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1.
Abstract– On the basis of morphological and petrographic characteristics, eight “giant” unmelted micrometeorites in the 300–1100 μm size range were selected from the Transantarctic Mountain micrometeorite collection, Victoria Land, Antarctica. Mineralogical and geochemical data obtained by means of scanning electron microscopy, electron probe microanalyses, and synchrotron X‐ray diffraction allow their classification as chondritic micrometeorites. The large size of the micrometeorites increases considerably the amount of mineralogical and geochemical information compared to micrometeorites in smaller size fractions, therefore allowing a better definition of their parent material. A large variety of material is observed: five micrometeorites are related to unequilibrated and equilibrated ordinary chondrite, one to CV chondrite, one to CM chondrite, and one to CI chondrite parent materials. Besides reporting the first occurrence of a CV‐like micrometeorite, our study shows that the abundance of chondritic material supports observations from recent studies on cosmic spherules that a large part of the micrometeorite flux in this size range is of asteroidal origin.  相似文献   

2.
Abstract— Bencubbin is an unclassified meteorite breccia which consists mainly of host silicate (～40 vol.%) and host metal (～60%) components. Rare (< 1%) ordinary chondrite clasts and a dark xenolith (formerly called a carbonaceous chondrite clast) are also found. A petrologic study of the host silicates shows that they have textures, modes, mineralogy and bulk compositions that are essentially the same as that of barred olivine (BO) chondrules, and they are considered to be BO chondritic material. Bulk compositions of individual host silicate clasts are identical and differ only in their textures which are a continuum from coarsely barred, to finely barred, to feathery microcrystalline; these result from differing cooling rates. The host silicates differ from average BO chondrules only in being angular clasts rather than fluid droplet-shaped objects, and in being larger in size (up to 1 cm) than most chondrules; but large angular to droplet-shaped chondrules occur in many chondrites. Bencubbin host metallic FeNi clasts have a positive Ni-Co trend, which coincides with that of a calculated equilibrium nebular condensation path. This appears to indicate a chondritic, rather than impact, origin for this component as well. The rare ordinary chondrite clast and dark xenolith also contain FeNi metal with compositions similar to that of the host metal. Two scenarios are offered for the origin of the Bencubbin breccia. One is that the Bencubbin components are chondritic and were produced in the solar nebula. Later brecciation, reaggregation and minor melting of the chondritic material resulted in it becoming a monomict chondritic breccia. The alternative scenario is that the Bencubbin components formed as a result of major impact melting on a chondritic parent body; the silicate fragments were formed from an impact-induced lava flow and are analogous to the spinifex-textured rocks characteristic of terrestrial komatiites. Both scenarios have difficulties, but the petrologic, chemical and isotopic data are more consistent with Bencubbin being a brecciated chondrite. Bencubbin has a number of important chemical and isotopic characteristics in common with the major components in the CR (Renazzo-type) chondrites and the unique ALH85085 chondrite, which suggests that their major components may be related. These include: (1) Mafic silicates that are similarly Mg-rich and formed in similar reducing environments. (2) Similarly low volatiles; TiO2, Al2O3 and Cr2O3 contents are also similar. (3) Similar metallic FeNi compositions that sharply differ from those in other chondrites. (4) Remarkable enrichments in 15N. (5) Similar oxygen isotopic compositions that lie on the same mixing line. Thus, the major components of the Bencubbin breccia are highly similar to those of the ALH85085 and CR chondrites and they may have all formed in the same isotopic reservoir, under similar conditions, in the CR region of the solar nebula.  相似文献   

