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1.
Abstract— The Nova 001 [= Nuevo Mercurio (b)] and Nullarbor 010 meteorites are ureilites, both of which contain euhedral graphite crystals. The bulk of the meteorites are olivine (Fo79) and pyroxenes (Wo9En73Fs18, Wo3En77Fs20), with a few percent graphite and minor amounts of troilite, Ni-Fe metal, and possibly diamond. The rims of olivine grains are reduced (to Fo91) and contain abundant blebs of Fe metal. Silicate mineral grains are equant, anhedral, up to 2 mm across, and lack obvious preferred orientations. Euhedral graphite crystals (to 1 mm x 0.3 mm) are present at silicate grain boundaries, along boundaries and protruding into the silicates, and entirely within silicate mineral grains. Graphite euhedra are also present as radiating clusters and groups of parallel plates grains embedded in olivine; no other ureilite has comparable graphite textures. Minute lumps within graphite grains are possibly diamond, inferred to be a result of shock. Other shock effects are limited to undulatory extinction and fracturing. Both ureilites have been weathered significantly. Considering their similar mineralogies, identical mineral compositions, and identical unusual textures, Nova 001 and Nullarbor 010 are probably paired. Based on olivine compositions, Nova 001 and Nullarbor 010 are in Group 1 (FeO-rich) of Berkley et al. (1980). Silicate mineral compositions are consistent with those of other known ureilites. The presence of euhedral graphite crystals within the silicate minerals is consistent with an igneous origin, and suggests that large proportions of silicate magma were present locally and crystallized in situ.  相似文献   

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

3.
Abstract— A petrologic and TEM study of a remarkable dark inclusion (DI) in the Ningqiang CV3 chondrite reveals that it is a mixture of highly primitive solar nebula materials. The DI contains two lithologies. The first, lithology A, contains micron‐sized olivine and pyroxene grains rimmed by amorphous materials with compositions similar to the underlying crystalline grains. The second, lithology B, appears to preserve the mineralogy of lithology A before formation of the amorphous rims. Overall, the Ningqiang DI appears to record the following processes: 1) formation (condensation and Fe‐enrichment) of olivine crystals in the nebula with compositions of Fo42–62; 2) irradiation, resulting in amorphitization of the olivine and pyroxene to varying degrees; 3) partial annealing, resulting in formation of fairly large, euhedral olivine and pyroxene grains with remnant amorphous sharply‐bounded rims; 4) in some cases, prolonged annealing, resulting in the formation of microcrystalline olivine or pyroxene rims. The latter annealing would have been a natural consequence of irradiation near the critical temperature for olivine; and 5) mixture of the above materials (lithology A) with nebular condensate high‐Ca pyroxene and olivine, which escaped nebular processing, to become lithology B. We suggest that the amorphous rims in lithology A formed in an energetic solar event such as a bi‐polar outflow or FU‐orionis flare.  相似文献   

4.
Haverö consists of large olivine areas with a pavement structure and single crystals of twinned clinopyroxene. Black veins with sharp boundaries traverse the silicates. They contain graphite, diamond, and kamacite. In olivine a reaction rim is formed around these veins containing Ni-poor metal and showing a lower FeO content than farther away from the vein. The CaO content of olivine and pyroxene, 0.27% and 1.7%, respectively, are higher than in these minerals in normal chondrites. The mole percent Fe + Ca/Fe + Ca + Mg in unchanged olivine and in pyroxene agree with the range of L-chondrites. Metal occurs in three types: a, larger grains in the course of the black veins, they contain 2 to 3% Ni; b, micron-sized grains inside the black veins and its reaction rim; c, medium-sized grains with ～0.7% Ni in olivine The interpretation of these observations is: a material similar to an L-chondrite was reheated and recrystallized (at this time it may have lost its feldspar, metal and troilite by partial melting), Ca was redistributed and partly retained in olivine and pyroxene due to rapid cooling, a late introduction of carbon into veins caused a partial reduction of FeO in olivine and formation of Ni-poor metal  相似文献   

