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1.
Abstract– This work describes two newly discovered eucrite breccias: three presumably paired meteorites, all named Northwest Africa (NWA) 6105, and NWA 6106. For each meteorite, major‐ and minor‐element compositions of minerals were determined using the electron microprobe. Pyroxene Fe‐Mn co‐variations and bulk‐rock oxygen isotope compositions confirm their classification as eucrites. Variations in mineral compositions and textures are attributed to differences in clast types present (i.e., basaltic or cumulate eucrite). The pyroxene compositions support the hypothesis that samples NWA 6105,1; 6105,2; and 6105,3 are paired polymict eucritic breccias, whereas sample NWA 6106 is a monomict basaltic eucritic breccia. Two‐pyroxene geothermometry yields temperatures too low for igneous crystallization. The variation in temperatures among samples suggests that metamorphism occurred prior to brecciation.  相似文献   

2.
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   

3.
We present a study of the petrology and geochemistry of basaltic shergottite Northwest Africa 2975 (NWA 2975). NWA 2975 is a medium‐grained basalt with subophitic to granular texture. Electron microprobe (EMP) analyses show two distinct pyroxene compositional trends and patchy compositional zoning patterns distinct from those observed in other meteorites such as Shergotty or QUE 94201. As no bulk sample was available to us for whole rock measurements, we characterized the fusion crust and its variability by secondary ion mass spectrometer (SIMS) measurements and laser ablation inductively coupled plasma spectroscopy (LA‐ICP‐MS) analyses as a best‐available proxy for the bulk rock composition. The fusion crust major element composition is comparable to the bulk composition of other enriched basaltic shergottites, placing NWA 2975 within that sample group. The CI‐normalized REE (rare earth element) patterns are flat and also parallel to those of other enriched basaltic shergottites. Merrillite is the major REE carrier and has a flat REE pattern with slight depletion of Eu, parallel to REE patterns of merrillites from other basaltic shergottites. The oxidation state of NWA 2975 calculated from Fe‐Ti oxide pairs is NNO‐1.86, close to the QFM buffer. NWA 2975 represents a sample from the oxidized and enriched shergottite group, and our measurements and constraints on its origin are consistent with the hypothesis of two distinct Martian mantle reservoirs: a reduced, LREE‐depleted reservoir and an oxidized, LREE‐enriched reservoir. Stishovite, possibly seifertite, and dense SiO2 glass were also identified in the meteorite, allowing us to infer that NWA 2975 experienced a realistic shock pressure of ~30 GPa.  相似文献   

4.
Abstract— We have carried out a detailed petrographic, mineralogical, and trace element study of Northwest Africa (NWA) 011. This meteorite bears many similarities to the eucrites it was initially identified with, although oxygen isotopic compositions rule out a genetic relationship. Like many eucrites, NWA 011 crystallized from a source with approximately chondritic proportions of REE, although a slightly LREE‐enriched bulk composition with a small positive Eu anomaly, as well as highly fractionated Fe/Mg ratios and depleted Sc abundances (Korotchantseva et al. 2003), suggest that the NWA 011 source experienced some pyroxene and/or olivine fractionation. Thermal metamorphism resulted in homogenization of REE abundances within grains, but NWA 011 did not experience the intergrain REE redistribution seen in some highly metamorphosed eucrites. Despite a similarity in oxygen isotopic compositions, NWA 011 does not represent a basaltic partial melt from the acapulcoite/lodranite parent body. The material from which NWA 011 originated may have been like some CH or CB chondrites, members of the CR chondrite clan, which are all related through oxygen isotopic compositions. The NWA 011 parent body is probably of asteroidal origin, possibly the basaltic asteroid 1459 Magnya.  相似文献   