3.
A new meteorite find, named Khatyrka, was recovered from eastern Siberia as a result of a search for naturally occurring quasicrystals. The meteorite occurs as clastic grains within postglacial clay‐rich layers along the banks of a small stream in the Koryak Mountains, Chukotka Autonomous Okrug of far eastern Russia. Some of the grains are clearly chondritic and contain Type IA porphyritic olivine chondrules enclosed in matrices that have the characteristic platy olivine texture, matrix olivine composition, and mineralogy (olivine, pentlandite, nickel‐rich iron‐nickel metal, nepheline, and calcic pyroxene [diopside‐hedenbergite solid solution]) of oxidized‐subgroup CV3 chondrites. A few grains are fine‐grained spinel‐rich calcium‐aluminum‐rich inclusions with mineral oxygen isotopic compositions again typical of such objects in CV3 chondrites. The chondritic and CAI grains contain small fragments of metallic copper‐aluminum‐iron alloys that include the quasicrystalline phase icosahedrite. One grain is an achondritic intergrowth of Cu‐Al metal alloys and forsteritic olivine ± diopsidic pyroxene, both of which have meteoritic (CV3‐like) oxygen isotopic compositions. Finally, some grains consist almost entirely of metallic alloys of aluminum + copper ± iron. The Cu‐Al‐Fe metal alloys and the alloy‐bearing achondrite clast are interpreted to be an accretionary component of what otherwise is a fairly normal CV3 (oxidized) chondrite. This association of CV3 chondritic grains with metallic copper‐aluminum alloys makes Khatyrka a unique meteorite, perhaps best described as a complex CV3 (ox) breccia.  相似文献   

4.
Micrometeorites provide a large range of samples sourced from a wide variety of planetary materials, thereby providing a scope for expanding the known inventory of solar system materials. Here we report the micrometeorite AAS62‐34‐P117 having the assemblage of corundum, hibonite, unknown Al‐rich phases, FeNi metal blebs, sulfide, and phosphate embedded in Al‐rich silicate composition, and Pt‐group element nuggets dispersed throughout the micrometeorite. Here, we report the presence of corundum in micrometeorites as a major refractory phase with sizes greater than ~10 μm. The Al‐rich phases have Al2O3 ~50–70%, such high Al phases are not known from meteoritic components either in chondrules or refractory inclusions. In addition, the Ca content is extremely poor to relate it directly to known refractory inclusions, but is very high in Al. The presence of corundum in Al‐rich phases indicates the micrometeorite to be early condensate from solar nebula that later got incorporated into Si‐rich materials leading to a transformation that produced the unusual Al‐rich and Ca‐poor phases different from the average solar composition. The observed texture and mineralogy of the micrometeorite appears to have evolved in a nebular setting that has compositional reservoirs different from those of any known components of meteorites.  相似文献   

5.
Abstract– Tucson is an enigmatic ataxitic iron meteorite, an assemblage of reduced silicates embedded in Fe‐Ni metal with dissolved Si and Cr. Both, silicates and metal, contain a record of formation at high temperature (～1800 K) and fast cooling. The latter resulted in the preservation of abundant glasses, Al‐rich pyroxenes, brezinaite, and fine‐grained metal. Our chemical and petrographic studies of all phases (minerals and glasses) indicate that they have a nebular rather than an igneous origin and give support to a chondritic connection as suggested by Prinz et al. (1987) . All silicate phases in Tucson apparently grew from a liquid that had refractory trace elements at approximately 6–20 × CI abundances with nonfractionated (solar) pattern, except for Sc, which was depleted (～1 × CI). Metal seems to have precipitated before and throughout silicate aggregate formation, allowing preservation of all evolutionary steps of the silicates by separating them from the environment. In contrast to most chondrites, Tucson documents coprecipitation of metal and silicates from the solar nebula gas and precipitation of metal before silicates—in accordance with theoretical condensation calculations for high‐pressure solar nebula gas. We suggest that Tucson is the most metal‐rich and volatile‐element‐poor member of the CR chondrite clan.  相似文献   

6.
We investigated the khatyrkite–cupalite holotype sample, 1.2 × 0.5 mm across. It consists of khatyrkite (Cu,Zn)Al2, cupalite (Cu,Zn)Al, and interstitial material with approximate composition (Zn,Cu)Al3. All mineral phases of the holotype sample contain Zn and lack Fe that distinguishes them from khatyrkite and cupalite in the Khatyrka meteorite particles (Bindi et al. 2009 , 2011 , 2012 , 2015 ; MacPherson et al. 2013 ; Hollister et al. 2014 ). Neither highly fractionated natural systems nor geo‐ or cosmochemical processes capable of forming the holotype sample are known so far. The bulk chemistry and thermal history of khatyrkite–cupalite assemblage in the holotype sample hint for its possible industrial origin. Likewise, the aluminides in the Khatyrka meteorite particles may also be derived from industrial materials and mixed with extraterrestrial matter during gold prospecting in the Listvenitovy Stream valley.  相似文献   

