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41.
Abstract— The Ilafegh 009 meteorite is an impact melt rock from an EL-chondritic parent body. Its mineralogic assemblage is the result of rapid crystallization after shock-induced melting. We report here an analytical transmission electron microscopy (ATEM) study of the major minerals of this meteorite (enstatite, plagioclase, Fe-Ni metal and sulfides). Based on this study, we discuss the crystallization sequence and the further evolution of the rock in the solid state. Microstructure and microanalyses confirm that the mineralogy of Ilafegh 009 results from the crystallization of an EL-chondritic melt. The high compositional variability of plagioclases and the presence of silica-rich glass pockets indicate fast cooling. During crystallization, the large enstatite grains trapped a large number of phases (plagioclase, silica-rich glass and enstatite nuclei). Sulfides (troilite, alabandite and daubreelite) form finely polycrystalline areas and reveal a complex crystallization sequence. Although Fe-Ni metal grains formed during rapid cooling, their microstructures show that some postsolidification process occurred in Ilafegh 009. A large number of tiny Ni-P-Si-rich precipitates were detected that formed as a result of exsolution of elements that become insoluble in kamacite at low temperature. Finally, the microstructure (dislocation arrangements and phase transformations) observed in enstatite and Fe-Ni metal attests that Ilafegh 009 also experienced a moderate postsolidification shock event. 相似文献
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Addi Bischoff Markus Patzek Tommaso Di Rocco Andreas Pack Aleksandra Stojic Jasper Berndt Stefan Peters 《Meteoritics & planetary science》2023,58(10):1385-1398
On February 13, 2023, a huge fireball was visible over Western Europe (fireball event 2023 CX1). After the possible strewn field was calculated, the first of several recovered samples, with a mass of about 100 g, was discovered just 2 days after the fireball event on the ground of the village of Saint-Pierre-le-Viger. Meanwhile, more than 60 samples with a total mass of more than 1 kg were recovered and a piece of one of these is studied here. The fall occurred 220 years after the historic meteorite fall of L'Aigle on April 26, 1803, <120 km south. L'Aigle is the closest meteorite fall to Saint-Pierre-le-Viger and belongs to the same chondrite group. Both meteorites are breccias containing only clasts of high metamorphic degree (type 5 and type 6). Since only 20% of the L chondrites are breccias this coincidence is remarkable. As just mentioned, both samples studied from these rocks in this work are ordinary chondrite breccias and consist of equilibrated and recrystallized lithologies of petrologic type 6. The brecciated texture in L'Aigle, resulting in a remarkable light–dark structure, is more pronounced than the brecciated features in Saint-Pierre-le-Viger, from which also type 5 fragments have been reported. The compositions of low-Ca pyroxene and olivine grains in Saint-Pierre-le-Viger (Fs21.2 and Fa23.4, respectively) clearly require an L-group classification. L'Aigle was classified as an L6 breccia in the past, and this has now been confirmed by new data on low-Ca pyroxene and olivine (Fs20.7 and Fa23.8, respectively). Saint-Pierre-le-Viger contains local thin shock veins, and both meteorites are moderately shocked. Most olivines in the studied samples have planar fractures, but the estimated abundance of mosaicized olivines of 30%–40% among the large grains require a S4 shock classification. Oxygen isotope and bulk chemical data of Saint-Pierre-le-Viger certainly support the L chondrite classification. Bulk spectral data of Saint-Pierre-le-Viger are dominated by silicate minerals, that is, Fe-bearing low-Ca pyroxene, olivine, and plagioclase. Isotopic, chemical, and spectral data of the L'Aigle meteorite are shown for comparison and are very similar, providing additional circumstantial evidence of Saint-Pierre-le-Viger's L chondritic nature. 相似文献
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Addi Bischoff Mike Komnik Jakob Storz Jasper Berndt 《Meteoritics & planetary science》2023,58(9):1260-1273
The lunar regolith breccia Dhofar 1769, which was found in 2012 as a single 125 g piece in the Zufar desert area of Oman, contains a relatively large, dark-colored impact melt breccia embedded in a fine-grained clastic matrix. The internal texture of the fragment indicates the repeated melt breccia formation on the lunar surface, their repeated brecciation, and mixing in second, third, and fourth generations of brecciated rock types. The chemical and mineralogical data reveal the incorporation of a feldspar-rich subophitic crystalline melt within a feldspar-rich microporphyritic crystalline melt breccia. This lithic paragenesis itself is embedded within a mafic, crystalline melt breccia. The entire breccia with the three different impact melts has been finally incorporated into the whole rock breccia. The three impact melts are mixtures of different source rocks and impact projectiles, based on the obtained minor and trace element compositions (in particular of Ni and the rare earth elements [REE]) of the impact melt lithologies. For all processes of impact melt formation, additional steps of their brecciation and re-lithification require a minimum number of seven impact processes. 相似文献
