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1.
Abstract— The cumulate eucrite meteorites are gabbros that are related to the eucrite basalt meteorites. The eucrite basalts are relatively primitive (nearly flat REE patterns with La ～ 8–30 × CI), but the parent magmas of the cumulate eucrites have been inferred as extremely evolved (La to > 100 × CI). This inference has been based on mineral/magma partitioning, and on mass balance considering the cumulate eucrites as adcumulates of plagioclase + pigeonite only; both approaches have been criticized as inappropriate. Here, mass balance including magma + equilibrium pigeonite + equilibrium plagioclase is used to test a simple model for the cumulate eucrites: that they formed from known eucritic magma types, that they consisted only of magma + crystals in chemical equilibrium with the magma, and that they were closed to chemical exchange after the accumulation of crystals. This model is tested for major and rare earth elements (REE). The cumulate eucrites Serra de Magé and Moore County are consistent, in both REE and major elements, with formation by this simple model from a eucrite magma with a composition similar to the Nuevo Laredo meteorite: Serra de Magé as 14% magma, 47.5% pigeonite, and 38.5% plagioclase; Moore County as 35% magma, 37.5% pigeonite, and 27.5% plagioclase. These results are insensitive to the choice of mineral/magma partition coefficients. Results for the Moama cumulate eucrite are strongly dependent on choice of partition coefficients; for one reasonable choice, Moama's composition can be modeled as 4% Nuevo Laredo magma, 60% pigeonite, and 36% plagioclase. Selection of parent magma composition relies heavily on major elements; the REE cannot uniquely indicate a parent magma among the eucrite basalts. The major element composition of Y-791195 can be fit adequately as a simple cumulate from any basaltic eucrite composition. However, Y-791195 has LREE abundances and La/Lu too low to be accommodated within the model using any basaltic eucrite composition and any reasonable partition coefficients. Postcumulus loss of incompatible elements seems possible. It is intriguing that Serra de Magé, Moore County, and Moama are consistent with the same parental magma; could they be from the same igneous body on the eucrite parent asteroid (4 Vesta)?  相似文献   

2.
Abstract— We report on major and trace element analyses of 17 eucrites, including three cumulate eucrites (Binda, Moore County, and Serra de Magé), determined by, respectively, inductively‐coupled plasma atomic emission spectrometry and inductively‐coupled plasma mass spectrometry. The results obtained for Binda and Moore County are consistent with the model of Treiman (1997) for the formation of cumulate eucrites, which holds that these meteorites were produced from a eucritic melt. Our sample of Serra de Magé contains unusually large amounts of pyroxene and probably an accessory phase rich in heavy rare earth elements and is therefore not representative of this eucrite as known from literature data. Our results for the noncumulate eucrites Bereba, Bouvante, Cachari, Caldera, Camel Donga, Ibitira, Jonzac, Juvinas, Lakangaon, Millbillillie, Padvarninkai, Pasamonte, Sioux County, and Stannern are in good agreement with literature data. The observed decoupling between major and trace elements for noncumulate eucrites can be explained by in situ crystallization during the differentiation of an asteroidal magma ocean. This model can further account for both the Nuevo Laredo and the Stannern trends but has as a consequence that none of the analyzed eucrites represents a primary melt.  相似文献   

