is the source of ureilite diamonds. Diamond in unbrecciated and brecciated ureilites have peak center wave numbers closer to terrestrial kimberlite diamond, but show a wider range of FWHM than Almahata Sitta. The larger peak shift observed in Almahata Sitta may indicate the presence of lonsdaleite. Alternatively, the lower values in brecciated ureilites may be evidence of an annealing step either following the initial diamond‐generating shock or as a consequence of heating during reconsolidation of the breccia. Graphite in Almahata Sitta shows a G‐band peak center range of 1569.1–1577.1 cm?1 and a G‐band FWHM range of 24.3–41.6 cm?1 representing a formation temperature of 990 ± 120 °C. Amorphous carbon was also found. We examine the different theories for diamond formation in ureilites, such as chemical vapor deposition and shock origin from graphite, and explore explanations for the differences between Almahata Sitta and other ureilites.  相似文献   

14.
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   

15.
Abstract— The mid-infrared (4000–450 cm?1; 2.5–22.2 μm) transmission spectra of seven Antarctic ureilites and 10 Antarctic H-5 ordinary chondrites are presented. The ureilite spectra show a number of absorption bands, the strongest of which is a wide, complex feature centered near 1000 cm?1 (10 μm) due to Si-O stretching vibrations in silicates. The profiles and positions of the substructure in this feature indicate that Mg-rich olivines and pyroxenes are the main silicates responsible. The relative abundances of these two minerals, as inferred from the spectra, show substantial variation from meteorite to meteorite, but generally indicate olivine is the most abundant (olivine:pyroxene = 60:40 to 95:5). Both the predominance of olivine and the variable olivine-to-pyroxene ratio are consistent with the known composition and heterogeneity of ureilites. The H-5 ordinary chondrites spanned a range of weathering classes and were used to provide a means of addressing the extent to which the ureilite spectra may have been altered by weathering processes. It was found that, while weathering of these meteorites produces some weak bands due to the formation of small amounts of carbonates and hydrates, the profile of the main silicate feature has been little affected by Antarctic exposure in the meteorites studied here. The mid-infrared ureilite spectra provide an additional means of testing potential asteroidal parent bodies for the ureilites. At present, the best candidates include the subset of S-type asteroids having low albedos and weak absorption features in the near infrared.  相似文献   

16.
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   

17.
Abstract— We studied the petrography and mineralogy of two monomict ureilites, Hammadah al Hamra 064 (HH064) and Jalanash, by using reflected light and scanning electron microscopy. Quantitative analyses were performed by electron microprobe and the microstructures were investigated with transmission electron microscopy (TEM). HH064 features two different textures, a poikilitic and a typical one, whereas Jalanash shows only the typical ureilite texture. Our synergetic chemical and microstructural investigations reveal a complex cooling history for both ureilites. The temperature for the first equilibrium deduced from the pigeonite‐augite assemblage in HH064 is ～1200°C. The presence of antiphase domains in low‐Ca pyroxenes proves that they are clearly pigeonite. The occurrences of tweed micro structure and orthopyroxene lamellae, which are incompletely developed, imply a faster cooling rate from the first equilibrium with a sudden end. Although both ureilites contain shock induced diamonds, dislocations in silicates are rare. This observation suggests that the meteorites were hot at the time of strong shock metamorphism or that they were heated after strong shock metamorphism. After this event, new microstructural features were generated by different cooling processes and were frozen by a final rapid decrease in temperature possibly due to excavation from the ureilite parent body, or bodies.  相似文献   

18.
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   

19.
Miller Range (MIL) 090340 and MIL 090206 are olivine‐rich achondrites originally classified as ureilites. We investigate their petrography, mineral compositions, olivine Cr valences, equilibration temperatures, and (for MIL 090340) oxygen isotope compositions, and compare them with ureilites and other olivine‐rich achondrites. We conclude that they are brachinite‐like achondrites that provide new insights into the petrogenesis of brachinite clan meteorites. MIL 090340,6 has a granoblastic texture and consists of ~97 modal % by area olivine (Fo = molar Mg/[Mg+Fe] = 71.3 ± 0.6). It also contains minor to trace augite, chromite, chlorapatite, orthopyroxene, metal, troilite, and terrestrial Fe‐oxides. Approximately 80% by area of MIL 090206,5 has a granoblastic texture of olivine (Fo 72.3 ± 0.1) plus minor augite and chromite, similar to MIL 090340 but also containing minor plagioclase. The rest of the section consists of a single crystal of orthopyroxene (~11 × 3 mm), poikilitically enclosing rounded grains of olivine (Fo = 76.1 ± 0.6), augite, chromite, metal, and sulfide. Equilibration temperatures for MIL 090340 and MIL 090206, calculated from olivine‐spinel, olivine‐augite, and two‐pyroxene thermometry range from ~800 to 930 °C. In both samples, symplectic intergrowths of Ca‐poor orthopyroxene + opaque phases (Fe‐oxides, sulfide, metal) occur as rims on and veins/patches within olivine. Before terrestrial weathering, the opaques were probably mostly sulfide, with minor metal. All petrologic properties of MIL 090340 and MIL 090206 are consistent with those of brachinite clan meteorites, and largely distinct from those of ureilites. Oxygen isotope compositions of olivine in MIL 090340 (δ18O = 5.08 ± 0.30‰, δ17O = 2.44 ± 0.21‰, and Δ17O = ?0.20 ± 0.12‰) are also within the range of brachinite clan meteorites, and well distinguished from ureilites. Olivine Cr valences in MIL 090340 and the granoblastic area of MIL 090206 are 2.57 ± 0.06 and 2.59 ± 0.07, respectively, similar to those of three brachinites also analyzed here (Brachina, Hughes 026, Nova 003). They are higher than those of olivine in ureilites, even those containing chromite. The valence systematics of MIL 090340, MIL 090206, and the three analyzed brachinites (lower Fo = more oxidized Cr) are consistent with previous evidence that brachinite‐like parent bodies were inherently more oxidized than the ureilite parent body. The symplectic orthopyroxene + sulfide/metal assemblages in MIL 090340, MIL 090206, and many brachinite clan meteorites have superficial similarities to characteristic “reduction rims” in ureilites. However, they differ significantly in detail. They likely formed by reaction of olivine with S‐rich fluids, with only minor reduction. MIL 090340 and the granoblastic area of MIL 090206 are similar in modal mineralogy and texture to most brachinites, but have higher Fo values typical of brachinite‐like achondrites. The poikilitic pyroxene area of MIL 090206 is more typical of brachinite‐like achondrites. The majority of their properties suggest that MIL 090340 and MIL 090206 are residues of low‐degree partial melting. The poikilitic area of MIL 090206 could be a result of limited melt migration, with trapping and recrystallization of a small volume of melt in the residual matrix. These two samples are so similar in mineral compositions, Cr valence, and cosmic ray exposure ages that they could be derived from the same lithologic unit on a common parent body.  相似文献   

20.
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   

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