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1.
Abstract— We report on a new angrite, Northwest Africa (NWA) 1296, a fine‐grained rock with a magmatic texture of rapid cooling. Dendritic olivine (?Fo50) crystallized first in association with anorthite microcrysts (An98–100) forming composite chains separated from one another by intergrown Al‐Fe diopside‐hedenbergite pyroxenes. In addition, some olivines with lower Mg# and increased CaO (up to 12%) are found between the chains as equant microphenocrysts. Pyroxenes and olivines are both normally zoned from Mg# = 0.52 to less than 0.01 in the rims. Ca‐rich olivines are surrounded by, intergrown with, or replaced by subcalcic kirschsteinite. They appear after plagioclase crystallization stopped, at the end of the crystallization sequence. Minor phases are pyrrhotite, F‐apatite, and titanomagnetite. Pyroxene is the last silicate phase to grow, interstitial to idiomorphic olivine‐kirschsteinite. Numerous small vesicles and some channels are filled with microcristalline carbonate. The mode (vol%) is about 28% olivine, 3% kirschsteinite, 32% anorthite, 34% pyroxene, and 3% of the minor phases—close to that reported previously for D'Orbigny and Sahara (SAH) 99555. The bulk chemical composition of NWA 1296 is similar to D'Orbigny and SAH 99555; NWA 1296 differs by its texture and mineralogy, which are interpreted as resulting from rapid crystallization—an evidence of impact melting. Angrites cannot be produced by partial melting of a CV source because segregation of a “planetary” core is necessary to explain the low FeO/MgO ratio of magnesian olivines. Neither the odd Ca/Al ratio nor the very low SiO2 content can be explained by conventional partial melting scenarios. We suggest that carbonate is the key to angrite genesis. This is supported by the striking similarities with terrestrial melilitites (low SiO2, superchondritic Ca/Al ratio, presence of carbonate). The lack of alkalies could be the result of either loss after impact melting or absence of alkalies in the source.  相似文献   

2.
Abstract— Angrites are generally believed to be fragments of a basaltic asteroid that differentiated under relatively oxidizing conditions. Almost all angrites (e.g., D'Orbigny, Lewis Cliff [LEW] 86010, and Sahara 99555) are composed predominately of anorthite, Al‐Ti diopside‐hedenbergite, and Ca‐rich olivine, except for the type specimen, Angra dos Reis, which is composed almost entirely of Al‐Ti diopside‐hedenbergite. D'Orbigny, LEW 86010, and Sahara 99555 also have spectral properties very different from Angra dos Reis. These newly measured angrites all have broad absorption features centered near 1 μm with very weak to absent absorption bands at ?2 μm, which is characteristic of some clinopyroxenes. The spectrum of Angra dos Reis has the characteristic 1 and 2 μm features due to pyroxene. One asteroid, 3819 Robinson, has similar spectral properties to the newly measured angrites in the visible wavelength region, but does not appear to spectrally match these angrites in the near‐infrared.  相似文献   

3.
Abstract— D'Orbigny is an exceptional angrite. Chemically, it resembles other angrites such as Asuka‐881371, Sahara 99555, Lewis Cliff (LEW) 87051, and LEW 86010, but its structure and texture are peculiar. It has a compact and porous lithology, abundant glasses, augite‐bearing druses, and chemical and mineralogical properties that are highly unusual for igneous rocks. Our previous studies led us to a new view on angrites: they can possibly be considered as CAIs that grew to larger sizes than the ones we know from carbonaceous chondrites. Thus, angrites may bear a record of rare and special conditions in some part of the early solar nebula. Here we report trace element contents of D'Orbigny phases. Trace element data were obtained from both the porous and the compact part of this meteorite. We have confronted our results with the popular igneous genetic model. According to this model, if all phases of D'Orbigny crystallized from the same system, as an igneous origin implies, a record of this genesis should be expressed in the distribution of trace elements among early and late phases. Our results show that the trace element distribution of the two contemporaneous phases olivine and plagioclase, which form the backbone of the rock, seem to require liquids of different composition. Abundances of highly incompatible elements in all olivines, including the megacrysts, indicate disequilibrium with the bulk rock and suggest liquids very rich in these elements (>10,000 x CI), which is much richer than any fractional crystallization could possibly produce. In addition, trace element contents of late phases are incompatible with formation from the bulk system's residual melt. These results add additional severe constraints to the many conflicts that existed previously between an igneous model for the origin of angrites and the mineralogical and chemical observations. This new trace element content data, reported here, corroborate our previous results based on the shape, structure, mineralogy, chemical, and isotopic data of the whole meteorite, as well as on a petrographic and chemical composition study of all types of glasses and give strength to a new genetic model that postulates that D'Orbigny (and possibly all angrites) could have formed in the solar nebula under changing redox conditions, more akin to chondritic constituents (e.g., CAIs) than to planetary differentiated rock.  相似文献   

