Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.213d&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 213-214
Other
1
Scientific paper
LEW88516 (LEW) is a shergottite, very similar to ALHA 77005 (ALH). Both meteorites contain 58-59% (vol.) olivine, 12-13% pigeonite, 10-15% augite, 15-12% plagioclase, 0.7-0.9% oxides, 1.7% phosphate, with traces of sulfide and silica minerals. (LEW88516,19 has slightly more augite and less plagioclase than ALH77005.) Mineral compositions are similar but not identical. LEW is more Fe-rich than ALH. Olivine in ALH77005 is typically Fo72-75 while that in LEW88516 is Fo65-70. Texturally LEW88516 has large pheno/xenocrysts (1-2+mm) of olivine in an olivine basalt matrix (50-500 micrometer grains), and is compatible with early crystallizing olivine pheno/xenocrysts being incorporated into a magnesian basalt. Pheno/xenocryst olivine in LEW also shows slightly normal zoning (Fo68-65) whereas olivine in ALH appears to be unzoned. Pyroxene compositions in LEW are also consistently about 3-5% (mol.) more Fe-rich than those in ALH. The compositions of the mafic phases in LEW overlap those of Chassigny. In ALH, the feldspathic phases are lathy maskelynite with more calcic plagioclase rims believed to have formed by thermal recrystallization of the felsic glass. In LEW, however, maskelynite is the only felsic phase (An56-45). MicroXANES measurement of the LEW maskelynite, suggests it contains more Fe^3+ than the ALH maskelynite. (The role of Antarctic weathering of such small meteorites remains to be assessed.) Almost identical assemblages of Ca-rich pyroxene, plagioclase composition glass, silica, phosphate, titan-chromite/ilmenite and sulfide occur in melt inclusions in the larger olivines of both meteorites. In LEW, the feldspathic phase from some these inclusions is slightly more K-rich and the inclusions contain more phosphate than ALH. The strong similarity of the two objects suggests, however, that they are very closely related melagabbros/peridotitic assemblages, but they are not identical. Compositional similarities imply that they may be from the same mafic complex and may have been physically close to each other on the parent body. The differences in the plagioclase physical states also imply slightly different thermal histories. The recrystallization of ALH maskelynite to crystalline feldspar, requires a longer post-shock thermal history than was experienced by LEW. If ALHA77005 was originally deeper than LEW88516 in a mafic complex that was impacted to produce the maskelynite, the slight differences in Fe/(Fe+Mg) and the slower cooling of ALH would be consistent. Simultaneous ejection of both objects by the same event that placed them in earth crossing orbit could well deliver them to the relatively close Lewis Cliffs and Allan Hills sites. The large olivines often have the alkali/phosphate-rich melt inclusions. The abundant olivine of the sample presumably reflects a cumulate history. The olivine of Chassigny is identical in composition to that of LEW88516, but its mesostasis is more alkalic. However, a more olivine rich cumulate precipitating from the LEW/ALH magma source might well resemble Chassigny. Chassigny contains much more augitic pyroxene than these shergottites, but the presence of very calcic cpx (Wo45) in melt inclusions in the phenocryst olivines indicates that the large early olivines precipitated from a magma capable of evolving to much more augitic endpoint than is represented by the shergottites. Indeed the inclusions in LEW88516 are fairly similar to melt inclusions in Chassigny olivine. LEW88516 is, therefore, similar to ALHA 77005 but not identical to it. The mafic minerals are slightly more iron rich and match those of Chassigny very closely. The texture suggests an early olivine cumulate-forming event that might lead to the formation of Chassignite-like lithologies and indeed melt inclusions in these early olivines are very comparable to those in Chassigny.
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