3.
Comparative planetary geochemistry provides insight into the origin and evolutionary paths of planetary bodies in the inner solar system. The eucrite and angrite achondrite groups are particularly interesting because they show evidence of early planetary differentiation. We present 147Sm‐143Nd and 176Lu‐176Hf analyses of eight noncumulate (basaltic) eucrites, two cumulate eucrites, and three angrites, which together place new constraints on the evolution and differentiation histories of the crusts of the eucrite and angrite parent bodies and their mantle mineralogies. The chemical compositions of both eucrites and angrites indicate similar evolutionary paths and petrogenetic models with formation and isolation of differentiated crustal reservoirs associated with segregation of ilmenite. We report a 147Sm‐143Nd mineral isochron age for the Moama cumulate eucrite of 4519 ± 34 Ma (MSWD = 1.3). This age indicates protracted magmatism within deep crustal layers of the eucrite parent body lasting up to about 50 Ma after the formation of the solar system. We further demonstrate that the isotopic compositions of constituent minerals are compromised by secondary processes hindering precise determination of mineral isochron ages of basaltic eucrites and angrites. We interpret the changes in geochemistry and, consequently, the erroneous 147Sm‐143Nd and 176Lu‐176Hf internal mineral isochron ages of basaltic eucrites and angrites as the result of metamorphic events such as impacts (effects from pressure, temperature, and peak shock duration) on the surfaces of the eucrite and angrite parent bodies.  相似文献   

4.
Volatile elements play a key role in the dynamics of planetary evolution. Extensive work has been carried out to determine the abundance, distribution, and source(s) of volatiles in planetary bodies such as the Earth, Moon, and Mars. A recent study showed that the water in apatite from eucrites has similar hydrogen isotopic compositions compared to water in terrestrial rocks and carbonaceous chondrites, suggesting that water accreted very early in the inner solar system given the ancient crystallization ages (~4.5 Ga) of eucrites. Here, the measurements of water (reported as equivalent H2O abundances) and the hydrogen isotopic composition (δD) of apatite from five basaltic eucrites and one cumulate eucrite are reported. Apatite H2O abundances range from ~30 to ~3500 ppm and are associated with a weighted average δD value of ?34 ± 67‰. No systematic variations or correlations are observed in H2O abundance or δD value with eucrite geochemical trend or metamorphic grade. These results extend the range of previously published hydrogen isotope data for eucrites and confirm the striking homogeneity in the H‐isotopic composition of water in eucrites, which is consistent with a common source for water in the inner solar system.  相似文献   

5.
Abstract— The isotopic composition and concentrations of noble gases were measured in the eucrites Bereba, Cachari, Caldera, Camel Donga, Chervony Kut, Ibitira, Jonzac, Juvinas, Millbillillie, Moore County, Padvarninkai, Pasamonte, Pomozdino, Serra de Magé, Sioux County, and Vetluga. The distribution of 81Kr-Kr exposure ages shows “clusters” at (7 ± 1) Ma, (10 ± 1) Ma, (14 ± 1) Ma, (22 ± 2) Ma, and (37 ± 1) Ma that agree with those for howardites, eucrites, and diogenites (HED) at (6 ± 1) Ma, (12 ± 2) Ma, (21 ± 4) Ma, and (38 ± 8) Ma. This most likely indicates a common origin of HED meteorites. Correlation equations for the shielding-sensitive cosmogenic ratios 78Kr/83Kr, 80Kr/83Kr, 82Kr/83Kr, and 124Xe/131Xe were obtained. Comparison with data from simulation experiments suggests that most eucrites were exposed to the cosmic radiation as somewhat large meteoroids with diameters of ～1 m or more. The shielding-dependence of the 78Kr and 126Xe production rates was found to be small, with a few exceptions the variations aren <10%–15%. Concentrations of spallogenic 3He indicate diffusive losses of up to 70% that can be, in first approximation, described by a model of quasi-continuous losses during the exposure to the cosmic radiation with a loss rate of the order of ～3 × 10?8 a?1. Radiogenic 4He shows additional substantial losses that occurred at the time of, or prior to, the separation of the meteoroids from their parent body. Typical 40Ar retention in eucrites is 50%–60% which corresponds to a 40Ar-K retention age of 3.4–3.6 Ga. In all analyzed unbrecciated eucrites, the retention is distinctly larger (70%–100%). The 244Pu fission ratio (86Kr/136Xe)Pu, was evaluated from the data on Pomozdino samples to be 0.039 ± 0.014.  相似文献   

