Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.231h&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 231
Mathematics
Logic
5
Scientific paper
LEW88516 (LEW) is a 13-g stone recovered from the Lewis Cliff ice tongue by the 1988-89 Antarctic Search for Meteorites project. Its small size and relatively "ordinary" appearance relegated it to secondary status, and as a result it was not first described until 1991 (Mason, 1991). Like its near-twin ALH77005 (ALH), LEW is a maskelynite-bearing olivine cumulate rock, and a member of the SNC family (Lindstrom et al., 1992). LEW is texturally heterogenous with distinct poikilitic (pigeonite oikocrysts enclosing subhedral olivine and euhedral chromite) and nonpoikilitic lithologies. Olivine grains in poikilitic areas are smaller than those in nonpoikilitic areas and are generally not in contact with each other. Pigeonite oikocrysts are large (mm- sized) and show patchy exsolution near their rims, while non- poikilitic pigeonite shows similar exsolution but occurs as smaller subhedral grains and mixed-composition overgrowths among cumulus olivine and intersertal maskelynite. Chromite (commonly altered toward ulvospinel compositions) is widespread in both lithologies. Other accessory minerals include troilite, whitlockite, ilmenite, and chlorapatite, most prevalent in non- poikilitic interstitial areas. Heterogenous shock features are pervasive. This general description is equally valid for ALH and points out the strong petrographic similarity between these meteorites (McSween et al., 1979; Berkley and Keil, 1981; Jagoutz, 1989; Lundberg et al., 1990). Calculated 3-pyroxene temperatures for LEW cluster around 1150 degrees C, identical to the equilibrium temperature for ALH (Ishii et al., 1979). Measurements of ^10Be for LEW also indicate a calculated exposure age identical (within error) to that of the other shergottites (G. Herzog, pers. comm.; Bogard et al., 1984). ^26Al measurements are expected to yield similar results. Details of the mineral chemistry, however, reveal some interesting distinctions. Pyroxenes in LEW appear to be slightly more Fe-rich and more variable in composition, but at an extremely fine level of distinction (Fig. 1a). LEW olivine is of distinctly higher Fe- content than ALH; more than 80% of olivine grains in LEW have compositions more Fe-rich than any ALH olivine observed (Fig. 1b). There is also a strong dichotomy between poikilitic and non- poikilitic areas, with olivine in nonpoikilitic areas (avg. Fa 36%) being substantially more Fe-rich than poikilitic olivine (avg. Fa 31%). In contrast, ALH shows less than 2% difference in average Fa content between poikilitic and nonpoikilitic areas. These differences could be attributed to LEW having experienced slightly more near- or subsolidus equilibration after accumulation. LEW and ALH clearly are derived from the same lithologic unit on their parent body, but it is not so clear that they came from the same fall. REFERENCES: Berkley J.L and Keil K. (1981) Am. Mineral. 66, 1233- 1236. Bogard D.D., Nyquist L.E., and Johnson P. (1984) Geochim. Cosmochim. Acta 48, 1723-1739. Ishii T., Takeda H., and Yanai K. (1979) Mineral. J. 9, 460-481. Jagoutz E. (1989) Geochim. Cosmochim. Acta 53, 2429-2441. Lindstrom M.M, Mittlefehldt D.W., Treiman A.H., Wentworth S.J., Gooding J.L., Morris R.V., Keller L.P., and McKay G.A. (1992) Lunar Planet. Sci. (abstract) 23, 783-784. Lundberg L.L., Crozaz G., and McSween H.Y. Jr (1990) Geochim. Cosmochim. Acta 54, 2535-2547. Mason B. (1991) Antarctic Meteorite Newsletter 14:2, 19. McSween H.Y. Jr, Taylor L.T., and Stolper E.M. (1979) Science 204, 1201-1203. Figure 1, which in the hard copy appears here, shows pyroxene and olivine compositions from LEW88516 and ALH77005.
Harvey Ralph P.
McSween Harry Y. Jr.
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