4.
Abstract— Northwest Africa (NWA) 1670, contains olivines of up to 5 mm in size representing about 30% of the studied section. With subordinate clinopyroxene and chrome‐spinel microphenocrysts (0.2‐0.5 mm), they represent a xenocrystic association. Phenocrysts are surrounded by a groundmass, predominantly comprising bundles of plagioclase and clinopyroxene (typically 20 × 200 μm crystals). Olivine and kirschsteinite are present in the groundmass in lesser amounts. The olivine xenocrysts (Fo90) are significantly fractured and show mosaicism for their major part, the remaining showing faint undulatory extinction. They are surrounded with a rim of 100–200 μm zoned down to Fo80 and overgrown with serrated olivine, Fo80 to Fo60 (about 100 μm). Olivine in the groundmass is zoned from Mg# 0.55 to 0.15; its CaO content ranges 2.0 to 8.4%. Subcalcic kirschsteinite is zoned from Mg# 0.13 to 0.03, CaO increasing from 15.8 to 21.3%. Pyroxenes xenocrysts (Mg# = 0.77) are superseded in the groundmass by less magnesian pyroxenes, Mg# 0.61 to 0.17, with an average FeO/ MnO of 98. Their compositions range from En30 Fs22 Wo27 Al‐Ts28 Ti‐Ts2 to En2 Fs37 Wo22 Al‐Ts40 Ti‐Ts1. Anorthite microcrysts (An99‐100) are restricted to the groundmass. Accessories are pyrrhotite, kamacite, Ca‐phosphate, titanomagnetite, hercynite and Ca‐carbonate. The bulk chemical composition confirms that NWA 1670 corresponds to a normal angrite melt that incorporated olivine. High Mg olivine xenocrysts and the associated mineralogy are typical of angrites. We suggest that it is an impact melt with relict phenocrysts. The strong silica undersaturation, the presence of Fo90 olivine xenocrysts and carbonate support their derivation as melilite‐like melts in the presence of carbonate.  相似文献   

5.
Abstract— We have conducted a detailed study of the Mn‐Cr systematics of the angrite D'Orbigny. Here, we report Cr isotopic abundances and Mn/Cr ratios in olivine, pyroxene, glass, chromite, and bulk rock samples from D'Orbigny. 53Cr excesses in these samples correlate well with their respective Mn/Cr ratios and define an isochron with a slope that corresponds to an initial 53Mn/55Mn ratio = (3.24 ± 0.04) × 10?6 and initial 53Cr/52Cr ratio of ?(53) = 0.30 ± 0.03 at the time of isotopic closure. The 53Mn/55Mn ratio of the D'Orbigny bulk rock is more than two‐fold the 53Mn/55Mn ratio of the angrites Lewis Cliff 86010 (LEW) and Angra dos Reis (ADOR) and implies an older Mn‐Cr age of 4562.9 ± 0.6 Ma for D'Orbigny relative to a Pb‐Pb age of 4557.8 ± 0.5 Ma for LEW and ADOR. One of the most unusual aspects of D'Orbigny is the presence of glass, a phase that has not been identified in any of the other angrites. The Mn‐Cr data for glass and a pyroxene fraction found in druses indicate that they formed contemporaneously with the main phases of the meteorite. Since the Mn‐Cr age of D'Orbigny is ?5 Ma years older than the angrites LEW and ADOR, D'Orbigny likely represents an earlier stage in the evolution of the angrite parent body.  相似文献   