6.
Major, minor and trace element abundances have been determined by instrumental neutron activation analysis (INAA) in whole rock and plagioclase separates of Serra de Magé (SdM). The whole rock contains 52% normative plagioclase and its chondritic normalized REE abundance pattern shows a large Eu anomaly, dominated by the plagioclase REE distribution, and nearly unfractionated La-Sm and Sm-Lu abundances. The plagioclase separates contained ～ 6% pyroxenes and exhibited a typical plagioclase REE distribution. The REE abundances in the derivative equilibrium magmas from which SdM and Moore County (MC) plagioclases crystallized have been estimated from the plagioclase data and the plagioclase mineral/liquid partition coefficients. The REE distributions in possibly earlier parental magmas were calculated by assuming that various degrees of plagioclase and pigeonite (plagioclase/pigeonite = 1) fractional crystallization had been operative prior to the crystallization of SdM and MC. The calculated La/Sm and Sm/Yb ratios for the earlier magmas are essentially the same as the equilibrium magmas over a wide range (10–95%) of the assumed fractional crystallization. Considering the REE distributions and the Fe/Fe+Mg ratios, calculation shows that there is no simple genetic relationship between MC and SdM via fractional crystallization processes. A hypothesis for the derivation of these cumulate eucrites in the plutonic environment from residual diogenitic liquid, which was produced by the extensive partial melting of an eucritic source material followed by the crystallization of diogenite, also fails to account for the fractionated REE patterns calculated for the equilibrium and the possible parental magmas for either SdM or MC. Equilibrium non-modal partial melting calculations indicate that SdM and MC could be genetically related by a factor ～ 6 difference in the degrees of partial melting from a similar source material. However, this common source material which should contain > 30% high-Ca clinopyroxene and has a chondritic normalized La/Yb ～ 3, is different than that proposed for the non-cumulate eucrites.  相似文献   

7.
Abstract— According to a currently popular model for petrogenesis on the howardite, eucrite, and diogenite (HED) parent asteroid, the diogenites are not comagmatic with most eucrites but instead formed in separate orthopyroxenite-dominated plutons. This model can be tested for consistency with mass balance for MgO and FeO, assuming the overall diogenite/(diogenite + eucrite) ratio, d, of the parent asteroid is at least comparable to the average d for the eucrite + diogenite dominated howardite regolith breccias. Average mg# (=MgO/[MgO + FeO]) is much lower for eucrites, especially noncumulate eucrites, than for diogenites. Unless the diogenite parent magmas eventually produced a large proportion of low-mg# residual basalt and gabbro (RBG), the implied initial magma's mg# is vastly higher than that of any noncumulate eucrite. Starting from a source previously depleted by putative primary eucrite genesis, melt mg# can be estimated as a function of the exchange reaction KD and degree of melting. Using several very conservative assumptions (e.g., assuming that the total [MgO + FeO] concentration is nearly the same in the nascent melt as in the residual solids), the degree of melting required to yield a melt with mg# high enough to satisfy mass balance, without implying an RBG component that accounts for >50% of all eucrites, is an implausibly high 60–80 wt%. The separate orthopyroxenitic plutons (SOP) model also seems inconsistent with the uniform density of melts across the diogenite-eucrite compositional spectrum (2.77–2.82 g/cm3), which implies that diogenitic magmas should have been as capable as eucrites of extruding to form lavas. This difficulty cannot be reduced by simply assuming that later-formed magmas were systematically both more plutonic and more MgO-rich than earlier ones, because the plutonic cumulate eucrites equilibrated with melts systematically lower in mg# than noncumulate eucrites. Conceivably, the bulk mg# of the asteroid's silicate system was increased between primary-melt eucrite genesis and SOP diogenite genesis by graphite-fueled reduction of FeO. However, the graphite oxidation process generates a huge proportion of gas, which would have enhanced the buoyancy of the nascent diogenite-parent magmas, thus exacerbating the difficulty of achieving the implied high degrees of partial melting. To avoid these difficulties but still form most eucrites as rapidly cooled extrusives, I propose the NERD (noncumulate eucrites as extruded residua of diogenites) model. In this model, the diogenites form as early cumulates from a large magma system (probably a global “magma ocean”) that yields a large proportion of eucritic melt as residuum. This residual melt zone undergoes relatively little crystallization during a period when it is episodically tapped to produce extrusions, dikes and sills of rapidly cooled noncumulate eucrites. Slight (～5–10%) porosity in the nascent eucritic crust keeps it marginally buoyant over the residual melt zone. The common thermal metamorphism of noncumulate eucrites results from baking by superjacent flows during the episodic venting of the melt zone. The NERD model's greatest advantage is that it does not require implausibly high degrees of localized melting in the mature stages of igneous evolution of the HED asteroid.  相似文献   