7.
E.A. Cloutis  P. Hudon  T. Hiroi  M.J. Gaffey 《Icarus》2012,217(1):389-407
Powdered samples of a suite of 14 CR and CR-like chondrites, ranging from petrologic grade 1 to 3, were spectrally characterized over the 0.3–2.5 μm interval as part of a larger study of carbonaceous chondrite reflectance spectra. Spectral analysis was complicated by absorption bands due to Fe oxyhydroxides near 0.9 μm, resulting from terrestrial weathering. This absorption feature masks expected absorption bands due to constituent silicates in this region. In spite of this interference, most of the CR spectra exhibit absorption bands attributable to silicates, in particular an absorption feature due to Fe2+-bearing phyllosilicates near 1.1 μm. Mafic silicate absorption bands are weak to nonexistent due to a number of factors, including low Fe content, low degree of silicate crystallinity in some cases, and presence of fine-grained, finely dispersed opaques. With increasing aqueous alteration, phyllosilicate: mafic silicate ratios increase, resulting in more resolvable phyllosilicate absorption bands in the 1.1 μm region. In the most phyllosilicate-rich CR chondrite, GRO 95577 (CR1), an additional possible phyllosilicate absorption band is seen at 2.38 μm. In contrast to CM spectra, CR spectra generally do not exhibit an absorption band in the 0.65–0.7 μm region, which is attributable to Fe3+–Fe2+ charge transfers, suggesting that CR phyllosilicates are not as Fe3+-rich as CM phyllosilicates. CR2 and CR3 spectra are uniformly red-sloped, likely due to the presence of abundant Fe–Ni metal. Absolute reflectance seems to decrease with increasing degree of aqueous alteration, perhaps due to the formation of fine-grained opaques from pre-existing metal. Overall, CR spectra are characterized by widely varying reflectance (4–21% maximum reflectance), weak silicate absorption bands in the 0.9–1.3 μm region, overall red slopes, and the lack of an Fe3+–Fe2+ charge transfer absorption band in the 0.65–0.7 μm region.  相似文献   

8.
Metal in various forms is common in almost all meteorites but considerably rare among micrometeorites. We report here the discovery of two metal micrometeorites, i.e., (1) an awaruite grain similar to those found in the metal nodules of CV chondrites and (2) a metal micrometeorite of kamacite composition enclosing inclusions of chromite and merrillite. This micrometeorite appears to be a fragment of H5/L5 chondrite. These metal micrometeorites add to the inventory of solar system materials that are accreted by the Earth in microscopic form. They also strengthen the argument that a large proportion of material accreted by the Earth that survives atmospheric entry is from asteroidal sources.  相似文献   

9.
Abstract— The maximum diameter of chromite (FeCr2O4) grains within L chondrites reflects the petrographic type of the sample. On the basis of our measurements of nine recent L chondrites, L3 chromite Dmax = 34–50 μm, L4 = 87–150 μm, L5 = 76–158 μm, and L6 = 253–638 μm. This variation reflects the crystallization of the chromite grains during parent body thermal metamorphism. We use this calibration to classify six fossil meteorites from the Middle Ordovician in Sweden as type 3 (or 4) to 6. The high flux of L chondrites at 470 Ma contained a range of petrographic types and may have had a higher proportion of lower petrographic type meteorites than are found in recent L chondrite falls. The fossil meteorites have in places preserved recognizable chondrule textures, including porphyritic olivine, barred olivine, and radiating pyroxene. A large relict clast and fusion crust have also been tentatively identified in one fossil meteorite. Apart from chromite, all of the original meteorite minerals have been replaced by carbonate (and sheet silicate and sulfate) during diagenesis within the limestone host. The preservation of chondrule definition has allowed us to measure the mean diameters of relict chondrules. The range (0.4–0.6 mm) is consistent with measurements made in the same way on recent L chondrites.  相似文献   

