ALHA 81011 -- an eucritic impact melt breccia formed 350 m.y. ago

Details ALHA 81011 -- an eucritic impact melt breccia formed 350 m.y. ago ALHA 81011 -- an eucritic impact melt breccia formed 350 m.y. ago

Jul 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29r.502m&link_type=abstract

Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 502-503

Physics

3

Breccia, Ejecta, Hypervelocity Impact, Lithology, Radioactive Age Determination, Degassing, Neutron Activation Analysis, Phase Transformations, Recrystallization, Scanning Electron Microscopy, Textures

Scientific paper

The ALHA 81011 meteorite has been described as a eucritic breccia consisting of mineral and lithic clasts embedded in a vesicular, dark glassy matrix. Lithic clasts are equilibrated and dominated by subophitic and granulitic texture, frequently with gradual textural transitions in a given clast. Both mineral and lithic clasts were shocked in excess of approximately 30 GPa, transforming plagioclase into maskelynite, followed by thermally induced recrystallization. The observation that plagioclase fragments are 'swirled' into the dark matrix leaving pyroxene fragments unaffected, indicates that the plagioclase fragments were transformed into maskelynite prior to admixing as well. Scanning Electron Microscopy (SEM) investigations revealed that the dark matrix represents a quenched melt with eutectic fabric consisting of parallel intergrowths of pyroxene and plagioclase crystals, interspersed with small vesicles and larger subangular cavities up to 0.6 cm. One basalt clast with a partly granulitic texture and a portion of the dark crystallized matrix were separated and analyzed by Instrumental Neutron Activation Analysis (INAA). We performed age determinations on the separated lithologies by applying the Ar-40/AR-39 method. ALHA 81011 represents a clast-rich eucritic impact melt breccia not older than 350 Ma. It was either part of a rapidly cooled larger impact melt formation or represents a melt 'bomb' that originates from a suevitic ejecta blanket formed by a large-scale impact on the Howardite Eucritic and Diogenite (HED) parent body surface.

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