5.
Knowledge of Martian igneous basaltic compositions is crucial for constraining mantle evolution, including early differentiation and mantle convection. Primitive magmas provide direct information about their mantle source regions, but most Martian meteorites either contain cumulate olivine or crystallized from fractionated melts. The recently discovered Martian meteorite Northwest Africa (NWA) 5789 is an olivine‐phyric shergottite. NWA 5789 has special significance among the Martian meteorites because it appears to represent one of the most magnesian Martian magmas known, other than Yamato (Y) 980459. Its most magnesian olivine cores (Fo85) are in Mg‐Fe equilibrium with a magma of the bulk rock composition, suggesting that the bulk represents a magma composition. Based on the Al/Ti ratio of its pyroxenes, we infer that the rock began to crystallize at a high pressure consistent with conditions in Mars’ lower crust/upper mantle. It continued and completed its crystallization closer to the surface, where cooling was rapid and produced a mesostasis of radiating sprays of plagioclase and pyroxene. The mineralogy, petrology, mineral chemistry, and bulk rock composition of NWA 5789 are very similar to those of Y‐980459. The similarities between the two meteorites suggest that NWA 5789 (like Y‐980459) represents a primitive, mantle‐derived magma composition. They also suggest the possibility that NWA 5789 and Y‐980459 formed in the same lava flow. However, based on the mineralogy and texture of its mesostasis, NWA 5789 must have cooled more slowly than Y‐980459. NWA 5789 will help elucidate the igneous geology and geochemistry of Mars.  相似文献   

6.
Abstract— Piplia Kalan is an equilibrated eucrite consisting of 60–80 vol% lithic clasts in a subordinate brecciated matrix. Ophitic/subophitic clasts fall into two groups: finer‐grained lithology A and coarser‐grained lithology B. Very fine‐grained clasts with equigranular textures (lithology C) also occur and originally were hypocrystalline in texture. The variety of materials represented in Piplia Kalan suggests cooling histories ranging from quenching to slower crystallization. Despite textural differences, clasts and matrix have similar mineral and bulk compositions. Thus, Piplia Kalan is probably best classified as a genomict breccia that could represent fragments of a single lava flow or shallow intrusive body, including fine‐grained or glassy outer margin and more slowly cooled coarser‐grained interior. Bulk composition suggests that the meteorite is most closely related to the main group eucrites, but it probably was affected by minor amounts of fractional crystallization. Piplia Kalan displays evidence of an early shock event, including brecciated matrix and areas of lithic clasts that contain very fine‐grained, granular pyroxene between deformed feldspar laths. The meteorite also displays evidence of at least one episode of extensive thermal metamorphism: hypocrystalline materials are recrystallized to hornfelsic textures and minerals throughout the meteorite contain abundant inclusions that are relatively large in size. Veins of brown glass transect both clasts and matrix and indicate a second, postmetamorphism shock event.  相似文献   

7.
Lunar breccias preserve the records of geologic processes on the Moon. In this study, we report the occurrence, petrography, mineralogy, and geologic significance of the observed secondary olivine veinlets in lunar feldspathic breccia meteorite Northwest Africa (NWA) 11273. Bulk‐rock composition measurements show that this meteorite is geochemically similar to other lunar highland meteorites. In NWA 11273, five clasts are observed to host veinlets that are dominated by interconnecting olivine mineral grains. The host clasts are mainly composed of mafic minerals (i.e., pyroxene and olivine) and probably sourced from a basaltic lithology. The studied olivine veinlets (~5 to 30 μm in width) are distributed within the mafic mineral host, but do not extend into the adjacent plagioclase. Chemically, these olivine veinlets are Fe‐richer (Fo41.4–51.9), compared with other olivine grains (Fo54.3–83.1) in lithic clasts and matrix of NWA 11273. By analogy with the secondary olivine veinlets observed in meteorites from asteroid Vesta (howardite–eucrite–diogenite group samples) and lunar mare samples, our study suggests that the newly observed olivine veinlets in NWA 11273 are likely formed by secondary deposition from a lunar fluid, rather than by crystallization from a high‐temperature silicate melt. Such fluid could be sulfur‐ and phosphorous‐poor and likely had an endogenic origin on the Moon. The new occurrence of secondary olivine veinlets in breccia NWA 11273 reveals that the fluid mobility and deposition could be a previously underappreciated geological process on the Moon.  相似文献   

