Mathematics – Logic
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..319b&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 319
Mathematics
Logic
6
Acfer 277, Achondrites, Antarctic Meteorites, Meteorite Classification, Orthopyroxene, Ungrouped Meteorites, Ureilites
Scientific paper
Three ureilite specimens in the EUROMET collection have been studied by mineralogical techniques as part of the consortium studies. Grady and Pillinger [1] reported a preliminary result on carbon and nitrogen geochemistry. Their carbon and nitrogen data for FRO90054 serve to reinforce the belief that this meteorite, if it is a ureilite, is most unusual. They gave the bulk carbon content of 0.24 wt%. Our mineralogical data also show intermediate nature between augite-bearing lodranites [2] and magnesian orthopyroxene-augite ureilites [3]. A PTS (13 X 8 mm in size) of FRO90054,007 shows a coarse-grained equigranular texture. FRO90054 contains considerable orthopyroxene (Opx) and augite (Aug). Opx crystals reach up to 2 X 1.5 mm in size, olivine up to 2 X 1.5 mm, and Aug up to 3 X 2 mm. Olivines have parallel fractures, indicating minerals were shocked. The Opx crystals poikilitically enclose rounded olivine and Aug and show good cleavage. The silicates contain very fine- grained dusty inclusions. The modal abundances (olivine 23 vol.%, Aug 22 %, Opx 48 %, troilite and opaque vein 7 %) show that this PTS is rich in Opx more than Aug. Carbonaceous matrices at grain boundaries are smaller than other ureilites. The compositions of olivine (Fa(sub)12) is more Mg-rich than common ureilites. The Aug (Ca(sub)38Mg(sub)56Fe(sub)7) and Opx (Ca(sub)4.8Mg(sub)84Fe(sub)11.2) compositions give an equilibration temperature of 1245 degrees C [4]. Some Opx at the rim or small grains show higher Fe concentrations. A PTS (7 X 6 mm in size) of FRO90036,008 shows heavily shocked textures of olivines and pigeonites. The olivine crystals were converted into mosaics of tiny grains (Ca 0.03 mm in diameter). The mineral chemistry (Fa(sub)12 to Fa(sub)23) and textures are similar to those observed in LEW86216 [5]. The PTS (8 X 6.5 mm in size) of ACF277 shows a coarse-grained texture with elongated crystals. The modal abundances of silicates are: olivine 45 vol% and pigeonite 55%. The compositions of olivine (Fa(sub)21) and pigeonite (Ca(sub)9.7Mg(sub)73.2Fe(sub)17.1) are uniform. This is a typical ordinary ureilite. Ordinary ureilites have been said to be olivine-pigeonite achondrites such as ACF277. However, recently Opx-bearing ureilites [3] and Aug-bearing one [6] have been found. According to the mineral compositions, Opx-bearing ones are magnesian ureilites [3]. Coexisting Opx and Aug are known for a magnesian ureilite (LEW85440). FRO90054 can be classified as Opx-Aug- bearing ureilite of the above group if it is a ureilite, because the FRO90054 Opx compositions are located at the Fe-rich extension of the ureilite Opx trend and at a little above the lodranite Opx. The fact that some of the magnesian ureilites and lodranites contain Opx as a major phase implies that these two groups of meteorites are more closely related in their classification. The mineral assemblage of FRO90054 is rather similar to Aug- bearing lodranite, MAC88177 than other ureilites. If the oxygen isotope ratios indicate otherwise, FRO90054 would be classified as a Aug-bearing lodranite such as MAC88177 with a little metal, because it consists of Opx, Aug and olivine with similar Mg/Fe ratios [2]. Only evidence indicating affinity to ureilites is that the CaO concentration of olivine (0.30 wt%) is within the range of ureilites and that the chromite has not been found. Although they have different oxygen isotope trends [7] and interstitial grain boundary materials, ureilites and lodranites may have formed from different source materials, but the formation mechanism may be similar. We thank EUROMET for the meteorite samples, and Drs. T. Ishii and H. Yoshida for their technical assistance. References: [1] Grady M. M. and Pillinger C. T. (1993) LPS XXIV 551-552. [2] Takeda H. et al. (1992) Proc. Japan Academy 68B, 115-120. [3] Takeda H. (1989) EPSL, 93, 181-194. [4] Ishii T. et al. (1979) Min. J., 9, 460-481. [5] Saito J. and Takeda H. (1990) Proc. NIPR Symp. Antarct. Meteorites, 3, 132-146. [6] Takeda H. et al. (1989) Meteoritics, 24, 73-81. [7] Clayton R. N. et al. (1992) LPS XXIII, 231-232.
Baba Takayasu
Saiki Koichiro
Takeda Hidenori
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