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On October 7, 2008, a small asteroid named 2008 TC3 was detected in space about 19 h prior to its impact on Earth. Numerous world-wide observations of the object while still in space allowed a very precise determination of its impact area: the Nubian Desert of northern Sudan, Africa. The asteroid had a pre-atmospheric diameter of ∼4 m; its weight is reported with values between ∼8 and 83 t, and the bulk density with ∼2–3 g/cm3, translating into a bulk porosity in the range of ∼20–50%. Several dedicated field campaigns in the predicted strewn field resulted in the recovery of more than 700 (monolithological) meteorite fragments with a total weight of ∼10.5 kg. These meteorites were collectively named “Almahata Sitta”, after the nearby train station 6, and initially classified as an anomalous polymict ureilite. Further work, however, showed that Almahata Sitta is not only a ureilite but a complex polymict breccia containing chemically and texturally highly variable meteorite fragments, including different ureilites, a ureilite-related andesite, metal-sulfide assemblages related to ureilites, and various chondrite classes (enstatite, ordinary, carbonaceous, Rumuruti-like). It was shown that that chondrites and ureilites derive from one parent body, i.e., asteroid 2008 TC3, making this object, in combination with the remotely sensed physical parameters, a loosely aggregated, rubble-pile-like object. Detailed examinations have been conducted and mineral-chemical data for 110 samples have been collected, but more work on the remaining samples is mandatory. 相似文献
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Emilie JANOTS Edwin GNOS Beda A. HOFMANN Richard C. GREENWOOD Ian A. FRANCHI Addi BISCHOFF 《Meteoritics & planetary science》2011,46(1):134-148
The Jiddat al Harasis (JaH) 422 ureilite was found in the Sultanate of Oman; it is classified as a ureilitic impact melt breccia. The meteorite consists of rounded polycrystalline olivine clasts (35%), pores (8%), and microcrystalline matrix (57%). Clasts and matrix have oxygen isotopic values and chemical compositions (major and trace elements) characteristic of the ureilite group. The matrix contains olivine (Fo83–90), low‐Ca pyroxene (En84–92Wo0–5), augite (En71–56Wo20–31), graphite, diamond, Fe‐metal, sulfides, chromite, and felsic glass. Pores are partly filled by secondary Fe‐oxihydroxide and desert alteration products. Pores are surrounded by strongly reduced silicates. Clasts consist of fine‐grained aggregates of polygonal olivine. These clasts have an approximately 250 μm wide reaction rim, in which olivine composition evolves progressively from the core composition (Fo79–81) to the matrix composition (Fo84–87). Veins crossing the clasts comprise pyroxene, Fe‐oxihydroxide, C‐phases, and chromite. Clasts contain Ca‐, Al‐, and Cr‐rich glass along olivine grain boundaries (<1 μm wide). We suggest that a significant portion of JaH 422, including olivine and all the pyroxenes, was molten as a result of an impact. In comparison with other impact‐melted ureilites, JaH 422 shows the highest melt portion. Based on textural and compositional considerations, clasts and matrix probably originated from the same protolith, with the clasts representing relict olivine that survived, but was recrystallized in the impact melt. During the melt stage, the high availability of FeO and elevated temperatures controlled oxygen fugacity at values high enough to stabilize olivine with Fo～83–87 and chromite. Along pores, high Mg# compositions of silicates indicate that in a late stage or after melt crystallization FeO became less available and fO2 conditions were controlled by C?CO + CO2. 相似文献
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Knut METZLER Addi BISCHOFF Richard C. GREENWOOD Herbert PALME Marko GELLISSEN Jens HOPP Ian A. FRANCHI Mario TRIELOFF 《Meteoritics & planetary science》2011,46(5):652-680