8.
Abstract— The newly found meteorite Northwest Africa 6234 (NWA 6234) is an olivine (ol)‐phyric shergottite that is thought, based on texture and mineralogy, to be paired with Martian shergottite meteorites NWA 2990, 5960, and 6710. We report bulk‐rock major‐ and trace‐element abundances (including Li), abundances of highly siderophile elements, Re‐Os isotope systematics, oxygen isotope ratios, and the lithium isotope ratio for NWA 6234. NWA 6234 is classified as a Martian shergottite, based on its oxygen isotope ratios, bulk composition, and bulk element abundance ratios, Fe/Mn, Al/Ti, and Na/Al. The Li concentration and δ7Li value of NWA 6234 are similar to that of basaltic shergottites Zagami and Shergotty. The rare earth element (REE) pattern for NWA 6234 shows a depletion in the light REE (La‐Nd) compared with the heavy REE (Sm‐Lu), but not as extreme as the known “depleted” shergottites. Thus, NWA 6234 is suggested to belong to a new category of shergottite that is geochemically “intermediate” in incompatible elements. The only other basaltic or ol‐phyric shergottite with a similar “intermediate” character is the basaltic shergottite NWA 480. Rhenium‐osmium isotope systematics are consistent with this intermediate character, assuming a crystallization age of 180 Ma. We conclude that NWA 6234 represents an intermediate compositional group between enriched and depleted shergottites and offers new insights into the nature of mantle differentiation and mixing among mantle reservoirs in Mars.  相似文献   

9.
Records of space weathering are important for understanding the formation and evolution of surface regolith on airless celestial bodies. Current understanding of space weathering processes on asteroids including asteroid‐4 Vesta, the source of the howardite–eucrite–diogenite (HED) meteorites, lags behind what is known for the Moon. In this study, we studied agglutinates, a vesicular glass‐coating lithic clast, and a fine‐grained sulfide replacement texture in the polymict breccia Northwest Africa (NWA) 1109 with electron microscopy. In agglutinates, nanophase grains of FeNi and FeS were observed, whereas npFe0 was absent. We suggested that the agglutinates in NWA 1109 formed from fine‐grained surface materials of Vesta during meteorite/micrometeorite bombardment. The fine‐grained sulfide replacement texture (troilite + hedenbergite + silica) should be a result of reaction between S‐rich vapors and pyroxferroite. The unique Fe/Mn values of relict pyroxferroite indicate a different source from normal HED pyroxenes, arguing that the reaction took place on or near the surface of Vesta. The fine‐grained sulfide replacement texture could be a product of nontypical space weathering on airless celestial bodies. We should pay attention to this texture in future returned samples by asteroid exploration missions.  相似文献   

10.
Northwest Africa (NWA) 4734 is an unbrecciated basaltic lunar meteorite that is nearly identical in chemical composition to basaltic lunar meteorites NWA 032 and LaPaz Icefield (LAP) 02205. We have conducted a geochemical, petrologic, mineralogic, and Sm‐Nd, Rb‐Sr, and Ar‐Ar isotopic study of these meteorites to constrain their petrologic relationships and the origin of young mare basalts. NWA 4734 is a low‐Ti mare basalt with a low Mg* (36.5) and elevated abundances of incompatible trace elements (e.g., 2.00 ppm Th). The Sm‐Nd isotope system dates NWA 4734 with an isochron age of 3024 ± 27 Ma, an initial εNd of +0.88 ± 0.20, and a source region 147Sm/144Nd of 0.201 ± 0.001. The crystallization age of NWA 4734 is concordant with those of LAP 02205 and NWA 032. NWA 4734 and LAP 02205 have very similar bulk compositions, mineral compositions, textures, and ages. Their source region 147Sm/144Nd values indicate that they are derived from similar, but distinct, source materials. They probably do not sample the same lava flow, but rather are similarly sourced, but isotopically distinct, lavas that probably originate from the same volcanic complex. They may have experienced slightly different assimilation histories in route to eruption, but can be source‐crater paired. NWA 032 remains enigmatic, as its source region 147Sm/144Nd definitively precludes a simple relationship with NWA 4734 and LAP 02205, despite a similar bulk composition. Their high Ti/Sm, low (La/Yb)N, and Cl‐poor apatite compositions rule out the direct involvement of KREEP. Rather, they are consistent with low‐degree partial melting of late‐formed LMO cumulates, and indicate that the geochemical characteristics attributed to urKREEP are not unique to that reservoir. These and other basaltic meteorites indicate that the youngest mare basalts originate from multiple sources, and suggest that KREEP is not a prerequisite for the most recent known melting in the Moon.  相似文献   