6.
Angrites are a small group of ancient basaltic achondrites, notable for their unusual chemistry and extreme volatile depletion. No comprehensive study of indigenous light elements currently exists for the group. Measurement of the abundances and isotopic composition of carbon and nitrogen could provide information pertaining to the evolution of the angrite parent body. Bulk‐sample stepped combustion analyses of five angrites and a glass separate from D'Orbigny were combined with earlier data and acid dissolution experiments of carbonates found in D'Orbigny to compile an inventory of indigenous carbon and nitrogen. Indigenous carbon combusted between 700 °C and 1200 °C, with abundances of 10–140 ppm and a mass‐weighted δ13C of ?25 to ?20‰ with the exception of D'Orbigny (δ13C approximately ?5‰). Nitrogen was released at 850–1200 ºC, 1–20 ppm with a δ15N ?3‰ to +4‰; again, D'Orbigny (δ15N approximately +20 to +25‰) was an exception. We interpret these components as largely indigenous and decoupled; the carbon in graphitic or amorphous form, while the nitrogen is present as a dissolved component in the silicates. No relationship with the textural sub‐classification of angrites is apparent. We suggest that the angrite parent body contains a reservoir of reduced carbon and thus may have undergone a change in redox conditions, although the timing and mechanism for this remain unclear.  相似文献   

7.
Cover     
Complex corona of anorthite + olivine around primary spinel in angrite NWA 3164. Left‐hand side image is general view (backscattered electron image): the anorthite corona developed preferentially at the expense of spinel and extends far from the contact between spinel and clinopyroxene; locally, secondary clinopyroxene and spinel developed between the anorthite corona and primary olivine. Traces of calcite are observed within the late veins. Scale bar: 100 μm. The blue box indicates the enlarged inset (right‐hand side image) showing a detailed view (false‐color image) red: spinel; light green: olivine; dark green: clinopyroxene; violet: anorthite; pink: kamacite and iron oxide; blue: calcite. Olivine in the corona is partly enveloped by clinopyroxene. Small clinopyroxene grains and spinel fi lms formed between the secondary olivine and anorthite that developed around primary spinel. Scale bar: 20 μm. For details see Baghdadi et al. on pp. 1873–1893.  相似文献   

8.
Northwest Africa 3164 is a coarse‐grained angrite that shows reaction coronas, a unique character among achondrites. Olivine (Fo57; 1.2 wt% CaO), fassaitic clinopyroxene, anorthite, and spinel account for 46–47, 28–29, 8–13, and 4–8 vol%, respectively; kamacite is an accessory phase. The spinel grains in contact with clinopyroxene are bounded by discontinuous 20 μm thick coronas of anorthite and olivine, indicating the reaction Cpx + Spl → Ol + An (R1). In addition, irregular coronas of clinopyroxene and spinel developed around the primary anorthite in contact with primary olivine, during the reaction Ol + An → Cpx + Spl (R2). R2 also generated clinopyroxene and spinel films between the secondary olivine and anorthite coronas produced during R1, implying that R1 preceded R2. Both are metamorphic reactions that developed in the solid state. Finally, the coronas are cross cut by μm‐thick veinlets due to a late shock. A mass‐balance study shows that R2 is almost the reverse of R1. The P–T metamorphic evolution of the rock, modeled by calculating a P–T isochemical diagram, indicates an equilibrium T of 940 ± 120 °C at P < 0.9 GPa for the initial assemblage, followed by an increase of T up to approximately 1000–1200 °C during reaction R1 and a subsequent cooling during R2. Several causes are envisaged to account for this metamorphic evolution. Contact metamorphism due to a hot magmatic intrusion in the angrite parent body is favored, as similar metamorphic coronas are well known in metamorphic terrestrial rocks. In addition to differentiation and magmatism, there is now evidence for metamorphism in the angrite parent body, which would have been a large asteroid or a planetary‐sized body.  相似文献   