Abstract– Northwest Africa (NWA) 869 consists of thousands of individual stones with an estimated total weight of about 7 metric tons. It is an L3–6 chondrite and probably represents the largest sample of the rare regolith breccias from the L–chondrite asteroid. It contains unequilibrated and equilibrated chondrite clasts, some of which display shock‐darkening. Impact melt rocks (IMRs), both clast‐free and clast‐poor, are strongly depleted in Fe,Ni metal, and sulfides. An unequilibrated microbreccia, two different light inclusions and two different SiO2‐bearing objects were found. Although the matrix of this breccia appears partly clastic, it is not a simple mixture of fine‐grained debris formed from the above lithologies, but mainly represents an additional specific lithology of low petrologic type. We speculate that this material stems from a region of the parent body that was only weakly consolidated. One IMR clast and one SiO2‐bearing object show Δ17O values similar to bulk NWA 869, suggesting that both are related to the host rock. In contrast, one light inclusion and one IMR clast appear to be unrelated to NWA 869, suggesting that the IMR clast is contaminated with impactor material. 40Ar‐39Ar analyses of a type 4 chondrite clast yield a plateau age of 4402 ± 7 Ma, which is interpreted to be the result of impact heating. Other impact events are recorded by an IMR clast at 1790 ± 36 Ma and a shock‐darkened clast at 2216 ± 40 Ma, demonstrating that NWA 869 escaped major reset in the course of the event at approximately 470 Ma that affected many L–chondrites. 相似文献
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Julia ROSZJAR Knut METZLER Addi BISCHOFF Jean‐Alix BARRAT Thorsten GEISLER Richard C. GREENWOOD Ian A. FRANCHI Stephan KLEMME 《Meteoritics & planetary science》2011,46(11):1754-1773
Abstract– We report on the bulk chemical composition, petrology, oxygen isotopic composition, trace element composition of silicates, and degree of self‐irradiation damage on zircon grains of the eucrite Northwest Africa (NWA) 5073 to constrain its formation and postcrystallization thermal history, and to discuss their implications for the geologic history of its parent body. This unequilibrated and unbrecciated meteorite is a new member of the rare Stannern‐trend eucrites. It is mainly composed of elongated, zoned pyroxene phenocrysts up to 1.2 cm, plagioclase laths up to 0.3 cm in length, and is rich in mesostasis. The latter contains zircon grains up to 30 μm in diameter, metal, sulfide, tridymite, and Ca‐phosphates. Textural observations and silicate compositions, coupled with the occurrence of extraordinary Fe‐rich olivine veins that are restricted to large pyroxene laths, indicate that NWA 5073 underwent a complex thermal history. This is also supported by the annealed state of zircon grains inferred from μ‐Raman spectroscopic measurements along with U and Th data obtained by electron probe microanalyses. 相似文献
49.
A. BISCHOFF 《Meteoritics & planetary science》2000,35(4):699-706
Abstract— Rumuruti (R) chondrites constitute a new, well‐established chondrite group different from the carbonaceous, ordinary, and enstatite chondrites. Many of these samples are gas‐rich regolith breccias showing the typical light‐dark structure and consist of abundant fragments of various parent‐body lithologies embedded in a fine‐grained olivine‐rich matrix. Unequilibrated type‐3 lithologies among these fragments have frequently been mentioned in various publications. In this study, detailed mineralogical data on seven primitive fragments from the R‐chondrites Dar al Gani 013 and Hughes 030 are presented. The fragments range from ～300 μ in size up to several millimeters. Generally, the main characteristics can be summarized as follows: (1) Unequilibrated type‐3 fragments have a well‐preserved chondritic texture with a chondrule‐to‐matrix ratio of ～1:1. Chondrules and chondrule fragments are embedded in a fine‐grained olivine‐rich matrix. Thus, the texture is quite similar to that of type‐3 carbonaceous chondrites. (2) In all cases, matrix olivines in type‐3 fragments have a significantly higher Fa content (44–57 mol%) than olivines in other (equilibrated) lithologies (38–40 mol% Fa). (3) Olivines and pyroxenes occurring within chondrules or as fragments are highly variable in composition (Fa0–65 and Fs0–33, respectively) and, generally, more magnesian than those found in equilibrated R chondrites. Agglomerated material of the R‐chondrite parent body (or bodies) was highly unequilibrated. It is suggested that the material that accreted to form the parent body consisted of chondrules and chondrule fragments, mainly having Mg‐rich silicate constituents, and Fe‐rich highly oxidized fine‐grained materials. The dominating phase of this fine‐grained material may have been Fa‐rich olivine from the beginning. The brecciated whole rocks, the R‐chondrite regolith breccias, were not significantly reheated subsequent to brecciation or during lithification, as indicated by negligible degree of equilibration between matrix components and Mg‐rich olivines and pyroxenes in primitive type‐3 fragments. 相似文献