10.
Abstract— A new grouplet of primitive, metal‐rich chondrites, here called the CB (C, carbonaceous; B, bencubbinite) chondrites, has been recognized. It includes Bencubbin, Weatherford, Hammadah al Hamra (HH) 237 and Queen Alexandra Range (QUE) 94411, paired with QUE 94627. Their mineral compositions, as well as their oxygen and nitrogen isotopic compositions, indicate that they are closely related to the CR and CH chondrites, all of which are members of the more inclusive CR clan. CB chondrites have much greater metal/silicate ratios than any other chondrite group, widely increasing the range of metal/silicate fractionation recorded in solar nebular processes. They also have the greatest moderately volatile lithophile element depletions of any chondritic materials. Metal has compositional trends and zoning patterns that suggest a primitive condensation origin, in contrast with metal from other chondrite groups. CB chondrites, as well as other CR clan chondrites, have much heavier nitrogen (higher 15N/14N) than that in other chondrite groups. The primitive characteristics of the CB chondrites suggest that they contain one of the best records of early nebular processes. Another chondrite, Grosvenor Mountains 95551, is petrographically similar to the CB chondrites, but its mineral and oxygen and nitrogen isotope compositions indicate that it formed from a different nebular reservoir.  相似文献   

11.
We measured the concentrations and isotopic compositions of He, Ne, and Ar in 14 fragments from 12 different meteorites: three carbonaceous chondrites, six L chondrites (three most likely paired), one H chondrite, one R chondrite, and one ungrouped chondrite. The data obtained for the CV3 chondrites Ramlat as Sahmah (RaS) 221 and RaS 251 support the hypothesis of exposure age peaks for CV chondrites at approximately 9 Ma and 27 Ma. The exposure age for Shi?r 033 (CR chondrite) of 7.3 Ma is also indicative of a possible CR chondrite exposure age peak. The three L chondrites Jiddat al Harasis (JaH) 091, JaH 230, and JaH 296, which are most likely paired, fall together with Hallingeberg into the L chondrite exposure age peak of approximately 15 Ma. The two L chondrites Shelburne and Lake Torrens fall into the peaks at approximately 40 Ma and 5 Ma, respectively. The ages for Bassikounou (H chondrite) and RaS 201 (R chondrite) are approximately 3.5 Ma and 5.8 Ma, respectively. Six of the studied meteorites show clear evidence for 3He diffusive losses, the deficits range from approximately 17% for one Lake Torrens aliquot to approximately 45% for RaS 211. The three carbonaceous chondrites RaS 221, RaS 251, and Shi?r 033 all have excess 4He, either of planetary or solar origin. However, very high 4He/20Ne ratios occur at relatively low 20Ne/22Ne ratios, which is unexpected and needs further study. The measured 40Ar ages fit well into established systematics. They are between 2.5 and 4.5 Ga for the carbonaceous chondrites, older than 3.6 Ga for the L and H chondrites, and about 2.4 Ga for the R chondrite as well as for the ungrouped chondrite. Interestingly, none of our studied L chondrites has been degassed in the 470 Ma break‐up event. Using the amount of trapped 36Ar as a proxy for noble gas contamination due to terrestrial weathering we are able to demonstrate that the samples studied here are not or only very slightly affected by terrestrial weathering (at least in terms of their noble gas budget).  相似文献   