9.
Angrite meteorites are samples of early planetesimal magmatic rocks, distinguished from more typical “basaltic eucrites” by compositions that are silica undersaturated, relatively oxidized, and with high CaO/Al2O3. The latter is not expected from nebular, chondritic materials that might form a primitive mantle, such as a source enriched in refractory inclusions with fixed CaO/Al2O3 (e.g., CV chondrite). Here we present results of “reversal” crystallization experiments for two possible parental angrite compositions (approximating the D'Orbigny meteorite) to investigate the role of spinel as a sink for Al2O3. This mineral has previously been produced with angritic melts during “forward” melting of CV chondrite and may be abundant in the angrite source. At oxidizing conditions, we confirm that spinel is a liquidus phase and that angritic magmas form near the olivine-anorthite-spinel-liquid peritectic. A stability gap separates Al-rich liquidus spinels and lower temperature spinels, the latter of which are similar to those in basaltic eucrites. Al-rich spinel is likely more abundant in the angritic source than other Fe-rich core-forming components such as metal or sulfide, and a CV chondrite-like composition generates most features of angrite magmas by fractionation of observed olivine and liquidus spinel. Direct CaO excess, via carbonate addition, is therefore limited. In this model, discrepancies remain for Li, Sc, Cr(-Al), and Ba, which may record local accretion conditions or early processing. The possible role of spinel as a sink for 26Al may have strong influence on the thermal evolution of the angrite parent body.  相似文献   

10.
Abstract— We measured rare earth element (REE) abundances in selected silicate phases in the angrites Angra dos Reis (AdoR) and Lewis Cliff (LEW) 86010 in order to further clarify the thermal history of AdoR. We also carried out a preliminary experimental study designed to examine apparent REE partitioning between silicates (fassaite, olivine, kirschsteinite, and melt) in synthetic analogs of angrites under disequilibrium conditions at liquidus temperatures. Silicates in AdoR are homogeneous with respect to major, minor, and trace elements, which is consistent with the interpretation that AdoR underwent extensive subsolidus equilibration. REE distributions in olivine and kirschsteinite in AdoR are similar to those in LEW 86010 and are consistent with the formation of kirschsteinite by exsolution from olivine during cooling and/or annealing. There is no evidence for a disequilibrium trace element signature that could have been inherited from rapid cooling at liquidus temperatures. This is supported by our petrographic observations of the occurrence of kirschsteinite within olivine aggregates in AdoR. Olivine/kirschsteinite pairs in AdoR record closure temperatures around 600–650 °C.  相似文献   

11.
Abstract— The crystal structure of druse clinopyroxene from the D'Orbigny angrite, (Ca0.944 Fe2+0.042 Mg0.010Mn0.004) (Mg0.469Fe2+0.317Fe3+0.035Al0.125Cr0.010Ti0.044) (Si1.742Al0.258) O6, a = 9.7684(2), b = 8.9124(2), c = 5.2859(1) Å, β = 105.903(1)°, V = 442.58 Å3, space group C2/c, Z = 2, has been refined to an R1 index of 1.92% using single‐crystal X‐ray diffraction data. The unit formula, calculated from electron microprobe analysis, and the refined site scattering values were used to assign site populations. The distribution of Fe2+and Mg over the M1 and M2 sites suggests a closure temperature of 1000 °C. Mössbauer spectroscopy measurements were done at room temperature on a single crystal and a powdered sample. The spectra are adequately fit by a Voigt‐based quadrupole‐splitting distribution model having two generalized sites, one for Fe2+with two Gaussian components and one for Fe3+with one Gaussian component. The two ferrous components are assigned to Fe2+at the M1 site, and arise from two different next‐nearest‐neighbor configurations of Ca and Fe cations at the M2 site: (3Ca,0Fe) and (2Ca,1Fe). The Fe3+/Fetot ratio determined by Mössbauer spectroscopy is in agreement with that calculated from the electron microprobe analysis. The results are discussed in connection with the redox and thermal history of D'Orbigny.  相似文献   

12.
Abstract— A detailed analysis of the reflectance spectrum of asteroid 3628 Bo?němcová, previously identified as a possible ordinary chondrite parent body, indicates that its surface consists of an assemblage dominated by clinopyroxene and plagioclase feldspar. The clinopyroxene is Fe2+‐bearing (likely in the range Fs?10–20), with >90% of the Fe2+ being present in the M1 crystallographic site (spectral type A). The clinopyroxene:plagioclase feldspar ratio is between ?2 and 3 (?55–75% clinopyroxene, ?20–33% plagioclase feldspar). If olivine is present, the clinopyroxene:olivine ratio is >?3 (<20% olivine). The derived mineralogy of Bo?němcová is most similar, but not identical, to the known angrite meteorites. The data suggest that Bo?němcová formed by melting and differentiation of an oxidized chondritic precursor and probably represents an unsampled angrite‐like body.  相似文献   