11.
The Emmaville eucrite is a relatively poorly studied basaltic achondrite with an anomalous oxygen isotope signature. In this study, we report comprehensive mineralogical, petrographic, and geochemical data from Emmaville in order to understand its petrogenesis and relationship with the basaltic eucrites. Emmaville is an unusually fine‐grained, hornfelsic‐textured metabasalt with pervasive impact melt veins and mineral compositions similar to those of typical basaltic eucrites. The major and trace element bulk composition of Emmaville is also typical of a basaltic eucrite. Three separated individual lithologies were also analyzed for O isotopes; a dark gray fraction (E1), a shocked lithology (E2), and a lighter gray portion (E3). Fractions E1 and E2 shared similar O isotope compositions to the bulk sample (E‐B), whereas the lighter gray portion (E3) is slightly elevated in Δ17O and significantly elevated in δ18O compared to bulk. No evidence for any exogenous material is observed in the thin sections, coupled with the striking compositional similarity to typical basaltic eucrites, appears to preclude a simple impact‐mixing hypothesis. The O‐isotopes of Emmaville are similar to those of Bunburra Rockhole, A‐881394, and EET 92023, and thus distinct from the majority of the HEDs, despite having similarities in petrology, mineral, and bulk compositions. It would, therefore, seem plausible that all four of these samples are derived from a single HED‐like parent body that is isotopically distinct from that of the HEDs (Vesta) but similar in composition.  相似文献   

12.
Olivine‐dominated (70–80 modal %) achondrite meteorite Lewis Cliff (LEW) 88763 originated from metamorphism and limited partial melting of a FeO‐rich parent body. The meteorite experienced some alteration on Earth, evident from subchondritic Re/Os, and redistribution of rhenium within the sample. LEW 88763 is texturally similar to winonaites, has a Δ17O value of ?1.19 ± 0.10‰, and low bulk‐rock Mg/(Mg+Fe) (0.39), similar to the FeO‐rich cumulate achondrite Northwest Africa (NWA) 6693. The similar bulk‐rock major‐, minor‐, and trace‐element abundances of LEW 88763, relative to some carbonaceous chondrites, including ratios of Pd/Os, Pt/Os, Ir/Os, and 187Os/188Os (0.1262), implies a FeO‐ and volatile‐rich precursor composition. Lack of fractionation of the rare earth elements, but a factor of approximately two lower highly siderophile element abundances in LEW 88763, compared with chondrites, implies limited loss of Fe‐Ni‐S melts during metamorphism and anatexis. These results support the generation of high Fe/Mg, sulfide, and/or metal‐rich partial melts from FeO‐rich parent bodies during partial melting. In detail, however, LEW 88763 cannot be a parent composition to any other meteorite sample, due to highly limited silicate melt loss (0 to <<5%). As such, LEW 88763 represents the least‐modified FeO‐rich achondrite source composition recognized to date and is distinct from all other meteorites. LEW 88763 should be reclassified as an anomalous achondrite that experienced limited Fe,Ni‐FeS melt loss. Lewis Cliff 88763, combined with a growing collection of FeO‐rich meteorites, such as brachinites, brachinite‐like achondrites, the Graves Nunataks (GRA) 06128/9 meteorites, NWA 6693, and Tafassasset, has important implications for understanding the initiation of planetary differentiation. Specifically, regardless of precursor compositions, partial melting and differentiation processes appear to be similar on asteroidal bodies spanning a range of initial oxidation states and volatile contents.  相似文献   

13.
The howardite‐eucrite‐diogenite (HED) clan of meteorites, which most likely originate from the asteroid Vesta, provide an opportunity to combine in‐depth sample analysis with the comprehensive remote‐sensing data set from NASA's recent Dawn mission. Miller Range (MIL) 11100, an Antarctic howardite, contains diverse rock and mineral fragments from common HED lithologies (diogenites, cumulate eucrites, and basaltic eucrites). It also contains a rare pyroxferroite‐bearing lithology—not recognized in HED until recently—and rare Mg‐rich (Fo86‐91) olivine crystals that possibly represent material excavated from the Vestan mantle. Clast components underwent different histories of thermal and impact metamorphism before being incorporated into this sample, reflecting the diversity in geological histories experienced by different parts of Vesta. The bulk chemical composition and petrography of MIL 11100 suggest that it is akin to the fragmental howardite meteorites. The strong lithological heterogeneity across this sample suggests that at least some parts of the Vestan regolith show heterogeneity on the mm‐scale. We combine the outcomes of this study with data from NASA's Dawn mission and hypothesize on possible source regions for this meteorite on the surface of Vesta.  相似文献   

