Mineralogy and petrology of Belgica-7904: A new kind of carbonaceous chondrite from Antarctica

A mineralogical and petrological study of Belgica-7904 (B-7904) shows that it can be classified into the CM group. B-7904 has a variety of chondrules and aggregates, where pyroxenes and mesostasis glass are completely replaced by phyllosilicates, but olivine remains little altered. It has a high abundance of troilite; most occur in submicron to micron grains dispersed in the matrix. Minor taenite occurs, but magnetite is rare and tochilinite is absent. The phyllosilicates have relatively high Na contents and lower (Mg+Fe)/(Si+Al) ratios than serpentine; thus, they may be intergrowths of serpentine and a smectite-like phyllosilicate. Microprobe analyses of the phyllosilicates show high analytical totals relative to ordinary phyllosilicates, being consistent with the idea that they were dehydrated by heating. The fine troilite grains in the matrix may have been transformed from a thermally labile phase such as tochilinite during thermal metamorphism. The opaque mineral assemblage suggests that the thermal metamorphism occurred in a reduced condition. B-7904 and Y-86720 are mineralogically similar, although the former is less affected by aqueous alteration than the latter. They were probably derived from similar precursors and experienced aqueous alteration and thermal metamorphism in a common environment. These meteorites and ordinary CM chondrites apparently experienced distinct alteration histories, suggesting that they came from different regions in a parent body or different parent bodies

A fine-grained dark inclusion in the Vigarano CV3 chondrite: Record of accumulation processes on the meteorite parent body

Mineralogy and petrography of an unusual dark inclusion (DI) from the Vigarano CV3 chondrite (AMNH 2226-7) has been studied in detail. Formerly, this DI was briefly described by JOHNSON et al. (Geochim. Cosmochim. Acta, 54,819,1990). Unlike most common types of DIs, AMNH 2226-7 consists almost exclusively of fine grains (< μm in diameter) of mainly Fe-rich olivine and is devoid of chondrules, large mineral fragments and Fe-rich olivine aggregates (chondrule pseudomorphs). The Fe-rich olivine grains are mostly granular and equidimensional in morphology, and acicular to fibrous olivine grains, common in other types of DIs, are absent. Lesser amounts of relatively coarse grains (10-60μm) of Mg-rich olivine, Fe-rich olivine, enstatite, spinel, melilite, and perovskite are dispersed throughout the DI. A most distinguished feature of this DI is a network of arcuate bands (each 10-500μm wide and 250μm-3.5mm long) which exhibit peculiar textures. The bands consist mainly of olivine grains which are slightly finergrained and more Fe-rich than those in other areas. Several bands commonly occur roughly parallel to each other with equal spacing, forming a set of consecutive parallel bands. Different sets of parallel bands crosscut one another, some almost orthogonally. These mineralogical and textural features differ from the major group of DIs which contain chondrules and/or chondrule pseudomorphs, and indicate that the formation of AMNH 2226-7 cannot be explained by the direct replacement of a precursor chondrite. Probably, the origin of this DI is not directly related to that of the major types of DIs. We support the interpretation of JOHNSON et al. (1990) that the arcuate bands resemble fluvial sedimentary deposits and may reflect particle settling processes on the meteorite parent body. We suggest AMNH 2226-7 to be the product of reaccumulation of a fragmented and comminuted C3 chondrite. The predominant occurrence of fine-grained components in this DI may have resulted from either a grain size sorting process during reaccumulation or the disaggregation of already fine-grained precursor material such as type B DIs

Ca-Al-rich inclusions in three Antarctic CO3 chondrites, Yamato-81020, Yamato-82050 and Yamato-790992: Record of lowtemperature alteration