8.
We report the results of a detailed study of the basaltic eucrite Northwest Africa (NWA) 7188, including its mineralogical and bulk geochemical characteristics, oxygen isotopic composition, and 147,146Sm‐143,142Nd mineral isochron ages. The texture and chemical composition of pyroxene and plagioclase demonstrate that NWA 7188 is a monomict eucrite with a metamorphic grade of type 4. The oxygen isotopic composition and the Fe/Mn ratios of pyroxene confirmed that NWA 7188 belongs to the howardite–eucrite–diogenite meteorite suite, generally considered to originate from asteroid 4 Vesta. Whole‐rock TiO2, La, and Hf concentrations and a CI chondrite‐normalized rare earth element pattern are in good agreement with those of representative Stannern‐group eucrites. The 147,146Sm‐143,142Nd isochrons for NWA 7188 yielded ages of 4582 ± 190 and 4554 +17/?19 Ma, respectively. The closure temperature of the Sm‐Nd system for different fractions of NWA 7188 was estimated to be >865 °C, suggesting that the Sm‐Nd decay system has either been resistant to reheating at ~800 °C during the global metamorphism or only partially reset. Therefore, the 146Sm‐142Nd age of NWA 7188 corresponds to the period of initial crystallization of basaltic magmas and/or global metamorphism on the parent body, and is unlikely to reflect Sm‐Nd disturbance by late reheating and impact events. In either case, NWA 7188 is a rare Stannern‐group eucrite that preserves the chronological information regarding the initial crustal evolution of Vesta.  相似文献   

9.
Abstract— We have investigated 10 new specimens of the Millbillillie eucrite to study its textures and mineral compositions by electron probe microanalyser and scanning electron microscope. Although originally described as having fine-grained texture, the new specimens show diversity of texture. The compositions (Mg/Fe ratios) of the host pigeonites and augite lamellae are homogeneous, respectively, in spite of the textural variation. In addition to their chemical homogeneity, pyroxenes in coarse and fine-grained clasts are partly inverted to orthopyroxene. Chemical zoning of plagioclase during crystal growth is preserved. This eucrite includes areas of granulitic breccias and impact melts. Large scale textures show a subparallel layering suggesting incomplete mixing and deposition of impact melt and lithic fragments. An 39Ar-40Ar age determination for a coarse-grained clast indicates a strong degassing event at 3.55 ± 0.02 Ga. We conclude that Millbillillie is among the most equilibrated eucrites produced by thermal annealing after impact brecciation. According to the classification of impact breccias, Millbillillie can be classified as a mixture of granulitic breccias and impact melts. The last significant thermal event is characterized by network-like glassy veins that run through clasts and matrices. Consideration of textural observations and requirements for Ar-degassing suggests that the 39Ar-40Ar age could in principle date either the earilier brecciation and annealing event or the event which produced the veins.  相似文献   