14.
Abstract— 40Ar‐39Ar data are presented for the unbrecciated lunar basaltic meteorites Asuka (A‐) 881757, Yamato (Y‐) 793169, Miller Range (MIL) 05035, LaPaz Icefield (LAP) 02205, Northwest Africa (NWA) 479 (paired with NWA 032), and basaltic fragmental breccia Elephant Moraine (EET) 96008. Stepped heating 40Ar‐39Ar analyses of several bulk fragments of related meteorites A‐881757, Y‐793169 and MIL 05035 give crystallization ages of 3.763 ± 0.046 Ga, 3.811 ± 0.098 Ga and 3.845 ± 0.014 Ga, which are comparable with previous age determinations by Sm‐Nd, U‐Pb Th‐Pb, Pb‐Pb, and Rb‐Sr methods. These three meteorites differ in the degree of secondary 40Ar loss with Y‐793169 showing relatively high Ar loss probably during an impact event ?200 Ma ago, lower Ar loss in MIL 05035 and no loss in A‐881757. Bulk and impact melt glass‐bearing samples of LAP 02205 gave similar ages (2.985 ± 0.016 Ga and 2.874 ± 0.056 Ga) and are consistent with ages previously determined using other isotope pairs. The basaltic portion of EET 96008 gives an age of 2.650 ± 0.086 Ga which is considered to be the crystallization age of the basalt in this meteorite. The Ar release for fragmental basaltic breccia EET 96008 shows evidence of an impact event at 631 ± 20 Ma. The crystallization age of 2.721 ± 0.040 Ga determined for NWA 479 is indistinguishable from the weighted mean age obtained from three samples of NWA 032 supporting the proposal that these meteorites are paired. The similarity of 40Ar‐39Ar ages with ages determined by other isotopic systems for multiple meteorites suggests that the K‐Ar isotopic system is robust for meteorites that have experienced a significant shock event and not a prolonged heating regime.  相似文献   

15.
Abstract— We report on the bulk composition and petrography of four new basaltic meteorites found in Antarctica—LAP (LaPaz Icefield) 02205, LAP 02224, LAP 02226, and LAP 02436—and compare the LAP meteorites to other lunar mare basalts. The LAP meteorites are coarse‐grained (up to 1.5 mm), subophitic low‐Ti basalts composed predominantly of pyroxene and plagioclase, with minor amounts of olivine, ilmenite, and a groundmass dominated by fayalite and cristobalite. All of our observations and results support the hypothesis that the LAP stones are mutually paired with each other. In detail, the geochemistry of LAP is unlike those of any previously studied lunar basalt except lunar meteorite NWA (Northwest Africa) 032. The similarities between LAP and NWA 032 are so strong that the two meteorites are almost certainly source crater paired and could be two different samples of a single basalt flow. Petrogenetic modeling suggests that the parent melt of LAP (and NWA 032) is generally similar to Apollo 15 low‐Ti, yellow picritic glass beads, and that the source region for LAP comes from a similar region of the lunar mantle as previously analyzed lunar basalts.  相似文献   

