Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.213d&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 213
Other
Scientific paper
The following article is a preliminary commentary on the petrographic characteristics found in our investigation of the recently discovered Esperance chondrite. The material on which this study is based consists of a thin section made from the near-surface portion of the meteorite, combined with a small sample displaying its exterior features. A thin, dull, dark brown fusion crust is present on one side of the small specimen, while the remainder of the sample shows a highly weathered and irregular interior surface with some cracking and numerous exposed chondrules and cavities. In thin section, however, the weathering effects are limited and mainly restricted to a superficially light brown staining on silicates and around metal, close to the penetrating fractures that traverse the sample. These fractures are filled with oxidation products and are a continuation of those seen in the hand sample. The thin section shows a close-packed aggregate of sharply defined chondrules and chondrule fragments set in a dark opaque matrix. A great variety of chondrule types are present with sizes up to approx. 3 mm in diameter, ranging in form from perfectly spherical to rather irregular shapes. Several examples of chondrules within chondrules as well as rimmed, armored, and dark-zoned varieties are well-displayed features seen in thin section. Other prominent characteristics are transparent pale brown glass within many chondrules and abundant polysynthetically twinned pyroxenes and zoned olivines. Metal alloy and iron sulphide, the dominant opaque mineral phases, occur within chondrules, as chondrule rims or in the matrix. Although they occupy the same mode of occurrence they are rarely seen in intimate intergrowth. A large fraction of the metal consists of several up to mm sized interstitial grains. The remaining metal and the iron sulphide both occur finely dispersed throughout the section associated with small but abundant chromite and phosphate grains and rare ilmenite. Chromite is also found within chondrules associated with both olivine and pyroxene. Within the matrix, following a rather straight narrow zone, numerous small globules of iron sulphide and metal result in thin, more or less interconnecting veins. These opaque veins are exclusively confined to matrix regions, although small globules are often seen in connection with aggregates that make up opaque chondrule rims. Quite often small vein-like strings are seen to occur in the immediate vicinity of large interstitial opaque grains. These rapidly thin out in abundance and size, a short distance away from the larger grains. Locally melt pockets are evident, either as completely isolated enclaves or as junctions between melt veins. Both melt pockets and the thin veins can be distinguished from the dark matrix by their slightly lighter colour. These features, combined with many of the larger olivine crystals showing sets of parallel planar fractures and undulatory to mosaic extinction, indicate a degree of shock deformation. According to the petrographic classification of progressive stages of shock metamorphism by Stoffler, Keil, and Scott (1991), the Esperance can at present be placed in the catergories of weakly to moderately shocked chondrites (S3-S4). A microprobe investigation of the silicate fraction of the meteorite has revealed a pronounced chemical zoning of both olivines and pyroxenes in many chondrules. Measured Fa variations in single olivine crystals have, in the most extreme cases, been found to range from Fa=5,9% in the core, to Fa=25,8% at the rim. The highest Fa content encountered at present is 27,6% in a homogeneous matrix crystal. A frequency plot of rim and matrix olivine Fa contents, groups around a peak of 25%. The chemical composition of Fs contents in pyroxenes are seen to range continuously from 0,6% to 29,2%. Several low-Ca pyroxenes show a notable Fe-Mg zoning that, however, does not exceed approx. 10% difference from core to rim. Frequently chemical compositions consistent with augite, endiopside and pigeonite are seen as overgrowths on the low-Ca pyroxenes, but can also been seen as discrete crystals. Also encountered, are examples of low-Ca pyroxenes showing Fe-Mg heterogeneity as described by Tsuchiyama et al. (1988). The above-mentioned data indicates that this meteorite belongs among the unequilibrated ordinary chondrites. It is probable that it belongs to the subgroup 3.4. Its specific petrographic type is, however, uncertain at present, as the preliminary data reveal that it lies close to the border of L/LL chondrites. A continued research program has been initiated and includes, among other things, a bulk analysis, thermoluminescence, and continued microprobe investigations. Modal and textural classification in vol%. PO/POP PP BO RP Gran. Comp. Metal Sulp.etc. Matrix & Fragments 20 6,5 6 10 3 15 2,5 7 30 References: Stoffler D., Keil K., Scott E. R. D. (1991) Geochim. Cosmochim. Acta, 55, 3845-3867. Tsuchiyama A., Fujita T., Morimoto N. (1988) Proc. NIPR Symp. Antarct. Meteorites, 1, 173-184.
Davis Edward D.
Petersen Anna Katinka
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