Petrology and mineralogy of the PCA 91082 chondrite and its comparison with the Yamato-793495(CR) chondrite

Petrographic and mineralogical study of the PCA 91082 CR chondrite was performed using a scanning electron microscope (SEM) and an electron microprobe analyzer (EPMA). The present study shows that this meteorite has some unique petrographic and mineralogical properties which have not been observed in other CR chondrites. (1) About 40% of chondrules contain silica-pods in their outermost parts. Silica was probably formed by the adhesion of SiO_2-rich materials during chondrule formation. (2) Chondrules in PCA 91082 experienced a lesser degree of aqueous alteration than those in other CR chondrites. Only the peripheries of chondrules are altered in PCA 91082. Fe contents in the phyllosilicates in chondrules are higher than the values reported in other CR chondrites in which chondrules are altered into their cores. In Yamato (Y)-793495,the extent of aqueous alteration is similar to that in PCA 91082. Fe contents of phyllosilicates in chondrules in Y-793495 are also higher than those in the more altered CR chondrites. (3) PCA 91082 contains constituent components which experienced various alteration conditions. One phyllosilicate clast in PCA 91082 contains a Ca-sulfate component with magnesian phyllosilicates. Probably the clast experienced aqueous alteration on a parent body. The aqueous alteration condition of the clast was probably similar to that of Cls

Estimation of three-dimensional internal structures of some barred olivine chondrules in Allende (CV3) chondrite

Three-dimensional internal structures of four BO chondrules in Allende were studied by stacking backscattered electron image photographs of their successive cross sections, which were taken by polishing very carefully these chondrules embedded in epoxy resin. The reconstructed structures show that the BO chondrules investigated are composed of several sets of parallel platy olivine crystals with smooth surfaces. The blanching of the crystals is relatively rare and the olivine plates occasionally have holes. The plates connect to the inner rims of the olivine shells. In a BO chondrule, altered mesostasis with unique texture was found in a restricted region of altered mesostasis, which contains "phlogopite". The "phlogopite"-bearing region is continuous to a sulfide nodule on the surface of the chondrule, which suggests that the volatile that formed "phlogopite" was introduced through a sulfide nodule on the surface of the chondrule

Petrology and mineralogy of CK chondrites: Implications for the metamorphism of the CK chondrite parent body

The petrology and mineralogy of four CK chondrites, Karoonda (CK4), Maralinga (CK4), Yamato(Y)-693(CK4), and Elephant Moraine (EET) 87507 (CK5) were investigated in detail to estimate the origin of their quite heterogeneous plagioclases and the metamorphic history of CK chondrite parent body. EPMA analyses and SEM observations revealed that plagioclases in chondrules, CAIs, and matrices in CK chondrites have different compositional variations and that plagioclases in matrices display distinct reverse zoning in regard to An content. Cores of matrix plagioclases (An_∿An_) may have formed by recrystallization of primary plagioclases. Rims of matrix plagioclases were probably formed during metamorphism, because the reverse zoning can be observed, regardless of degree of recrystallization. The occurrences and compositions of pyroxenes are different among less recrystallized CK chondrites and recrystallized CK chondrites. Low-Ca pyroxenes in recrystallized matrices are enclosed in matrix plagioclases and different in CaO and Al_2O_3 contents from those in chondrules. Compositions of low-Ca pyroxenes in matrix plagioclases suggest that they include Fe^. They were probably formed during metamorphism under high oxygen fugacities. Highly oxidized metamorphic conditions are supported by the existence of Fe^ in spinels and high NiO contents in olivines. Olivine-spinel geothermometry and pyroxene compositions suggest that Maralinga, Y-693,EET87507,and some fragments in Karoonda were heated to the temperatures as high as those of type 5 to 6 ordinary chondrites (750 to 850℃). Slow cooling below∿400℃ of these meteorites was estimated by sulfide mineral assemblages. Because Maralinga has many different properties from other CK chondrites investigated, it is an anomalous CK chondrite