12.
Abstract— To test whether aubrites can be formed by melting of enstatite chondrites and to understand igneous processes at very low O fugacities, we have conducted partial melting experiments on the Indarch (EH4) chondrite at 1000–1500 °C. Silicate melting begins at 1000 °C, and Indarch is completely melted by 1500 °C. The metal-sulfide component melts completely at 1000 °C. Substantial melt migration occurs at 1300–1400 °C, and metal migrates out of the silicate charge at 1450 °C and ～50% silicate partial melting. As a group, our experiments contain three immiscible metallic melts (Si-, P-, and C-rich), two immiscible sulfide melts (Fe- and FeMgMnCa-rich), and silicate melt. Our partial melting experiments on the Indarch (EH4) enstatite chondrite suggest that igneous processes at low fO2 exhibit several unique features. The complete melting of sulfides at 1000 °C suggests that aubritic sulfides are not relics. Aubritic oldhamite may have crystallized from Ca and S complexed in the silicate melt. Significant metal-sulfide melt migration might occur at relatively low degrees of silicate partial melting. Substantial elemental exchange occurred between different melts (e.g., S between sulfide and silicate, Si between silicate and metal), a feature not observed during experiments at higher fO2. This exchange may help explain the formation of aubrites from known enstatite chondrites.  相似文献   

13.
Abstract— We have investigated the mineralogy, petrography, bulk chemistry, and light element isotope composition of the ungrouped chondrites North West Africa (NWA) 1152 and Sahara 00182. NWA 1152 contains predominantly type 1 porphyritic olivine (PO) and porphyritic olivinepyroxene (POP) chondrules. Chondrule silicates are magnesium‐rich (Fo98.8 ± 1.2, n = 36; Fs2.3 ± 2.1 Wo1.2 ± 0.3, n = 23). Matrix comprises ?40 vol% of the sample and is composed of a micron sized silicate groundmass with larger silicate, sulfide, magnetite, and Fe‐Ni metal (Ni ?50 wt%) grains. Phyllosilicates were not observed in the matrix. Refractory inclusions are rare (0.3 vol%) and are spinel pyroxene aggregates or amoeboid olivine aggregates; melilite is absent from the refractory inclusions. Sahara 00182 contains predominantly type 1 PO chondrules, POP chondrules are less common. Most chondrules contain blebs of, and are often rimmed with, Fe‐Ni metal and sulfide. Chondrule phenocrysts are magnesium‐rich (Fo92.2 ± 0.6, n = 129; Fs4.4 ± 1.8 Wo1.3 ± 1.1, n = 16). Matrix comprises ?30 vol% of the meteorite and is predominantly sub‐micron silicates, with rare larger silicate gains. Matrix Fe‐Ni metal (mean Ni = 5.8 wt%) and sulfide grains are up to mm scale. No phyllosilicates were observed in the matrix. Refractory inclusions are rare (1.1 vol%) and melilite is absent. The oxygen isotope composition of NWA 1152 falls within the range of the CV chondrites with δ17O = ?3.43%0 δ18O = 0.70%0 and is similar to Sahara 00182, δ17O = ?3.89%0, δ18O = ?0.19%0 (Grossman and Zipfel 2001). Based on mineralogical and petrographic characteristics, we suggest NWA 1152 and Sahara 00182 show many similarities with the CR chondrites, however, oxygen isotopes suggest affinity with the CVs. Thus, neither sample can be assigned to any of the currently known carbonaceous chondrite groups based on traditionally recognized characteristics. Both samples demonstrate the complexity of inter‐ and intra‐group relationships of the carbonaceous chondrites. Whatever their classification, N WA 1152 and Sahara 00182 represent a source of relatively pristine solar system material.  相似文献   

14.
The mineralogical and chemical compositions of meteorites from the Motta di Conti, Vercelli, Italy, shower (February 29, 1868) have been determined. Microprobe analyses, of olivine (Fa19,6) and orthopyroxene (Fs17,8), as well as the bulk chemical composition, particularly the ratios of SiO2/MgO (1.50), Fe°/Ni° (11.03), Fetotal/SiO2 (0.81), Fe°/Fetotal (0.70) and the content of Fetotal (28.60%) classify the meteorite as an H-group chondrite. The percentage of total metallic nickel-iron (22.06%) is somewhat higher than the average in H-group chondrites. The texture of our stone shows evidence of metamorphism. The integration between matrix and chondrules is advanced and may suggest a high petrographic grade, but the identification of several microscopic features (e.g. small grains of monoclinic twinned pyroxene, FeNi-FeS intergrowths, globules and mosaic) leads to the conclusion that a variety of petrographic types (4–6) are present. Metamorphic equilibration in chondrites is discussed and a preliminary hypothesis for H4–6 chondrites is suggested.  相似文献   