13.
Caleta el Cobre (CeC) 022 is a Martian meteorite of the nakhlite group, showing an unbrecciated cumulate texture, composed mainly of clinopyroxene and olivine. Augite shows irregular core zoning, euhedral rims, and thin overgrowths enriched in Fe relative to the core. Low‐Ca pyroxene is found adjacent to olivine. Phenocrysts of Fe‐Ti oxides are titanomagnetite with exsolutions of ilmenite/ulvöspinel. Intercumulus material consists of both coarse plagioclase and fine‐grained mesostasis, comprising K‐feldspars, pyroxene, apatite, ilmenite, Fe‐Ti oxides, and silica. CeC 022 shows a high proportion of Martian aqueous alteration products (iddingsite) in olivine (45.1 vol% of olivine) and mesostasis. This meteorite is the youngest nakhlite with a distinct Sm/Nd crystallization age of 1.215 ± 0.067 Ga. Its ejection age of 11.8 ± 1.8 Ma is similar to other nakhlites. CeC 022 reveals contrasted cooling rates with similarities with faster cooled nakhlites, such as Northwest Africa (NWA) 817, NWA 5790, or Miller Range 03346 nakhlites: augite irregular cores, Fe‐rich overgrowths, fine‐grained K‐feldspars, quenched oxides, and high rare earth element content. CeC 022 also shares similarities with slower cooled nakhlites, including Nakhla and NWA 10153: pyroxene modal abundance, pyroxenes crystal size distribution, average pyroxene size, phenocryst mineral compositions, unzoned olivine, and abundant coarse plagioclase. Moreover, CeC 022 is the most magnetic nakhlite and represents an analog source lithology for the strong magnetization of the Martian crust. With its particular features, CeC 022 must originate from a previously unsampled sill or flow in the same volcanic system as the other nakhlites, increasing Martian sample diversity and our knowledge of nakhlites.  相似文献   

14.
Abstract— LaPaz Icefield (LAP) 02205, 02226, and 02224 are paired stones of a crystalline basaltic lunar meteorite with a low‐Ti (3.21–3.43% TiO2) low‐Al (9.93–10.45% Al2O3), and low‐K (0.11–0.12% K2O) composition. They consist mainly of zoned pyroxene and plagioclase grains, with minor ilmenite, spinel, and mesostasis regions. Large, possibly xenocrystic, forsteritic olivine grains (<3% by mode) contain small trapped multiphase melt inclusions. Accessory mineral and mesostasis composition shows that the samples have experienced residual melt crystallization with silica oversaturation and late‐stage liquid immiscibility. Our section of LAP 02224 has a vesicular fusion crust, implying that it was at one time located sufficiently close to the lunar surface environment to have accumulated solar‐wind‐implanted gases. The stones have a comparable major element composition and petrography to low‐Ti, low‐Al basalts collected at the Apollos 12 and 15 landing sites. However, the LAP stones also have an enriched REE bulk composition and are more ferroan (Mg numbers in the range of 31 to 35) than similar Apollo samples, suggesting that they represent members of a previously unsampled fractionated mare basalt suite that crystallized from a relatively evolved lunar melt.  相似文献   

15.
We report in situ LA‐ICP‐MS trace element analyses of silicate phases in olivine‐bearing chondrules in the Sahara 97096 (EH3) enstatite chondrite. Most olivine and enstatite present rare earth element (REE) patterns comparable to their counterparts in type I chondrules in ordinary chondrites. They thus likely share a similar igneous origin, likely under similar redox conditions. The mesostasis however frequently shows negative Eu and/or Yb (and more rarely Sm) anomalies, evidently out of equilibrium with olivine and enstatite. We suggest that this reflects crystallization of oldhamite during a sulfidation event, already inferred by others, during which the mesostasis was molten, where the complementary positive Eu and Yb anomalies exhibited by oldhamite would have possibly arisen due to a divalent state of these elements. Much of this igneous oldhamite would have been expelled from the chondrules, presumably by inertial acceleration or surface tension effects, and would have contributed to the high abundance of opaque nodules found outside them in EH chondrites. In two chondrules, olivine and enstatite exhibit negatively sloped REE patterns, which may be an extreme manifestation of a general phenomenon (possibly linked to near‐liquidus partitioning) underlying the overabundance of light REE observed in most chondrule silicates relative to equilibrium predictions. The silicate phases in one of these two chondrules show complementary Eu, Yb, and Sm anomalies providing direct evidence for the postulated occurrence of the divalent state for these elements at some stage in the formation reservoir of enstatite chondrites. Our work supports the idea that the peculiarities of enstatite chondrites may not require a condensation sequence at high C/O ratios as has long been believed.  相似文献   