5.
Baszkówka is an equilibrated, apparently low‐shock, unusually porous chondrite. Some earlier studies were undertaken to understand whether the porosity in Baszkówka, and similar porous chondrites, is a relic of a primordial feature or rather the effect of atypical reprocessing on the parent body. Neither of the studies reconstructed the accurate thermal and deformational evolution of chondrites, however, while it is known that shock‐induced compaction is the main means to affect chondritic porosity. Here we use a combination of 3‐D and 2‐D petrographic examination to understand how the evolution of pores correlates with thermal and shock history recorded in the Baszkówka chondrite. The grain framework silicates in Baszkówka contain healed shock fractures—a clear recorder of significant shock process and postshock annealing. Simultaneously, metal grains do not exhibit any preferred orientation or fabric, which would be expected to develop in response to the deformation as recorded by silicates. We interpret this as evidence for re‐agglomeration and annealing of shocked material. Pore spaces in Baszkówka are connected and decorated by fine‐grained plagioclase‐dominated mass and bulky euhedral olivine crystals, which exhibit growth steps on crystal surfaces. The euhedral olivine must have formed owing to the condensation of a vapor, while plagioclase most likely crystallized from melted chondritic matrix. During the shock event, fine‐grained matrix in Baszkówka was melted and vaporized. Vapor expansion added to ballistic velocity led to ejection and opening of the pore spaces. After re‐agglomeration in a hot ejecta blanket the rock was annealed, melted material circulated in created pore spaces and vapor condensed.  相似文献   

6.
Several aubrites (e.g., LAP 03719, Bishopville, Khor Temiki, ALH 83015) contain orthopyroxene grains that exhibit more‐pronounced shock effects than associated olivine grains. The orthopyroxene grains in these samples have clinoenstatite lamellae on (100) and exhibit weak mosaic extinction, characteristic of shock stage S4; the olivine grains exhibit either sharp optical extinction, characteristic of shock stage S1 (as in LAP 03719), or undulose extinction (shock stage S2), as in Bishopville and ALH 83015. The Khor Temiki regolith breccia contains S1 and S2 olivine grains. Because literature data show that diffusion is much slower in orthopyroxene than in olivine, it seems likely that aubrites experienced postshock, impact‐induced annealing. After differentiation, the aubrite parent asteroid suffered major collisions that caused extensive brecciation of near‐surface materials and damaged orthopyroxene and olivine crystal lattices. As a result of these impact events, some aubrites were shocked and buried within warm ejecta blankets or beneath fallback debris under the crater floor. Entombed olivine crystal lattices healed (and became unstrained, reaching shock stage S1), but orthopyroxene lattices retained their S4‐level shock‐damaged features. Aubrites with S4 orthopyroxene and S2 olivine were probably very weakly shocked again after olivine was annealed to S1.  相似文献   

7.
We found a large (~2 mm) compound object in the primitive Yamato 793408 (H3.2‐an) chondrite. It consists mostly of microcrystalline material, similar to chondrule mesostasis, that hosts an intact barred olivine (BO) chondrule. The object contains euhedral pyroxene and large individual olivine grains. Some olivine cores are indicative of refractory forsterites with very low Fe‐ and high Ca, Al‐concentrations, although no 16O enrichment. The entire object is most likely a new and unique type, as no similar compound object has been described so far. We propose that it represents an intermediate stage between compound chondrules and macrochondrules, and formed from the collision between chondrules at low velocities (below 1 m s?1) at high temperatures (around 1550 °C). The macrochondrule also trapped and preserved a smaller BO chondrule. This object appears to be the first direct evidence for a genetic link between compound chondrules and macrochondrules. In accordance with previous suggestions and studies, compound chondrules and macrochondrules likely formed by the same mechanism of chondrule collisions, and each represents different formation conditions, such as ambient temperature and collision speed.  相似文献   

8.
Abstract— The Nakhla meteorite, commonly accepted to have originated from Mars, is a cumulus clinopyroxenite with ～10 vol% of Fe‐rich olivine. Almost all olivine grains in Nakhla contain dark lamellar inclusions (less than 2–3 μm wide). High‐resolution scanning and transmission electron microscopy revealed that the inclusions are complex intergrowths of augite and magnetite. Such a symplectic intergrowth of augite and magnetite in olivine was known in some terrestrial rocks, lunar rocks, and a few meteorites. The inclusion in Nakhla olivine is the first symplectite found in a martian rock. Apparently, the presence of Fe3+ in olivine under an oxidizing condition on Mars caused symplectic exsolution at high temperature (>900 °C) during cooling.  相似文献   