10.
Abstract— Eucrite meteorites are igneous rocks that derived from a large asteroid, probably 4 Vesta. Past studies have shown that after most eucrites formed, they underwent metamorphism in temperatures up to ≥800°C. Much later, many were brecciated and heated by large impacts into the parent body surface. The less common basaltic, unbrecciated eucrites also formed near the surface but, presumably, escaped later brecciation, while the cumulate eucrites formed at depths where metamorphism may have persisted for a considerable period. To further understand the complex HED parent body thermal history, we determined new 39Ar‐40Ar ages for 9 eucrites classified as basaltic but unbrecciated, 6 eucrites classified as cumulate, and several basaltic‐brecciated eucrites. Precise Ar‐Ar ages of 2 cumulate eucrites (Moama and EET 87520) and 4 unbrecciated eucrites give a tight cluster at 4.48 ± 0.02 Gyr (not including any uncertainties in the flux monitor age). Ar‐Ar ages of 6 additional unbrecciated eucrites are consistent with this age within their relatively larger age uncertainties. By contrast, available literature data on Pb‐Pb isochron ages of 4 cumulate eucrites and 1 unbrecciated eucrite vary over 4.4–4.515 Gyr, and 147Sm‐143Nd isochron ages of 4 cumulate and 3 unbrecciated eucrites vary over 4.41–4.55 Gyr. Similar Ar‐Ar ages for cumulate and unbrecciated eucrites imply that cumulate eucrites do not have a younger formation age than basaltic eucrites, as was previously proposed. We suggest that these cumulate and unbrecciated eucrites resided at a depth where parent body temperatures were sufficiently high to cause the K‐Ar and some other chronometers to remain as open diffusion systems. From the strong clustering of Ar‐Ar ages at ?4.48 Gyr, we propose that these meteorites were excavated from depth in a single large impact event ?4.48 Gyr ago, which quickly cooled the samples and started the K‐Ar chronometer. A large (?460 km) crater postulated to exist on Vesta may be the source of these eucrites and of many smaller asteroids thought to be spectrally or physically associated with Vesta. Some Pb‐Pb and Sm‐Nd ages of cumulate and unbrecciated eucrites are consistent with the Ar‐Ar age of 4.48 Gyr, and the few older Pb‐Pb and Sm‐Nd ages may reflect an isotopic closure before the large cratering event. One cumulate eucrite gives an Ar‐Ar age of 4.25 Gyr; 3 additional cumulate eucrites give Ar‐Ar ages of 3.4–3.7 Gyr; and 2 unbrecciated eucrites give Ar‐Ar ages of ?3.55 Gyr. We attribute these younger ages to a later impact heating. Furthermore, the Ar‐Ar impact‐reset ages of several brecciated eucrites and eucritic clasts in howardites fall within the range of 3.5–4.1 Gyr. Among these, Piplia Kalan, the first eucrite to show evidence for extinct 26Al, was strongly impact heated ?3.5 Gyr ago. When these data are combined with eucrite Ar‐Ar ages in the literature, they confirm that several large impact heating events occurred on Vesta between ?4.1–3.4 Gyr ago. The onset of major impact heating may have occurred at similar times for both Vesta and the moon, but impact heating appears to have persisted for a somewhat later time on Vesta.  相似文献   

11.
Some eucrites contain up to 10 vol% silica minerals; however, silica minerals have not been studied in detail so far. We performed a mineralogical study of silica minerals in three cumulate eucrites (Moore County, Moama, and Yamato [Y] 980433). Monoclinic tridymite was common in all three samples. Moama contained orthorhombic tridymite as lamellae within monoclinic tridymite grains. Y 980433 included quartz around an impact melt vein. The presence of orthorhombic tridymite in Moama indicates that Moama cooled more rapidly than the other two samples at low temperatures (<400 °C). This result is different from the slower cooling rates of Moama (?0.0004 °C yr?1) than that of Moore County (>0.3 °C yr?1, after the shock event) at high temperatures (>500 °C) estimated from compositional profiles of pyroxene exsolution lamellae. The difference of the cooling rates may reflect their geological settings. Y 980433 cooled slowly at low temperature, as did Moore County. Quartz in Y 980433 could be a local product transformed from monoclinic tridymite by a shock event. We suggest that silica minerals in meteorites record thermal histories at low temperatures and shock events.  相似文献   