16.
Abstract— In addition to the Mg‐, Al‐, 16O‐rich spinels that are known to occur in refractory inclusions, the Murchison meteorite contains Cr‐rich, 16O‐poor spinels, most of whose sources are unknown because they are rarely found in situ. Here we report the in situ occurrence in Allende of Cr‐rich spinels, found in 13 chondrules and 4 “olivine‐rich objects”. The Allende spinels exhibit major and minor element contents, isotopic compositions, and zoning of Cr2O3 contents like those of the Cr‐spinels from Murchison. Some chondrules contain patchy‐zoned spinel (Simon et al., 1994), which suggests that such grains did not form by sintering but perhaps by formation of overgrowths on relic grains. Unlike the olivine‐rich objects, phases in all three chondrules that were analyzed by ion microprobe have uniform, near‐normal O‐isotopic compositions. One olivine‐rich object, ALSP1, has a huge (1 mm) fragment of chevron‐zoned spinel. This spinel has near‐normal O‐isotopic compositions that are quite distinct from those of adjacent forsteritic olivine, which are relatively 16O‐rich and plot on the calcium‐aluminum‐inclusion (CAI) line, like some isolated forsterite grains found in Allende. The spinel and olivine in this object are therefore not genetically related to each other. Another olivine‐rich object, ALSP11A, contains a rectangular, 150 ×s 100 μm, homogeneous spinel grain with 50 wt% Cr2O3 and 23 wt% FeO in a vuggy aggregate of finer‐grained (5–90 μm), FeO‐rich (Fo47–55) olivine. The magnesian core of one olivine grain has a somewhat 16O‐rich isotopic composition like that of the large spinel, whereas the FeO‐rich olivine is relatively 16O‐poor. The composition of the spinel in ALSP11A plots on the CAI line, the first Cr‐rich spinel found to do so. Chevron‐zoned spinel has not been observed in chondrules, and it is unlikely that either ALSP1 or ALSP11A were ever molten. Calculations show that a spinel with the composition of that in ALSP1 can condense at 1780 K at a Ptot of 10?3 atm and a dust/gas ratio of 100 relative to solar. The Cr‐rich spinel in ALSP11A could condense at ～1420 K, but this would require a dust/gas enrichment of 1000 relative to solar. The data presented here confirm that, as in Murchison, the coarse Cr‐rich spinels in Allende are relatively 16O‐depleted and are isotopically distinct from the 16O‐enriched MgAl2O4 from CAIs. Sample ALSP11A may represent a third population, one that is Cr‐rich and plots on the CAI line. That the O‐isotopic composition of ALSP1 is like those of Cr‐rich spinels from chondrules indicates that O‐isotopic compositions cannot be used to distinguish whether grains from such unequilibrated objects are condensates or are fragments from a previous generation of chondrules.  相似文献   

17.
Northwest Africa (NWA) 10414 is an unusual shergottite with a cumulate texture. It contains 73% coarse prismatic pigeonite, plus 18% interstitial maskelynite, 2% Si‐rich mesostasis, 2% merrillite, and minor chromite‐ulvöspinel. It contains no olivine, and only ~3% augite. Phase compositions are pigeonite (En68‐43Fs27‐48Wo5‐15) and maskelynite An~54‐36, more sodic than most maskelynite in shergottites. Chromite‐ulvöspinel composition plots between the earliest and most fractionated spinel‐group minerals in olivine‐phyric shergottites. NWA 10414 mineralogically resembles the contact facies between Elephant Moraine 79001 lithologic units A and B, with abundant pigeonite phenocrysts, though it is coarser grained. Its most Mg‐rich pigeonite also has a similar composition to the earliest crystallized pyroxenes in several other shergottites, including Shergotty. The Shergotty intercumulus liquid composition crystallizes pigeonite with a similar composition range to NWA 10414 pigeonite, using PETROLOG. Olivine‐phyric shergottite NWA 6234, with a pure magma composition, produces an even better match to this pigeonite composition range, after olivine crystallization. These observations suggest that after the accumulation of olivine from an olivine‐phyric shergottite magma, the daughter liquid could precipitate pigeonite locally to form this pigeonite cumulate, before the crystallization of overlying liquid as a normal basaltic shergottite.  相似文献   