9.
The mechanism by which olivine grains became embedded within iron‐nickel alloy in pallasite meteorites continues to be a matter of scientific debate. Geochemical and textural observations have failed to fully elucidate the origin and history of the olivine crystals; however, little research attention has been devoted to their crystallographic orientations within the metal matrix. Using electron backscatter diffraction, we have collected crystallographic orientation data for 296 crystals within ～65 cm2 sample surface from Springwater. Though no global crystallographic preferred orientation exists, very low misorientations are observed among [100] axes of olivine crystals within specific texturally defined domains. Combined with a thorough characterization of large‐scale Springwater textures, the definitively nonrandom spatial distribution of olivine orientations provides clues regarding the nature of the olivine's initial formation environment as well as the sequence of events subsequent to metal incorporation.  相似文献   

10.
Here we report in situ structural and chemical analyses of four presolar grains and the matrices of the Meteorite Hills (MET) 00526 L3.05 and Queen Alexandra Range (QUE) 97008 L3.05 unequilibrated ordinary chondrites (UOCs). The presolar grains in MET 00526 include one Fe-rich single crystal olivine, and one olivine grain that contains both amorphous and polycrystalline material. The single crystal olivine likely has origins in the circumstellar envelope (CSE) of a red giant branch (RGB) or asymptotic giant branch (AGB) star, and the amorphous/polycrystalline olivine has an O-isotopic composition consistent with origins in a type II supernova. The presolar grains from QUE 97008 are Fe rich and include one crystalline, stoichiometric olivine that contains a Ca-rich core and one crystalline, stoichiometric pyroxene grain, both of which have O-isotopic compositions consistent with origins in the CSEs of low-mass AGB/RGB stars. The matrices of both UOCs are mineralogically diverse with evidence for unaltered material in the form of amorphous silicates and a C-rich nanoglobule and altered material in the form of Ni-rich sulfides, abundant Fe-rich olivine, and Fe-Mg zoning in matrix silicates. No phyllosilicates were observed. The Fe-rich olivine grains are the dominant alteration phase in both UOCs and likely replaced primary amorphous silicates in the presence of an Fe-rich fluid during parent body alteration. Our work suggests that the ordinary and carbonaceous chondrites received a similar inventory of dust with comparable structures and chemistries.  相似文献   

11.
Abstract— The Kobe CK4 chondrite, like most metamorphosed CK chondrites, exhibits pronounced silicate darkening of matrix and chondrule mesostases. Our petrographic and scanning electron microscopic study reveals that the matrix of Kobe consists mostly of intermixtures of two types of fine‐grained olivine. One forms subhedral to anhedral normal crystals. The other fills interstices of the subhedral to anhedral olivine crystals, exhibiting a complex network of veinlets. The latter type of olivine contains high densities of small spherical vesicles (<0.05‐3 μm in diameter) and grains (<0.05‐5 μm) of magnetite and pentlandite as well as round to anhedral grains (1–10 μm) of plagioclase, low‐Ca pyroxene, diopside and chlorapatite. The vesicular olivine is particularly abundant in regions of matrix that exhibit a relatively high degree of darkening and commonly fills chondrule mesostases. The vesicular olivine is clearly the principal cause of the silicate darkening in Kobe. The internal texture of the vesicular olivine closely resembles those of local melts produced from the matrices of experimentally and naturally shocked carbonaceous chondrites. The occurrence and texture of the vesicular olivine suggest that it resulted from recrystallization of partially melted matrix olivine by shock. Kobe exhibits light shock effects in olivine that are consistent with shock stage S2 that is too low to explain the occurrence of olivine melting. We suggest that the vesicular olivine in Kobe was produced by shock metamorphism at a relatively mild shock pressure (<25 GPa) and a high temperature (>600 °C). Thus, it is probable that the shock effects in olivine, manifest as fracturing and deformation, were relatively minor, but heating was strong enough to cause partial melting of matrix olivine.  相似文献   