16.
Abstract— Mineralogy, major element compositions of minerals, and elemental and oxygen isotopic compositions of the whole rock attest to a lunar origin of the meteorite Northwest Africa (NWA) 032, an unbrecciated basalt found in October 1999. The rock consists predominantly of olivine, pyroxene and chromite phenocrysts, set in a crystalline groundmass of feldspar, pyroxene, ilmenite, troilite and trace metal. Whole‐rock shock veins comprise a minor, but ubiquitous portion of the rock. Undulatory to mosaic extinction in olivine and pyroxene phenocrysts and micro‐faults in groundmass and phenocrysts also are attributed to shock. Several geochemical signatures taken together indicate unambiguously that NWA 032 originated from the Moon. The most diagnostic criteria include whole‐rock oxygen isotopic composition and ratios of Fe/Mn in the whole rock, olivine, and pyroxene. A lunar origin is documented further by the presence of Fe‐metal, troilite, and ilmenite; zoning to extremely Fe‐rich compositions in pyroxene; the ferrous oxidation state of all Fe in pyroxene; and the rare earth element (REE) pattern with a well‐defined negative europium anomaly. This rock is similar in major element chemistry to basalts from Apollo 12 and 15, but is enriched in light REE and has an unusually high Th/Sm ratio. Some Apollo 14 basalts yield a closer match to NWA 032 in REE patterns, but have higher concentrations of Al2O3. Ar‐Ar step release results are complex, but yield a whole‐rock age of ?2.8 Ga, suggesting that NWA 032 was extruded at 2.8 Ga or earlier. This rock may be the youngest sample of mare basalt collected to date. Noble gas concentrations combined with previously collected radionuclide data indicate that the meteorite exposure history is distinct from currently recognized lunar meteorites. In short, the geochemical and petrographic features of NWA 032 are not matched by Apollo or Luna samples, nor by previously identified lunar meteorites, indicating that it originates from a previously unsampled mare deposit. Detailed assessment of petrographic features, olivine zoning, and thermodynamic modelling indicate a relatively simple cooling and crystallization history for NWA 032. Chromite‐spinel, olivine, and pyroxene crystallized as phenocrysts while the magma cooled no faster than 2 °C/h based on the polyhedral morphology of olivine. Comparison of olivine size with crystal growth rates and preserved Fe‐Mg diffusion profiles in olivine phenocrysts suggest that olivine was immersed in the melt for no more than 40 days. Plumose textures in groundmass pyroxene, feldspar, and ilmenite, and Fe‐rich rims on the phenocrysts formed during rapid crystallization (cooling rates ?20 to 60 °C/h) after eruption.  相似文献   

17.
The petrologic and oxygen isotopic characteristics of calcium‐aluminum‐rich inclusions (CAIs) in CO chondrites were further constrained by studying CAIs from six primitive CO3.0‐3.1 chondrites, including two Antarctic meteorites (DOM 08006 and MIL 090010), three hot desert meteorites (NWA 10493, NWA 10498, and NWA 7892), and the Colony meteorite. The CAIs can be divided into hibonite‐bearing inclusions (spinel‐hibonite spherules, monomineralic grains, hibonite‐pyroxene microspherules, and irregular/nodular objects), grossite‐bearing inclusions (monomineralic grains, grossite‐melilite microspherules, and irregular/nodular objects), melilite‐rich inclusions (fluffy Type A, compact type A, monomineralic grains, and igneous fragments), spinel‐pyroxene inclusions (fluffy objects resembling fine‐grained spinel‐rich inclusions in CV chondrites and nodular/banded objects resembling those in CM chondrites), and pyroxene‐anorthite inclusions. They are typically small (98.4 ± 54.4 µm, 1SD) and comprise 1.54 ± 0.43 (1SD) area% of the host chondrites. Melilite in the hot desert and Colony meteorites was extensively replaced by a hydrated Ca‐Al‐silicate during terrestrial weathering and converted melilite‐rich inclusions into spinel‐pyroxene inclusions. The CAI populations of the weathered COs are very similar to those in CM chondrites, suggesting that complete replacement of melilite by terrestrial weathering, and possibly parent body aqueous alteration, would make the CO CAIs CM‐like, supporting the hypothesis that CO and CM chondrites derive from similar nebular materials. Within the CO3.0‐3.1 chondrites, asteroidal alteration significantly resets oxygen isotopic compositions of CAIs in CO3.1 chondrites (?17O: ?25 to ?2‰) but left those in CO3.0‐3.05 chondrites mostly unchanged (?17O: ?25 to ?20‰), further supporting the model whereby thermal metamorphism became evident in CO chondrites of petrologic type ≥3.1. The resistance of CAI minerals to oxygen isotope exchange during thermal metamorphism follows in the order: melilite + grossite < hibonite + anorthite < spinel + diopside + forsterite. Meanwhile, terrestrial weathering destroys melilite without changing the chemical and isotopic compositions of melilite and other CAI minerals.  相似文献   