15.
Based on sediment‐dispersed extraterrestrial spinel grains in the Bottaccione limestone section in Italy, we reconstructed the micrometeorite flux to Earth during the early Paleocene. From a total of 843 kg of limestone, 86 extraterrestrial spinel grains (12 grains > 63 μm, and 74 in the 32–63 μm fraction) have been recovered. Our results indicate that the micrometeorite flux was not elevated during the early Paleocene. Ordinary chondrites dominated over achondritic meteorites similar to the recent flux, but H chondrites dominated over L and LL chondrites (69%, 22%, and 9%, respectively). This H‐chondrite dominance is similar to that recorded within an enigmatic 3He anomaly (70, 27, and 3%) in the Turonian, but different from just before this 3He anomaly and in the early Cretaceous, where ratios are similar to the recent flux (~45%, 45%, and 10%). The K‐Ar isotopic ages of recently fallen H chondrites indicate a small impact event on the H‐chondrite parent body ~50 to 100 Ma ago. We tentatively suggest that this event is recorded by the Turonian 3He anomaly, resulting in an H‐chondrite dominance up to the Paleocene. Our sample spanning the 20 cm above the Cretaceous–Paleogene (K–Pg) boundary did not yield any spinel grains related to the K–Pg boundary impactor.  相似文献   

16.
We have discovered in a Stardust mission terminal particle a unique mineralogical assemblage of symplectically intergrown pentlandite ((Fe,Ni)9S8) and nanocrystalline maghemite (γ‐Fe2O3). Mineralogically similar cosmic symplectites (COS) have only been found in the primitive carbonaceous chondrite Acfer 094 and are believed to have formed by aqueous alteration. The O and S isotopic compositions of the Wild 2 COS are indistinguishable from terrestrial values. The metal and sulfide precursors were thus oxidized by an isotopically equilibrated aqueous reservoir either inside the snow line, in the Wild 2 comet, or in a larger Kuiper Belt object. Close association of the Stardust COS with a Kool mineral assemblage (kosmochloric Ca‐rich pyroxene, FeO‐rich olivine, and albite) that likely originated in the solar nebula suggests the COS precursors also had a nebular origin and were transported from the inner solar system to the comet‐forming region after they were altered.  相似文献   

17.
Based on the chemical heterogeneity of chondrule and matrix olivine, Northwest Africa (NWA) 5343 is the least metamorphosed CK chondrite reported so far. To better constrain the lower limit of metamorphism in the CK chondrites, we performed a detailed analysis of matrix material in NWA 5343, including characterization of the texture and bulk composition and analyses of individual silicate minerals. Results suggest that NWA 5343 is petrologic type 3.6 or 3.7. Although silicate minerals in the matrix seem to be equilibrated to roughly the same extent throughout the sample, there are recognizable differences in grain size and shape. These textural differences may be the result of transient heating events during impacts, which would be likely on the CK chondrite parent body. The difference between the extent of chemical equilibration and texture may also suggest that grain size and shape are still sensitive to metamorphism at petrologic subtypes where silicate mineral equilibration is nearly complete (e.g., >3.7). Carbonate material present in NWA 5343 is a product of terrestrial weathering; however, infiltration of a Ca-bearing fluid did not influence the composition of silicate minerals in the matrix. To evaluate the possibility of a continuous metamorphic sequence between the CV and CK chondrites, the bulk matrix composition of NWA 5343 is compared to the CVred chondrite, Vigarano. Although the matrix composition of NWA 5343 could be derived by secondary processing of a Vigarano-like precursor, porosity and texture of matrix olivine in NWA 5343 are hard to reconcile with a continuous metamorphic sequence.  相似文献   