12.
Abstract— The unusual composition of the nakhlites, a group of pyroxenitic martian meteorites with young ages, presents an opportunity to learn about nonbasaltic magmatic activity on another planet. However, the limited number of these meteorites makes unraveling their history difficult. A promising terrestrial analog for the formation of the nakhlites is Theo's Flow in Ontario, Canada. This atypical, 120 m-thick flow differentiated in place, forming distinct layered lithologies of peridotite, pyroxenite, and gabbro. Theo's pyroxenite and the nakhlites share strikingly similar petrographies, with concentrated euhedral to subhedral augite grains set in a plagioclase-rich matrix. These two suites of rocks also share specific petrologic features, mineral and whole-rock compositional features, and size and spatial distributions of cumulus grains. The numerous similarities suggest that the nakhlites formed by a similar mechanism in a surface lava flow or shallow intrusion. Their formation could have involved settling of crystals in a phenocryst-laden flow or in situ nucleation and growth of pyroxenes in an ultramafic lava flow. The latter case is more likely and requires steady-state nucleation and growth of clusters of pyroxene grains (and olivine in the nakhlites), circulating in a strongly convecting melt pool, followed by settling and continued growth in a thickening cumulate pile. Trapped pockets of intercumulus liquid in the pile gradually evolved, finally growing Fe-enriched rims on cumulus grains. With sufficient evolution, the melt reached plagioclase supersaturation, causing rapid growth of plagioclase sprays and late-stage mesostasis growth.  相似文献   

13.
Comets and the chondritic porous interplanetary dust particles (CP IDPs) that they shed in their comae are reservoirs of primitive solar nebula materials. The high porosity and fragility of cometary grains and CP IDPs, and anomalously high deuterium contents of highly fragile, pyroxene-rich Cluster IDPs imply these aggregate particles contain significant abundances of grains from the interstellar medium (ISM). IR spectra of comets (3–40 μm) reveal the presence of a warm (near-IR) featureless emission modeled by amorphous carbon grains. Broad andnarrow resonances near 10 and 20 microns are modeled by warm chondritic (50% Feand 50% Mg) amorphous silicates and cooler Mg-rich crystalline silicate minerals, respectively. Cometary amorphous silicates resonances are well matched by IRspectra of CP IDPs dominated by GEMS (0.1 μm silicate spherules) that are thought to be the interstellar Fe-bearing amorphous silicates produced in AGB stars. Acid-etched ultramicrotomed CP IDP samples, however, show that both the carbon phase (amorphous and aliphatic) and the Mg-rich amorphous silicate phase in GEMS are not optically absorbing. Rather, it is Fe and FeS nanoparticles embedded in the GEMS that makes the CP IDPs dark. Therefore, CP IDPs suggest significant processing has occurred in the ISM. ISM processing probably includes in He+ ion bombardment in supernovae shocks. Laboratory experiments show He+ ion bombardment amorphizes crystalline silicates, increases porosity, and reduces Fe into nanoparticles. Cometary crystalline silicate resonances are well matched by IR spectra of laboratory submicron Mg-rich olivine crystals and pyroxene crystals. Discovery of a Mg-pure olivine crystal in a Cluster IDP with isotopically anomalous oxygen indicates that a small fraction of crystalline silicates may have survived their journey from AGB stars through the ISM to the early solar nebula. The ISM does not have enough crystalline silicates (<5%), however, to account for the deduced abundance of crystalline silicates in comet dust. An insufficient source of ISMMg-rich crystals leads to the inference that most Mg-rich crystals in comets are primitive grains processed in the early solar nebula prior to their incorporation into comets. Mg-rich crystals may condense in the hot (～1450 K), inner zones of the early solar nebula and then travel large radial distances out to the comet-forming zone. On the other hand, Mg-rich silicate crystals may be ISM amorphous silicates annealed at ～1000 K and radially distributed out to the comet-forming zone or annealed in nebular shocks at ～5-10 AU. Determining the relative abundance of amorphous and crystalline silicatesin comets probes the relative contributions of ISM grains and primitive grains to small, icy bodies in the solar system. The life cycle of dust from its stardust origins through the ISM to its incorporation into comets is discussed.  相似文献   