12.
Abstract— Cosmogenic radionuclides, particle tracks and rare gases have been measured in two fragments of the Piplia Kalan eucrite that fell in Rajasthan, India on 1996 June 20. The cosmic-ray exposure age of the meteorite is calculated to be 23 Ma, which is similar to ages of some other eucrites. The track density in feldspars and pyroxenes varies between 0.2 × 106 to ～4.5 × 106 cm?2. The mass ablation of the meteorite, based on the distribution of track density in near-surface samples of the two fragments, is calculated to be ～75%, which corresponds to an entry velocity of ～17 km/s. The orbital parameters of the eucrite have been computed from the radiant of the meteor trail and the geocentric velocity. The best estimates are a = 2.47 AU, e = 0.62 and i = 7.54°, which is similar to the orbital elements of other meteorites, most of which have been inferred to originate within 2.6 AU of the Sun. The activity of the radionuclide 26Al agrees with the expected production rate; whereas the shortlived radionuclides 22Na, 54Mn, 46Sc etc. have levels that are consistent with the galactic cosmic-ray fluxes that are expected during the solar minimum period before the time of fall. All the cosmogenic effects (i.e., radio- and stable- nuclides and particle tracks) are consistent with the meteoroid having had a simple, one-stage exposure history in interplanetary space. Lower radio genic ages of U, Th-He (0.7 Ga) and K-Ar (3.6 Ga) indicate severe losses of 4He and 40Ar, as observed in most eucrites. A Pu-Xe age, concordant with Angra dos Reis, shows that Piplia belongs to the “old” eucrite group.  相似文献   

13.
Abstract— Meteoritical Bulletin No. 83 lists information for 898 newly described meteorites. These include 473 from Antarctica, 341 from the Sahara, and 22 from dry lakes in the southwestern United States. Seven of the meteorites are falls: Kunya-Urgench (H5), Lohawat (howardite), Ourique (H4), Portales Valley (H6), San Pedro de Quiles (L6), Talampaya (eucrite), and Zag (H3-6). Also included are a dozen new iron meteorites; several mesosiderites; a pallasite; several HED meteorites; several ureilites; a variety of CM, CO, CV, CR, and R chondrites; and numerous unequilibrated ordinary chondrites. All shock classifications are after Stöffler et al. (1991) and weathering grades are after Wlotzka (1993), except as noted. All italicized abbreviations refer to addresses tabulated at the end of this document.  相似文献   

14.
Glass particles have been separated from the Bununu howardite microbreccia and analyzed with the electron microprobe. Preliminary SEM studies of the glass reveal fragments, spherules, teardrops, and rods: particles reminiscent of glasses recovered from the lunar surface. When plotted, individual glass analyses from both the Bununu and Malvern howardites range through the howardite group and extend into the eucrite group with the average glass compositions slightly enriched in CaO and depleted in MgO when compared with the bulk chemical analyses. These glasses presumably represent quenched, impact-melted rocks, or partial melts of the major rock types and/or matrix in Bununu and Malvern. Shock-produced features which have been observed in known terrestrial and lunar impact breccias are also present in Bununu. Crystal deformation, maskeylenite and glass veining in clasts and glass spherules and shards in the matrix point to impact brecciation as the likely mechanism to form the features observed in Bununu and other howardites.  相似文献   

