Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.278p&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 278
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3
Scientific paper
A selection of irregular particles of the 50-100 micrometer size fraction of the 1991 EUROMET micrometeorite (MM) collection from the Antarctic ice sheet near Cap Prudhomme (1,2) has been investigated by optical microscopy, SEM, and EMP. Two polished mounts (91/1 and 91/2) contain 198 particles of which 143 (72%) are of extraterrestrial origin. Of these 65 (45%) are almost unmelted, phyllosilicate-dominated and 22 (15) are anhydrous crystalline. Of the phyllosilicate-bearing MMs 24 (37%) contain anhydrous silicates. In addition 22 partially melted scoriaceous particles contain anhydrous primary silicates. The unmelted phyllosilicate-bearing MMs contain mainly low-Ca pyroxene whereas the scoriaceous and crystalline particles contain olivine and pyroxene in about equal quantities. Olivines: FeO-contents range from 0,23 to 47 wt% but cluster between 1 and 7%. Most olivines contain appreciable amounts of CaO (cluster around 0,2 wt% Cr2O3 (up to 0,7%), MnO (mostly 0,05- 0,4%), and NiO (cluster at ~0,04%?. They are mostly poor in Al2O3. The FeO/MnO ratios are usually below 50 except for some FeO-rich olivines. NiO-rich olivines coexist with plagioclase and magnetite in one crystalline MM. Pyroxene in Antarctic MMs is typically enstatite. Its FeO content ranges from 0,2 wt% to 20% and it is generally rich in minor elements (Ca, Al, Cr, Mn). Other phases: Spinels have been found in three MMs. Low-Cr spinel (which encloses perovskite), Cr-bearing spinel (~2,8% Cr2O3), and a high-Cr (~25% Cr2O3) variety of hercynite (picotite). Chromites are present in scoriaceous and crystalline particles. Plagioclase is usually Ca-rich (An 43-70), K-feldspars are occasionally present (intergrown with plagioclase). Magnetite is very common as a secondary phase. Primary magnetites with morphologies typical for CI-chondrites (framboids and plaquettes) are usually poor in minor elements (3). Metal is present in 11 MMs (1 unmelted, 2 scoriaceous, and 8 crystalline). It is typically Ni-poor (~5 wt%). Sulfides (pyrrhotite and pentlandite) are fairly common in all types of MMs. Discussion and conclusion: The anhydrous silicate component of the phyllosilicate-bearing MMs is dominated by enstatites that are rich in minor elements (as are the olivines) covering the range observed in silicates from CMs, UOCs, and IDPs (4-8). However, the very low olivine/pyroxene ratio, the elevated FeO contents of the silicates, and the total lack of "blue," Ca, Al-rich olivines in MMs clearly set them apart from any known meteorite class. The closest match could probably be the unique chondrite Kaidun (4,9). Some CI-like particles (3) as well as one CK-like one are present. References: (1) Maurette, M. et al. (1992) LPSC XXIIl 859. (2) Maurette, M. et al. (1992) Meteoritics, in print. (3) Kurat, G. et al. (1992) LPSC XXIII, 747. (4) Brandstatter, F. et al. (1992) this volume. (5) Steele, I.M. (1986) GCA 50,.1379. (6) Steele, I.M. (1990) Meteoritics 25, 301. (7) Klock et al. (1989) Nature 339, 126. (8) Brandstatter, F. et al. (1991) Europ. J. Min., Beih. 3/1, 40. (9) Ivanov, A.V. et al. (1989) Geochim. Internat.. 26, 84.
Kurat Gero
Maurette Michel
Presper Th.
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