18.
Here, the petrological features of numerous primitive achondrites and highly equilibrated chondrites are evaluated to review and expand upon our knowledge of the chondrite–achondrite transition, and primitive achondrites in general. A thermodynamic model for the initial silicate melting temperature and progressive melting for nearly the entire known range of oxidation states is provided, which can be expressed as Tm = 0.035Fa2?3.51Fa + 1109 (in °C, where Fa is the proportion of fayalite in olivine). This model is then used to frame a discussion of textural and mineralogical evolution of stony meteorites with increasing temperature. We suggest that the metamorphic petrology of these meteorites should be based on diffusive equilibration among the silicate minerals, and as such, the chondrite–achondrite transition should be defined by the initial point of silicate melting, not by metal–troilite melting. Evidence of silicate melting is preserved by a distinctive texture of interconnected interstitial plagioclase ± pyroxene networks among rounded olivine and/or pyroxene (depending on ?O2), which pseudomorph the former silicate melt network. Indirectly, the presence of exsolution lamellae in augite in slowly cooled achondrites also implies that silicate melting occurred because of the high temperatures required, and because silicate melt enhances diffusion. A metamorphic facies series is defined: the Plagioclase Facies is equivalent to petrologic types 5 and 6, the Sub‐calcic Augite Facies is bounded at lower temperatures by the initiation of silicate melting and at higher temperatures by the appearance of pigeonite, which marks the transition to the Pigeonite Facies.  相似文献   

19.
Abstract— We present a new single‐crystal X‐ray diffraction (XRD) study performed on a suite of six orthopyroxene grains from the low‐shocked H6 Trenzano meteorite. The quenched intracrystalline Fe2+‐Mg ordering state in orthopyroxene preserves the memory of the cooling rate near closure temperature Tc, thus yielding useful constraints on the last thermal event undergone by the host rock. The orthopyroxene Tc of 522 ± 13 °C, calculated using a new calibration equation obtained by Stimpfl (2005b), is higher than in previously published H chondrite data. The orthopyroxene cooling rate at this Tc is about 100 °C/kyr. This fast rate is inconsistent with the much slower cooling rate expected for H6 in the onion shell structural and thermal model of chondrite parent bodies. A petrographic study carried out at the same time indicated that the Trenzano meteorite is an H5 chondrite and not an H6 chondrite, as it is officially classified. Furthermore, the two‐pyroxene equilibrium temperature of Trenzano (824 ± 24 °C), calculated with QUILF95, is similar to the two‐pyroxene temperature of 750–840 °C obtained for the Carcote (H5) chondrite (Kleinschrot and Okrusch 1999).  相似文献   

20.
Abstract— Here we report the transmission electron microscopy (TEM) observations of the mineral assemblages and textures in shock‐induced melt veins from seven L chondrites of shock stages ranging from S3 to S6. The mineral assemblages combined with phase equilibrium data are used to constrain the crystallization pressures, which can be used to constrain shock pressure in some cases. Thick melt veins in the Tenham L6 chondrite contain majorite and magnesiowüstite in the center, and ringwoodite, akimotoite, vitrified silicate‐perovskite, and majorite in the edge of the vein, indicating crystallization pressure of ?25 GPa. However, very thin melt veins (5–30 μm wide) in Tenham contain glass, olivine, clinopyroxene, and ringwoodite, suggesting crystallization during transient low‐pressure excursions as the shock pressure equilibrated to a continuum level. Melt veins of Umbarger include ringwoodite, akimotoite, and clinopyroxene in the vein matrix, and Fe2SiO4‐spinel and stishovite in SiO2‐FeO‐rich melt, indicating a crystallization pressure of ?18 GPa. The silicate melt veins in Roy contain majorite plus ringwoodite, indicating pressure of ?20 GPa. Melt veins of Ramsdorf and Nakhon Pathon contain olivine and clinoenstatite, indicating pressure of less than 15 GPa. Melt veins of Kunashak and La Lande include albite and olivine, indicating crystallization at less than 2.5 GPa. Based upon the assemblages observed, crystallization of shock veins can occur before, during, or after pressure release. When the assemblage consists of high‐pressure minerals and that assemblage is constant across a larger melt vein or pocket, the crystallization pressure represents the equilibrium shock pressure.  相似文献   

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