15.
Abstract The 244Pu-fission-136Xe retention ages of howardites, eucrites, and diogenites (HEDs) show that these meteorites have retained Xe since they were formed about 4500 Ma ago. For the Garland diogenite and the Millbillillie eucrite, we obtain fission Xe ages of 4525 ± 40 Ma and 4486 ± 40 Ma, respectively. If Xe isotope data reported by other workers are also considered, we conclude that the monomict equilibrated eucrites Camel Donga, Juvinas, and Millbillillie formed about 40 Ma later than Pasamonte, a polymict unequilibrated eucrite. Stannern, a monomict equilibrated brecciated eucrite, yields a 244Pu-136Xe age of 4442 Ma. The 40K-40Ar retention ages fall, for most HEDs, into the 1000–4000 Ma age range, indicating that 40Ar is generally not well retained. The good retentivity for Xe of HEDs allows us to study primordial trapped Xe in these meteorites. Except for Shalka, in which other authors found Kr and Xe from terrestrial atmospheric contamination only, we present for the first time Kr and Xe isotopic data for diogenites. We studied Ellemeet, Garland, Ibbenbühren, Shalka, and Tatahouine. We show that Tatahouine contains two types of trapped Xe: a terrestrial contamination acquired by an irreversible adsorption process and released at pyrolysis temperatures up to 800 °C, and indigenous primordial Xe released primarily between 800 °C and 1200 °C. The isotopic composition of this primordial Xe is identical to that proposed earlier to be present in primitive achondrites and termed U-Xe or “primitive” Xe, but it has not been directly observed in achondrites until now. This type of primitive Xe is important for understanding the evolution of other Xe reservoirs in the Solar System. Terrestrial atmospheric Xe (corrected for fission Xe and radiogenic Xe from outgassing of the Earth) is related to it by a mass dependent fractionation favoring the heavier Xe isotopes. This primitive Xe is isotopically very similar to solar Xe except for 134Xe and 136Xe. Solar Xe appears to contain an enrichment of unknown origin for these isotopes relative to the primitive Xe.  相似文献   

16.
Abstract— A wholly encrusted single stone that fell in Vissannapeta, Andhra Pradesh, India has been identified as a cumulate eucrite based on its primary texture and mineral composition: anorthite(An92.4–94.6), orthopyroxene(En49.1–51.8Fs44.2–49.7Wo1.2–4.0), and clinopyroxene (En38.8–46.8Fs14.8–33.6Wo19.6–46.4). The stone is pyramidal in shape, and the crust shows rib‐like flow features indicating that it had an oriented passage through the atmosphere towards the terminal stage of its flight. Conditions of its fall, mineralogical characteristics, and results of measurements of cosmogenic radioactivity (26Al, 22Na, and 54Mn) and track density are described. Aluminum‐26 and 22Na in Vissannapeta are ～75% of the expected values and also lower by a similar factor compared to the activities measured in Piplia Kalan, another eucrite, which fell ～18 months before Vissannapeta. Because higher activity of 22Na and 54Mn would be expected from solar cycle modulation of galactic cosmic rays, these results, as well as the track density gradient, indicate that Vissannapeta was a small body (≤120 kg) in the interplanetary space wherein the nuclear cascade due to galactic cosmic rays did not develop fully. Tracks, surface morphology, and crustal features indicate at least two fragmentation events in the atmosphere.  相似文献   

17.
Regolithic howardites are analogs for the surface materials of asteroid 4 Vesta, recently mapped by the Dawn spacecraft. Rigorously evaluating pairing of howardites recovered in 1995 in the Grosvenor Mountains (GRO 95), Antarctica, enables an examination of a larger, more representative regolith sample. Previous work on two of the howardites studied here concluded that GRO 95602 and GRO 95535 are solar wind‐rich surface regolith samples and that they are not paired with each other, leading to uncertainty regarding pairing relationships between the other GRO 95 howardites. Based on petrology, cosmic‐ray exposure history, and terrestrial age, four GRO 95 howardites are paired. The paired howardites (GRO 95534, 95535, 95574, 95581) were from a meteoroid with radius of 10–15 cm, a preatmospheric size comparable to that of Kapoeta, the largest known regolithic howardite. The paired GRO 95 howardites contain clasts of at least 18 separate HED lithologies, providing evidence they were assembled from diverse source materials. The total eucrite:diogenite mixing ratio (ratio of all eucrite lithologies to all diogenite lithologies) in the paired GRO 95 howardites is ~2:1. Petrographically determined basaltic eucrite:cumulate eucrite ratios in regolithic howardites, studied here and previously, vary more widely than total eucrite:diogenite ratios. Relative to eucritic pyroxene, plagioclase is depleted in these howardites, which provides evidence that plagioclase is preferentially comminuted in the vestan regolith. The extent of plagioclase depletion could be an indicator of regolith maturity.  相似文献   