18.
Abstract— Dar al Gani 872 (DaG 872) is a new meteorite from Libya that we classified by means of Instrumental Neutron Activation Analysis (INAA), electron microprobe, and optical microscopy. According to our results, DaG 872 is a Mg‐rich main group eucrite, i.e., a monomict noncumulate basaltic eucrite displaying a predominant coarse‐grained relict subophitic and a fine‐grained granulitic texture. The meteorite also shows pockets of late‐stage mesostasis and is penetrated by several calcite veins due to terrestrial weathering. Finally, it exhibits shock phenomena of stage 1–2 including heavily fractured mineral components, undulose extinction of plagioclase, kinked lamellae, and mosaicism in pyroxenes corresponding to peak pressures of ?20 GPa. In view of petrographic criteria as well as compositional and exsolution characteristics of its pyroxenes, the sample represents a metamorphic type 5 eucrite. Assuming the metamorphic type to be a function of burial depth on the parent body and taking into account the relatively high shock stage, the excavation of DaG 872 was likely induced by a major impact event. Prior to this point, DaG 872 apparently underwent a 4‐stage geological evolution that is reflected by intricate textural and mineralogical features.  相似文献   

19.
Abstract– Northwest Africa (NWA) 1068 is one of the few olivine‐phyric shergottites (e.g., NWA 1068, Larkman Nunatak [LAR] 06319, and Roberts Massif [RBT] 04262) that is not depleted in light rare earth elements (LREE). Its REE pattern is similar to that of the basaltic shergottite Shergotty, suggesting a possible connection between the olivine‐phyric and the basaltic shergottites. To test this possible link, we have investigated the high‐pressure near‐liquidus phase equilibria for the NWA 1068 meteorite bulk composition. Our results show that the NWA 1068 bulk composition does not represent an unmodified mantle‐derived melt; the olivine and pyroxene in our near‐liquidus experiments are more magnesian than in the rock itself, which suggests that NWA 1068 contains cumulate minerals (extra olivine). We have then used these experimental results combined with the pyroxene compositions in NWA 1068 to constrain the possible high‐pressure crystallization history of the parental magma. These results suggest that NWA 1068 had a complex polybaric history. Finally, we have calculated a model parental magma composition for the NWA 1068 meteorite. The calculated parental magma is an evolved basaltic composition which is too ferroan to be a primitive melt directly derived from the mantle. We suggest that it ponded and crystallized at approximately the base of the crust. This provided an opportunity for the magma to become contaminated by an “enriched” crustal component prior to crystallization. The results and modeling from these experiments are applicable not only to the NWA 1068 meteorite, but also to LAR 06319 and possibly any other enriched olivine‐phyric shergottite.  相似文献   

20.
High‐precision isotope data of meteorites show that the long‐standing notion of a “chondritic uniform reservoir” is not always applicable for describing the isotopic composition of the bulk Earth and other planetary bodies. To mitigate the effects of this “isotopic crisis” and to better understand the genetic relations of meteorites and the Earth‐forming reservoir, we performed a comprehensive petrographic, elemental, and multi‐isotopic (O, Ca, Ti, Cr, Ni, Mo, Ru, and W) study of the ungrouped achondrites NWA 5363 and NWA 5400, for both of which terrestrial O isotope signatures were previously reported. Also, we obtained isotope data for the chondrites Pillistfer (EL6), Allegan (H6), and Allende (CV3), and compiled available anomaly data for undifferentiated and differentiated meteorites. The chemical compositions of NWA 5363 and NWA 5400 are strikingly similar, except for fluid mobile elements tracing desert weathering. We show that NWA 5363 and NWA 5400 are paired samples from a primitive achondrite parent‐body and interpret these rocks as restite assemblages after silicate melt extraction and siderophile element addition. Hafnium‐tungsten chronology yields a model age of 2.2 ± 0.8 Myr after CAI, which probably dates both of these events within uncertainty. We confirm the terrestrial O isotope signature of NWA 5363/NWA 5400; however, the discovery of nucleosynthetic anomalies in Ca, Ti, Cr, Mo, and Ru reveals that the NWA5363/NWA 5400 parent‐body is not the “missing link” that could explain the composition of the Earth by the mixing of known meteorites. Until this “missing link” or a direct sample of the terrestrial reservoir is identified, guidelines are provided of how to use chondrites for estimating the isotopic composition of the bulk Earth.  相似文献   

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