18.
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   

19.
Northwest Africa (NWA) 7397 is a newly discovered, enriched, lherzolitic shergottite, the third described example of this group. This meteorite consists of two distinct textural lithologies (1) poikilitic—comprised of zoned pyroxene oikocrysts, with chadacrysts of chromite and olivine, and (2) nonpoikilitic—comprised of olivine, low‐Ca and high‐Ca pyroxene, maskelynite, and minor abundances of merrillite, spinel, ilmenite, and pyrrhotite. The constant Ti/Al ratios of pyroxene oikocrysts suggests initial crystallization of the poikilitic lithology at depth (equivalent to pressures of approximately 10 kbar), followed by crystallization of the nonpoikilitic lithology at shallower levels. Oxygen fugacity conditions become more oxidizing during crystallization ranging from fO2 conditions of approximately QFM‐2 to QFM‐0.7. Magma calculated to be in equilibrium with the major rock‐forming minerals is LREE‐enriched relative to depleted or intermediate shergottites and has flat overall profiles. Therefore, we suggest that the parental magma for NWA 7397 had sampled an enriched, oxidized, Martian geochemical source, similar to that of other enriched basaltic and olivine‐phyric shergottites. We present a polybaric formation model for the lherzolitic shergottite NWA 7397, to account for the petrologic constraints. Three successive stages in the development of NWA 7397 are discussed (1) formation of a REE‐enriched parental magma from a distinct Martian mantle reservoir; (2) magma ponding and development of a staging chamber concomitant with initial crystallization of the poikilitic lithology; and (3) magma ascent to the near surface, with entrainment of cumulates from the staging chamber and subsequent crystallization of the nonpoikilitic lithology en route to the surface.  相似文献   

20.
Abstract— Anorthite‐rich chondrules in CR and CH carbonaceous chondrites consist of magnesian low‐Ca pyroxene and forsterite phenocrysts, FeNi‐metal nodules, interstitial anorthite, Al‐Ti‐Cr‐rich low‐Ca and high‐Ca pyroxenes, and crystalline mesostasis composed of silica, anorthite and high‐Ca pyroxene. Three anorthite‐rich chondrules contain relic calcium‐aluminum‐rich inclusions (CAIs) composed of anorthite, spinel, ±Al‐diopside, and ± forsterite. A few chondrules contain regions which are texturally and mineralogically similar to magnesian (type I) chondrules and consist of forsterite, low‐Ca pyroxene and abundant FeNi‐metal nodules. Anorthite‐rich chondrules in CR and CH chondrites are mineralogically similar to those in CV and CO carbonaceous chondrites, but contain no secondary nepheline, sodalite or ferrosilite. Relatively high abundances of moderately‐volatile elements such as Cr, Mn and Si in the anorthite‐rich chondrules suggest that these chondrules could not have been produced by volatilization of the ferromagnesian chondrule precursors or by melting of the refractory materials only. We infer instead that anorthite‐rich chondrules in carbonaceous chondrites formed by melting of the reduced chondrule precursors (olivine, pyroxenes, FeNi‐metal) mixed with the refractory materials, including relic CAIs, composed of anorthite, spinel, high‐Ca pyroxene and forsterite. The observed mineralogical and textural similarities of the anorthite‐rich chondrules in several carbonaceous chondrite groups (CV, CO, CH, CR) may indicate that these chondrules formed in the region(s) intermediate between the regions where CAIs and ferromagnesian chondrules originated. This may explain the relative enrichment of anorthite‐rich chondrules in 16O compared to typical ferromagnesian chondrules (Russell et al., 2000).  相似文献   

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