14.
The Adams County, Colorado, H5 chondrite contains a lithic fragment, 1 cm in size, that is texturally and mineralogically quite different from the chondritic host. It is composed of: a groundmass of fine-grained euhedral to subhedral olivine (3–15 μm) and interstitial glass enclosing larger olivine and pyroxene grains (0.15-0.5 mm; about 15 vol %); an assemblage of enstatite grains (subfragment within) and an assemblage of olivine plus orthopyroxene (a second subfragment); and about 11 vol % grains of mixed troilite and nickel-iron metal. Analyses yielded these results: (i) olivine grains of the fragment groundmass have a compositional range (Fa12–45) and most grains contain substantial CaO and Cr2O3 (～ 0.20 and 0.30 avg. wt%, respectively); interstitial glass has ～ 55 wt% SiO2; (ii) larger olivine grains of the fragment are similarly high in CaO and Cr2O3 and also have a wide FeO/MgO range; one unusual pyroxene is an Mg-rich pigeonite; (iii) the metal is martensite in composition (11–14 wt% Ni); and (iv) major and trace element analyses by INAA indicate an H-group bulk composition for the entire 1 cm lithic fragment. On the basis of its texture and bulk and mineral compositions, the fragment is interpreted to represent unequilibrated H-group material that was partly melted by impact. The Ca- and Cr-enriched groundmass olivine and interstitial glass resulted from rapid crystallization of the chondritic melt. The Ca- and Cr-enriched larger silicate grains, including the enstatite sub-fragment and the pigeonite grain, are residual, unmelted clasts from the target material (this is supported by the presence of similar material in actual H3 chondrites). Further impact brecciation of the clast-laden melt material, and resultant impact-splashing accounts for the presence of the fragment in the H-group Adams County host and documents the coexistence of unequilibrated and equilibrated H-group material as surface regolith on one parent body.  相似文献   

15.
Abstract— We have used the Manchester ISOLAB 54 ion microprobe to make in situ measurements of the 17O/16O and 18O/16O ratios of olivine grains in the Julesburg (L3.6) and Allende (CV3) chondrites. We have discovered a population of olivines in Julesburg characterised by (1) the most 16O-rich compositions yet reported for olivine from an ordinary chondrite; (2) cores of low-Fa olivine, which frequently shows blue cathodoluminesce; (3) thick coats of more Fa-rich (Fa ～20) olivine, which is also 16O-enriched. In an O isotopic plot, the Julesburg 16O-rich grains form a roughly linear array that is offset from the Allende mixing line. The presence of very low Fa olivine and, sometimes, well-defined Fa-rich coats indicates that these grains experienced significantly less thermal metamorphism than most of the olivine in the meteorite. Some 16O-rich Julesburg grains are associated with minor feldspar or pyroxene and are probably chondrule fragments. They are isotopically indistinguishable from forsterite in Allende; however, Allende forsterite grains do not have the thick Fa20 coats typical of those in Julesburg. These 16O-rich forsterite grains appear to be related to the “blue olivine” of Steele (1986). Both cores and coats of 16O-rich grains in Julesburg are isotopically distinct from olivine in Semarkona group A and group B chondrules.  相似文献   

16.
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17.
Abstract– The nakhlites, a subgroup of eight clinopyroxenites thought to come from a single geological unit at the Martian surface, show melt inclusions in augite and olivine. In contrast to olivine‐hosted melt inclusions, augite‐hosted melt inclusions are not surrounded by fractures, and are thus considered preferential candidates for reconstructing parent liquid compositions. Furthermore, two types of augite‐hosted melt inclusion have been defined and characterized in four different nakhlites (Northwest Africa [NWA] 817, Nakhla, Governador Valadares, and NWA 998): Type‐I isolated inclusions in augite cores that contain euhedral to subhedral augite, Ti‐magnetite, and pigeonite plus silica‐rich glass and a gas bubble; Type‐II microinclusions that form trails crosscutting host augite crystals. Fast‐heating experiments were performed on selected pristine primary augite‐hosted melt inclusions from these four samples. Of these, only data from Nakhla were considered robust for reconstruction of a nakhlite parental magma composition (NPM). Based upon careful petrographic selection and consideration of iron‐magnesium ratios, our data are used to propose an NPM, which is basaltic (49.1 wt% SiO2), of high Ca/Al (1.95), and K2O‐poor (0.32 wt%). Thermodynamic modeling at an oxygen fugacity one log unit below the QFM buffer using the MELTS and PETROLOG programs implies that Mg‐rich olivine was not a liquidus phase for this composition. Our analysis is used to suggest that olivine megacrysts found in the nakhlites are unlikely to have coprecipitated with augite, and thus may have been introduced during or subsequent to accumulation in the magma chamber, possibly from more evolved portions of the same chamber.  相似文献   