Ca-Al-rich inclusions (CAIs) in three Antarctic CO chondrites, Yamato (Y)-81020,Y-82050 and Y-790992 have similar overall textures; most CAIs are rimmed or concentric objects. However, there are considerable differences in mineralogy; almost all of the CAIs in Y-81020 consist of primary high-temperature phases such as melilite, anorthite, fassaite and spinel, while most of the CAIs in Y-82050 and Y-790992 contain major amounts of nepheline and lesser amounts of melilite and anorthite. CAIs in Y-790992 contain more nepheline than those in Y-82050. Small grains (<1μm in diameter) of troilite commonly coexist with nepheline. Spinel in Y-81020 is almost free of Fe, while those in Y-82050 and Y-790992 contain variable amounts of Fe (6 to 60 and 42 to 65 FeAl_2O_4 mol%, respectively). The texture of CAIs suggests that nepheline and troilite are secondary alteration products formed by replacing mainly melilite and anorthite. Spinel in CAIs and olivine in amoeboid olivine aggregates probably became enriched in Fe along with the alteration. As the alteration proceeded, fassaite and spinel were also replaced by nepheline, and perovskite was replaced by ilmenite and ulvospinel. Diopside remained unaltered even in the most heavily altered inclusions. Comparison of mineralogy before and after the alteration suggests that considerable amounts of Na, Fe, S and Cl were introduced into CAIs and some amounts of Ca and Mg were lost. Based on the amounts of nepheline, the relative degrees of alteration of CAIs are as follows : Y-81020<Y-82050<Y-790992. In order to compare the relationship between alteration of CAIs and thermal metamorphism on the parent body, metamorphic grades of the three CO chondrites were petrographically determined. The results indicate that the metamorphic grade increases in the same order as that of the degree of alteration of CAIs, i. e. Y-81020<Y-82050<Y-790992. Therefore, the results raise the possibility that alteration of CAIs may be related to thermal metamorphism which occurred on the meteorite parent body, although further investigation is still needed to verify this hypothesis

Yamato-82162: A new kind of CI carbonaceous chondrite found in Antarctica

A petrographic and mineralogical study of the Yamato-82162 (Y-82162) meteorite shows that it may be the first CI carbonaceous chondrite that has ever been found in Antarctica. Y-82162 consists largely of fine-grained, phyllosilicaterich matrix and contains a large amount of Ni-bearing pyrrhotite and small amounts of framboidal, platy, spheroidal, and anhedral magnetite, Mg-Fe-rich carbonates, and Ca-phosphate. Isolated clusters of coarsely crystallized phyllosilicates are also present. The presence of these minerals supports the idea that Y-82162 is a CI carbonaceous chondrite. However, this meteorite shows several mineralogical features that apparently differ from non-Antarctic CI chondrites : it has much higher abundances of coarse phyllosilicates and pyrrhotite than non-Antarctic CI chondrites, and has no veins of sulfates and carbonates, suggesting that it was derived from different primary materials and has experienced a different aqueous alteration history from non-Antarctic CI chondrites. The Y-82162 matrix also contains abundant fine grains of olivine, which contrasts with non-Antarctic CI chondrites. The textures suggest that matrix phyllosilicates were dehydrated and altered to olivine by heating. Thus, this meteorite probably has been affected by mild thermal metamorphism. These results indicate that Y-82162 has experienced a distinct late history from the non-Antarctic CI chondrites, suggesting that it may have been derived from a different source from that of non-Antarctic CI chondrites

Mineralogy and petrology of the CK chondrites Yamato-82104, Yamato-693 and a Carlisle Lakes-type chondrite Yamato-82002

We have studied the mineralogy and petrology of three metamorphosed chondrites, Yamato (Y)-82104,Y-693,and Y-82002,which were previously classified as C5,C4 and C5,respectively. The results indicate that Y-82104 and Y-693 should be included with the CK carbonaceous chondrites. Y-82104 and Y-693 are remarkably similar in mineralogy and texture (e.g., virtually identical compositions of olivines, Fa 29.2±0.4vs. Fa 29.4±0.7,respectively). Thus, they are probably paired. The compositional homogeneity of olivine and pyroxene and degree of recrystallization of olivine and plagioclase are consistent with Y-82104 and Y-693 being petrologic type 5. Y-82002 differs from most CK chondrites in mineralogy and petrology, but is similar to Carlisle Lakes-type chondrites which were recently proposed to be a new grouplet of chondrites (A. E. RUBIN and G. W. KALLEMEYN, Geochim. Cosmochim., 53,3035,1989). We believe that Y-82002 is a member of this grouplet. Y-82002 was previously classified as petrologic type 5,but the relatively inhomogeneous compositions of olivine (e. g., Fa 35.1±5.1 for chondrules) and low degree of recrystallization of olivine and plagioclase suggest that Y-82002 should be classified as petrologic type 3.8-3.9

A dark inclusion in the Manych LL (3.1) ordinary chondrite: A product of strong shock metamorphism