Texture and chemical composition of pyroxenes in chondrules in carbonaceous and unequilibrated ordinary chondrites

The texture and composition of pyroxenes in chondrules in unequilibrated ordinary chondrites (UOCs) (petrologic subtype less than 3.5) and carbonaceous chondrites (CCs) were studied with SEM and EPMA. Four kinds of pyroxene can be recognized on the basis of the mode of occurrence and chemical composition; they are protopyroxene (now inverted to twinned low-Ca clinopyroxene) (Wo10 (typically more than 20). In contrast, the Fs content of pyroxene in UOCs ranges from 0 to 50 continuously. A TiO_2 vs. Al_2O_3 plot for augite in CCs shows two trends with different TiO_2/Al_2O_3 ratios. Augites with a high TiO_2/Al_2O_3 ratio coexist with pigeonite in CCs except for Y-790112. Y-790112 and UOCs rarely have augites with a high TiO_2/Al_2O_3 ratio, though some augites coexist with pigeonite. Minor elements in augite in chondrules show that considerable amounts of Na^+ and Fe^ were probably introduced into chondrules after crystallization of pyroxenes. The oxygen fugacity at which pyroxene in CC chondrules crystallized was lower that that for UOCs. During cooling the oxygen fugacity around CC chondrules became higher than that around UOC chondrules

Petrology and mineralogy of the Coolidge meteorite (CV4)

The Coolidge meteorite is important because it is the only equilibrated carbonaceous chondrite which does not belong to the CK group. The petrology and mineralogy of the Coolidge meteorite were investigated by SEM and EPMA. Coolidge has some similarities to the "reduced group" of CV3 chondrites. First, abundant Fe-Ni metal and sulfide are kamacite, taenite, and troilite. Second, AOIs, CAIs, and chondrules are depleted in Na and K. Third, the main constituent the mineral of matrix is olivine. In addition to these similarities, Coolidge also has some unique properties. First of all, matrix as well as AOIs, CAIs, and chondrules is depleted in Na. This corresponds to the depletions of Na abundances in bulk compositions of Coolidge relative to CV3 chondrites. The depletion was probably established before agglomeration of this meteorite. Second, there are some plagioclase pyroxene chondrules in this meteorite. Different from type IV chondrules, they include various amounts of silica mineral. Third, bulk compositions of CAIs are rich in FeO and Cr_2O_3. Fourth, compositional variations of spinel in Coolidge are very wide. The Cr/(Cr+Al) ratio varies from 0.01 to 0.7,and the Mg/(Mg+Fe) ratio varies from 0.45 to 0.77. Fifth, correlations between some elements in spinel and those in pyroxenes in Coolidge are not similar to the "reduced group" of CV3 chondrites but similar to EOCs. The last three differences were probably established during thermal metamorphism. Olivine-spinel geothermometry suggests that Coolidge was heated to temperatures as high as ∿900℃. The duration of thermal metamorphism was short enough to preserve many petrographic features such as Fe-Mg compositional zoning in low-Ca pyroxene. Distribution of Co between kamacite and taenite suggest fast colling around ∿500℃

Sample processing and initial analysis techniques for Antarctic micrometeorites

Based on our initial visual selection (IVS) criteria and low vacuum (LV)-SEM/EDS data, we successfully selected candidates of unmelted chondritic Antarctic micrometeorites (AMMs) from the unsorted sample set collected by Maurette\u27s group in 1991. AEM data of some of the selected microparticles show that they are unmelted but probably dehydrated AMMs. An AMM candidate, 91-1-C4,contains ferroan (around Fo30) olivine grains which have planar channels parallel to (001). Their textures quite resemble those shown in the initial stage of iddingsite formation of olivine phenocrysts in terrestrial basalts. These findings indicate that some olivine crystals in AMMs can be altered even in the Antarctic environment