18.
By means of nanoscale surface observation, we have proposed a new approach for investigating fine crystals of cosmic materials to reveal their origin and growth conditions. Several different morphologies of polyhedral fine olivines with faceted faces have been found in Allende carbonaceous chondrite (4.5 byr in geochronological age). In the present work, molecular level topography of the faceted matrix olivine by Atomic Force Microscopy (AFM) has successfully been performed. The matrix olivine found to have preserved growth step pattern on its surface even though quite long time has passed since they formed in the early Solar System. The surface pattern suggests that the faceted matrix olivine could have been condensed from the gas phase, and possibly that these olivine crystals had continued to grow under a rapid cooling condition (0.1-1 K s−1). The estimated cooling rate agrees well with predictions based on hypothetical rapid heating and cooling events such as shock wave heating.  相似文献   

19.
Abstract— A “carbonaceous chondrite” clast from Bencubbin was studied by analytical transmission electron microscopy and other electron beam techniques. In section, the clast consists of oval augen, with a preferred orientation, set in fine-grained matrix. The augen comprise olivine microphenocrysts in fine-grained to glassy mesostases. The olivines are heavily deformed, giving rise to mosaicism. Many sub-grains have high densities of dislocations with [001] Burgers vectors. In some regions the dislocation configurations are recovered, causing a reduced dislocation density; recrystallization is rarer. Severe cataclasis is absent; there are few open fractures and little intergranular porosity except where a second phase occurs. Such porosity in olivine mainly occurs as finescale negative crystals, which appear to be healed cracks. Some mesostases consist of small amounts of microporous oxides and feldspathic glass but glassy veins are absent. Pyrhottite and Fe/Ni sulfides are major mesostasis constituents that rarely form tongues between the olivines, which commonly include smaller sulfide blebs. The matrix of the clast has abundant sulfides and fine-grained, poorly crystalline Fe- and Fe/Ni-oxides, with more sparse ferrihydrite. The oxides mostly occur in contact with, or within, aluminous and siliceous glassy material in which crystals of melilite, spinel, Ca-pyroxene, feldspar, and other minor silicate phases have grown. Small patches of fibrous and/or sheet-like Mg/Fe silicates with layer morphologies also occur. They are microporous, poorly crystalline and lack the layer spacings of phyllosilicates, of which they may be relics. Grains of anhydrite and calcite are interlaced with fibrous silicates. There is evidence that one, or more, intense shock-heating event(s) produced local melting. A later shock event(s) involved less severe shock-heating, to about 900 °C. The mineralogy indicates that the clast may have originated from a CM2 precursor or from material like Allan Hills 85085, although it is just possible that terrestrial weathering produced some of the diagnostic minerals.  相似文献   

20.
Petrographic examination of the xenolith and xenocryst populations in the olivine‐phyric shergottite Elephant Moraine 79001 Lithology A shows more chemical heterogeneity than previously documented. Analyses of olivine grains in 18 megacrysts and in 4 lithic fragments show that these two populations either do not have the same source or that this source is heterogeneous in terms of its time–temperature history. Additionally, among the four lithic fragments analyzed, two are distinctive (1) one contains a major‐element‐equilibrated, euhedral olivine grain and (2) the second contains high‐magnesium pyroxene cores. Furthermore, two populations of ferroan olivine were identified (1) one more ferroan than any other reported in EET 79001 and (2) a slightly less ferroan, unequilibrated olivine, but with a restricted range in Mg#. We have also observed an equilibrated pyroxene grain associated with a zoned olivine megacryst. As a result, we recognize that the xenolith/xenocryst population does not represent the incorporation of a single xeno‐lithology into Lithology A, and propose that it be subdivided into a suite of seven identified lithologies, with the understanding that more are likely to be identified with further study. The abundance of Ca in olivine in the xeno‐lithologies suggests a set of crustal, rather than deep mantle, lithologies. Diffusion rates in olivine suggest that the lithologies were incorporated shortly before rapid cooling of the host magma, preserving preexisting mineral chemical zoning. These mineral chemical zones could have been preserved at lower crustal temperatures for up to 10s of Ka. Trace‐element studies of these distinct populations would be required to test whether they are related by igneous processes from a common source magma.  相似文献   

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