A dark inclusion (MNC-1) from the Manych LL (3.1) chondrite contains elliptical objects ("augen"; 50-500μm in long axis) embedded in a fine-grained matrix. Most augen are composed of parallel intergrowths of fine laths of Fe-rich olivine, high-Ca pyroxene, and Na-rich feldspathic glass that commonly enclose coarse grains (5-80μm in diameter) of Fe-rich olivine (Fo_). The matrix of MNC-1 consists predominantly of interlocking, granular grains (), interstitial high-Ca pyroxene and Na-rich feldspathic glass. Both the Manych host and MNC-1 have experienced strong shock metamorphism. The texture and mineralogy of MNC-1 suggest that the matrix of MNC-1 experienced shock melting, whereas the "augen" represent unmelted precursor material. Bulk major element composition of MNC-1 is generally similar to the average values for LL chondrites, except that MNC-1 is considerably depleted in S and enriched in FeO. The large S depletion may be due to volatilization during impact-induced melting. Texture and mineralogy of the augen are distinct from those of the Manych host, so MNC-1 may not be a melt dike which formed in situ in the host rock. MNC-1 appears to be a xenolith that solidified elsewhere on the LL chondrite parent body and was later incorporated to the present location

Yamato-86720: A CM carbonaceous chondrite having experienced extensive aqueous alteration and thermal metamorphism

A petrographic and mineralogical study of Yamato-86720 (Y-86720) shows that it may be a CM carbonaceous chondrite that has experienced a considerably different alteration history from most CM chondrites. Y-86720 has an unusually high abundance of troilite (～9 vol%); most troilite occurs in submicron to micron grains dispersed throughout the meteorite. Ca-Mg carbonates and minor Fe-Ni metal, mostly taenite, are present, but magnetite and PCP are absent. This meteorite contains chondrules and aggregates that were completely replaced by optically translucent materials, presumably phyllosilicates. Thus Y-86720 was probably extensively affected by aqueous alteration; the degree of alteration may be the highest of the CM chondrites. Matrix consists largely of fine grains of Mg-Fe olivine, a nearly amorphous Si-Mg-Fe-rich material, and an Fe-rich material; the latter may be ferrihydrite. Phyllosilicates are rare. The textures suggest that the olivine and the Si-Mg-Fe-rich material were produced by alteration of phyllosilicates by heating. The replacement products of chondrules and aggregates show lower (Mg+Fe)/(Si+Al) ratios than serpentine and relatively high Na contents (up to 2.0wt% as Na_2O), suggesting the presence of not only serpentine but another type of phyllosilicate. However, the replacement products show consistently high analytical totals; thus they may have been dehydrated and partially altered to anhydrous phases. These results suggest that Y-86720 has experienced mild thermal metamorphism after the aqueous alteration; it appears to have been heated above 500℃ in a reduced condition. Most fine-grained troilite probably segregated from matrix during the metamorphic process. Many of the troilites are replaced by a ferrihydrite-like material; thus, Y-86720 may have been affected by additional mild aqueous alteration after the thermal metamorphism

Mineralogical alteration of CM carbonaceous chondrites: A view

CM carbonaceous chondrites have been considerably affected by aqueous alteration, probably on the regolith of their parent body or bodies. The aqueous alteration resulted in the alteration of anhydrous silicates, metal, and sulfides, producing a complex mixture of Fe-Mg serpentines, Fe-Ni-S-O phase (tochilinite), and minor Fe-rich oxides and Fe-Ni sulfides. We here present a review of recent petrographic and mineralogical studies of CM carbonaceous chondrites and interpretations of their mineralogical alteration process

Fe-bearing phases in Antarctic carbonaceous chondrites Yamato-82162 and Yamato-86720: A Mossbauer study

We report the results of variable temperature Mossbauer spectroscopy measurements on samples of Yamato-82162 and Yamato-86720 carbonaceous chondrites. Transmission Mossbauer spectra were taken through the temperature range from 4.2K to 300K. The Mossbauer spectra at room temperature clearly indicate the presence of magnetic splits and quadrupole doublets in both meteorites, which can be attributed to troilite and olivine, respectively. The measurements also indicate the presence of a superparamagnetic component, probably ferryhidrite, in Y-86720 but not in Y-82162. These results are mostly consistent with those obtained by mineralogical studies. However, our Mossbauer measurements indicate the presence of a previously unknown magnetic component having a field of 250kOe in both meteorites; the mineral is unidentified so far. Our study supports the view that these meteorites were affected by thermal metamorphism, and that Y-82162 was less thermally metamorphosed than Y-86720