18.
Cover     
Engraving of the Orgueil meteorite fall published at the beginning of 1865 in l'Annuaire Mathieu (de la Drôme), indicateur du temps rédigé par les sommités scientifiques (from the collection at the Bibliothéque nationale de France). M. Gounelle and M. E. Zolensky discuss Orgueil in their article on pp. 1769–1794.  相似文献   

19.
Abstract— The C contents and isotopic compositions of four eucrites, four diogenites and two howardites have been determined. Stepped heating in an O atmosphere was employed to convert selectively different carbonaceous materials to CO2 gas at various temperatures. This technique successfully distinguishes between terrestrial contaminants and indigenous C. With the exception of the Kapoeta howardite, the howardite, eucrite, and diogenite (HED) meteorites contain ～10–30 ppm indigenous C with δ13C between ?29% and ?19%. Kapoeta (a regolith breccia) has an elevated C content and δ13C, due to the presence of 13C-enriched carbonate minerals (δ13C ～ +20%) in CM2- or CR2-like clasts. The range in δ13C displayed by HED samples is similar to that of other solar system basalts, such as lunar rocks and Martian meteorites but distinctly different from that of the terrestrial mantle. The diogenites have a slightly lower total C yield and higher δ13C than the eucrites, which is a result of degassing of trapped CO/CC2/CO2–3 from the silicate lattice during metamorphism or annealing. However, three out of the four diogenites studied appear to contain a discrete component, possibly of graphitic C coating silicate grains, that is seemingly unaffected by the extended annealing period experienced by the diogenites. It is possible that this component might host the indigenous primitive Xe recently identified in diogenites.  相似文献   

20.
Abstract— This work reports on the noble gas inventory of 3 new acapulcoites, 3 brachinites, 2 new eucrites from the Dar al Gani region in Libya, the unique achondrite Dar al Gani (DaG) 896 from the same locality, the new eucrite‐like achondrite Northwest Africa (NWA) 011, and the controversial sample Tafassasset. We determined cosmic ray exposure and gas retention ages, evaluated shielding conditions, and discuss the trapped noble gas component of the specimens. All exposure ages are within the known range of stony meteorites and partly confirm previously established age clusters. Shielding conditions vary, suggesting substantial shielding for all 3 brachinites and Tafassasset. We cannot exclude, however, that the Mg‐rich composition of brachinites simply simulates heavy shielding. Regarding the trapped component, we found Q‐like compositions only for the acapulcoite Thiel Mountains (TIL) 99002. The brachinite Elephant Moraine (EET) 99402 yields a high, subsolar 36Ar/132Xe ratio of ?400 along with a slightly elevated 84Kr/132atio, indicating minor atmospheric contamination. All the other samples, particularly the eucrite DaG 983, are characterized by clearly elevated Ar/Kr/Xe ratios due to significant terrestrial alteration. Tafassasset exhibits noble gas parameters that are different from those of CR chondrites, including a relatively high cosmic ray exposure age, the absence of a solar component, low 132Xe concentrations, a low trapped 36Ar/132Xe ratio of ?30, and a noticeable amount of radiogenic 129Xe. Similar attributes have been observed for some primitive achondrites. These attributes are also consistent with the metamorphic character of the sample. We, therefore, consider Tafassasset's noble gas record to be inconclusive as to its classification (primitive achondrite versus metamorphosed CR chondrite).  相似文献   

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