Mineralogy of Antarctic micrometeorites recovered from the Dome Fuji Station

Mineralogy of six micrometeorites (MMs) was investigated by a TEM for electron petrography. These MMs were selected based on X-ray diffraction analysis by a Gandolfi X-ray camera, qualitative analysis by SEMEDS of the surfaces of the MMs, and based on textural observation and quantitative analysis of "polished" sections of the MMs by a SEM and an EPMA. We found one saponite-bearing MM F96CI024 among 83 MMs. It has textural and mineralogical characteristics similar to a saponite-bearing MM BI 91/3-108. From the mineralogical point of view, they are different from CI, CM, CR, and hydrated CV chondrites. F96CI020 and F96DI021 contain lepidocrocite, which was formed by terrestrial weathering. MMs are susceptible to terrestrial weathering even in snow (or ice). A magnesium-and iron-bearing mineral (probably (Mg, Fe) O) was found in the phyllosilicate-bearing MM and two pyroxene-rich MMs. In the latter MMs, the mineral coexists with pyroxene that was formed newly during atmospheric entry of the MMs

Petrology and mineralogy of the Yamato-86720 carbonaceous chondrite

Phyllosilicate clasts in Yamato-86720 consist mainly of dehydrated serpentine (or chlorite) and sodian talc (or saponite) components, which are the most homogeneous in chemical compositions among those in CM chondrites. They were produced from chondrules by intense hydrous alteration, resulting in the homogeneous composition of the phyllosilicates. Metal spherules and troilite grains in original chondrules have also altered to ovoidal phyllosilicate inclusions and unusual carbonate-phyllosilicates inclusions, respectively. The CaO content of the original chondrules was retained within them as the unusual carbonate-phyllosilicate inclusions, which resulted in the low CaO content of the Y-86720 matrix. The matrix is different in composition from the clast phyllosilicates, suggesting that the two were produced in conditions different from each other. Pyrrhotite grains in clasts and matrix have altered probably to ferrihydrite, which have produced Fe-rich halos around the pyrrhotite grains after the agglomeration of Y-86720 and prior to a heating event. The heating event took place in the final stage of the chondrite formation, resulting in dehydration of phyllosilicates and reduction of ferrihydrite to Co-Ni-poor kamacite

Effects of terrestrial weathering on the matrix mineralogy of Colony CO3 chondrite

Colony is the least metamorphosed CO chondrite (CO3.0). However, it is badly weathered. In order to show both primary and secondary features of the matrix and discuss the effects of terrestrial weathering on the matrix mineralogy of Colony, we performed combined SEM, EPMA, CL, and TEM studies on this meteorite. EPMA data of the matrix of Colony show that matrix composition was changed by terrestrial weathering. The matrix composition can be represented by three hypothetical components : an Fe-rich component (Fe hydroxide-rich weathering products), a component with olivine-like composition, and an Fe-poor, Si-rich component. TEM observation reveals that the matrix contains abundant Fe-rich poorly crystalline interstitial materials, abundant ferroan olivine (Fa_-Fa_), less abundant magnesian low-Ca pyroxene (En_), magnetite, and minor amounts of spinel group minerals, troilite, and rare Ca-rich pyroxene. Among these phases, anhydrous minerals are primary minerals. The Fe-rich component estimated from EPMA data is composed of abundant Fe-enriched, nearly amorphous material, minute phyllosilicates (mainly saponite, serpentine, and minor amounts of chlorite and montomorillonite), and goethite. The Fe-poor, Si-rich component estimated from EPMA data is also composed of basically the same but seems to contain more phyllosilicates than the Fe-rich one. Matrix olivine crystals often contain planar defects parallel to (001). Such planar defects were probably formed in the initial stage of iddingsite formation by terrestrial weathering

Size distribution of Antarctic micrometeorites stored in surface snow near Dome Fuji Station

The Tenth Symposium on Polar Science/Special session: [OA] Antarctic meteorites, Thur. 5 Dec. / 3F Multipurpose conference room, National Institute of Polar Researc