Mathematics – Logic
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.547m&link_type=abstract
Meteoritics, vol. 30, no. 5, page 547
Mathematics
Logic
2
Eucrites, Granulites, Howardites, Meteorites, Allan Hills 81011, Bialystok, Elephant Moraine 87503, Pasamonte, Petersburg, Yamato 7308, Yamato 74450, Yamato 790260, Zmenj, Rocks, Impact Melt
Scientific paper
Introduction: The bulk textures of HED meteorites indicate intensive post-igneous deformation, brecciation and thermal metamorphism [e.g.1,2]. Furthermore, the impact activity on the HED asteroid lead to the formation of impact melt rocks (IMRs; impact melts and clast-laden melt breccias), that are of special interest since their chemistry reflects the compositional variations of the various target materials. Additionally, some IMRs contain contaminations of chondritic projectile material, clearly indicating their impact origin [1,3]. The observation that several eucritic granulites are contaminated by chondritic material, as well, indicates that a certain fraction of these rocks are closely related to the suite of IMRs [1]. Samples and analytical techniques: Polished thin sections of 5 polymict eucrites (Pasamonte, Petersburg, Bialystok, Y-74450, Y-790260), three howardites (Zmenj, EET-87503, Y-7308) and the eucritic impact melt breccia ALHA-81011 [4] were studied by electron microprobe and optical and electron microscopy. Petrography and chemistry: The textures of the investigated clast-free and clast-bearing IMRs are highly variable, including devitrified glassy textures, quench textures characterized by skeletal pyroxene and plagioclase crystals and fine-grained subophitic textures. The variation of the melt compositions of some IMRs is shown in Fig. 1 (right). The chemical compositions follow a mixing line between eucritic lithologies and diogenites as it is observed for the bulk meteorites (Fig. 1, left). The compositions of IMRs either resemble the chemistry of the host breccias or are enriched in a eucritic component compared to host rock chemistry. We found an increasing frequency of diogenitic pyroxene fragments in the IMRs with increasing diogenitic component of the host breccia. IMRs richer in a diogenitic component than the host meteorite and pure diogenitic melt rocks were never found. Granulites (granulitic rocks and clast-laden granulitic breccias) were observed in all investigated samples, but Y-74450. These lithologies exhibit variable "basaltic" compositions, show significant variations in grain size, and in some granulitic clasts a pristine igneous texture (subophitic) is partially preserved. Conclusions: The fact that the melt compositions of IMRs follow the mixing line between eucritic lithologies and diogenites (Fig. 1) reflects impact-induced melting and mixing of these two lithological units. This observation clearly supports the validity of the two-component mixing model between eucritic lithologies and diogenites [e.g.5] derived from the observed mechanical mixture of these components in polymict HED breccias. Like lunar impact glasses [e.g.6], many HED IMRs appear to represent melts formed locally by impact-induced fusion of regolith material. Extended orthopyroxenites were never exposed at such crustal levels of the parent body where pure diogenitic whole-rock impact melts could have been formed. This could explain the lack of pure diogenitic IMRs in the investigated polymict HED breccias. Eucritic granulites mainly represent recrystallized portions of mechanically stressed basalts or recrystallized regions of clastic matrix breccias [1,2]. Additionally, a certain fraction of granulites represent recrystallized IMRs, indicated by the occurrence of chondritic projectile contaminations [1]. References: [1] Metzler K. et al. (1995) Planet. Space Sci., 43, in press. [2] Yamaguchi et al. (1995) LPS XXVI, 1531. [3] Delaney J. S. et al. (1982) in LPI Tech. Rpt. 82-02, 36. [4] Metzler K. et al. (1994) Meteoritics, 29, 502. [5] McCarthy T. S. et al. (1972), 86. [6] Delano et al. (1991) LPS XXII, 309.
Metzler Knut
Stäffler Dieter
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