Textural variations and impact history of the Millbillillie eucrite

Details Textural variations and impact history of the Millbillillie eucrite Textural variations and impact history of the Millbillillie eucrite

Mar 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29..237y&link_type=abstract

Meteoritics (ISSN 0026-1114), vol. 29, no. 2, p. 237-245

Computer Science

23

Achondrites, Breccia, Meteoritic Composition, Textures, Basalt, Grain Size, Mineralogy, Meteorites, Texture, Impact Effects, Millbillillie, Eucrites, Age, Samples, Meteorite, Laboratory Studies, Mineralogy, Epma, Sem, Eds, Composition, Microprobe Methods, Electron Microscopy, Equilibration, Chemistry, Origin, Formation, Chronology, Stony Meteorites, Isotopes, Argon Dating, Achondrites, Matrix, Isotopic Ratios, Brecciation, Impact Effects

Scientific paper

We have investigated 10 new specimens of the Millbillillie eucrite to study its textures and mineral compositions by electron probe microanalyser and scanning electron microscope. Although originally described as having fine-grained texture, the new specimens show diversity of texture. The compositions (Mg/Fe ratios) of the host pigeonites and augite lamellae are homogeneous, respectively, in spite of the textural variation. In addition to their chemical homogeneity, pyroxenes in coarse and fine-grained clasts are partly inverted to orthopyroxene. Chemical zoning of plagioclase during crystal growth is preserved. This eucrite includes areas of granulitic breccias and impact melts. Large scale textures show a subparallel layering suggesting incomplete mixing and deposition of impact melt and lithic fragments. An Ar-39-Ar-40 age determination for a coarse-grained clast indicates a strong degassing event at 3.55 +/- 0.02 Ga. We conclude that Millbillillie is among the most equilibriated eucrites produced by thermal annealing after impact brecciation. According to the classificcation of impact breccias, Millbillillie can be classified as a mixture of granulitic breccias and impact melts. The last significant thermal event is characterized by network-like glassy veins that run through clasts and matrices. Consideration of textual observations and requirements for Ar-degassing suggests that the Ar-39-Ar-40 age could in principle date either the earlier brecciation and annealing event or the event which produced the veins.

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