Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.514g&link_type=abstract
Meteoritics, vol. 30, no. 5, page 514
Other
1
Aluminum 26, Chondrites, Enstatite, Chondrules, Aluminum-Rich, Chronology, Meteorites, Allan Hills 85119, Lewis Cliff 87220, Lewis Cliff 87234, Macalpine Hills 88136, Pecora Escarpment 91020, Plagioclase, Solar System, Early
Scientific paper
LEW 87234 (paired with LEW 87223, LEW 87057, and LEW 87220) is an unusual E3 chondrite with unique textural and compositional properties [1]. Although it was described as an anomalous EL3 with an unusual history [2], and it does have such characteristic features as only ~0.5% Si in metal and an EL3 size-frequency distribution of chondrules, LEW87234 also contains extraordinarily high abundances of several classes of objects that are rare or absent in EL3's. It is better classified as an ungrouped E3 chondrite. Among its unusual components are a wide variety of Ca-Al-rich objects, including chondrules that contain primary (igneous) calcic plagioclase, isolated plagioclase grains, and plagioclase-rich rims around metal grains; some of these occurrences were first noted by [3]. Besides being noteworthy features of this meteorite in their own right, these aluminous objects represent the first opportunity to obtain Al-Mg isotopic data on material from an enstatite chondrite, albeit an odd one. We found ~12 Al-rich chondrules in three thin sections of LEW 87234 by detailed x-ray mapping, all containing crystalline plagioclase. A large metal grain with a thick, plagioclase-rich rim was found. A large (~230 micrometers max.), isolated, triangular fragment of a single plagioclase crystal was also found in LEW 87234. Similar analyses of single thin sections of MAC 88136, ALH 85119 and PCA 91020, all EL3 chondrites, revealed only "normal" chondrules with aluminous mesostases (also present in LEW 87234), and no plagioclase-rich objects or rims. Petrographic analysis showed that LEW 87234 has 28 vol.% chondrules that are larger than 250 micrometers (in thin section), similar to the EL3 group mean (including EET 90299 and the 3 listed above) of 32 vol.%, but much higher than the EH3 mean of only 12 vol.% large chondrules (for Qingzhen, Kota-Kota, and ALHA 77295). Four plagioclase-rich chondrules from one thin section of LEW 87234 and the plagioclase fragment were analyzed. The chondrules range in size from ~170-310 micrometers in diameter. All are characterized by plagioclase laths that project inward from the rims of the chondrules, to varying lengths into the interiors. In some chondrules the laths are skeletal - even dendritic - in shape. Enstatite (En~100, and, in some cases, olivine; En~100) crystals are interstitial and subordinate in shape to that of the plagioclase laths. Plagioclase ranges in composition from An~60 to An~100; in some chondrules the larger individual crystals show optical zonation. The textures and mineral chemistry indicate unambiguously that the plagioclase is not only a primary igneous phase in these chondrules, but is the liquidus phase as well. Two of these chondrules and the plagioclase crystal fragment were analyzed for Al-Mg isotopes using the St. Louis ion microprobe. The chondrules show no resolvable excess 26Mg at 27Al/24Mg as high as ~230 (Fig. 1). The plagioclase crystal also contains no resolvable excesses, even at 27Al/24Mg as high as ~3000 (only <=400 shown on figure). These results are consistent with analyses of aluminum-rich chondrules in carbonaceous and ordinary chondrites, and which also show little or no evidence for the former presence of 26Al [5,6]. Despite clear evidence for shock metamorphism of LEW 87234, textural evidence suggests that the Mg isotopic signatures of the calcic plagioclase are primary. The chondrules (at least) probably formed in the nebula, probably much later than objects with evidence for live 26Al in other chondrite classes. References: [1] Grossman J. N. et al. (1993) Meteoritics, 28, 358. [2] Zhang Z. et al. (1995) JGR, 100, 9417-9438. [3] DeHart J. M. and Lofgren G. (1993) LPS XXIV, 387. [4] MacPherson G. J. et al. (1995) Meteoritics, 30, in press. [5] Hutcheon I. D. et al. (1994) LPS XXV, 587. [6] Sheng Y. J. et al. (1991) GCA, 55, 581.
Grossman Jeffrey N.
Hsu Weibiao
MacPherson Glenn J.
Zinner Ernst K.
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