SHOCK EFFECTS AND PETROLOGICAL FEATURES OF THE OHABA CHONDRITE

Author:OVIDIU GABRIEL IANCU

Co-authors:Yasunori Miura, Gero Kurat, Franz Brandtstaetter

Abstract/Description:
The Ohaba meteorite fell in October 11, 1857, in Transylvania (Alba District, Romania). After appearance of a fireball, followed by detonations, a stone of 16.25 kg was found by a priest (Graham et al., 1985). The fragments recovered are kept in 16 museums from 11 countries. The Museum of Natural History from Vienna is the repository of the main mass (15.73 kg). The meteorite was previously classified as an H5 veined ordinary chondrite, based on olivine composition â€" Fa20 published by Mason (1963).
Two polished thin sections representing the Ohaba chondrite were studied under the microscope in both transmitted and reflected light. In order to determine the shock degree of this chondrite, in every thin section, ten to twenty of the largest, randomly distributed olivine single crystals and the largest plagioclase grains were examined with 20x- or 40x- objectives (cf. StÃ¶ffler et al., 1991) and with the electron microprobe. Quantitative chemical analyses of the constituent minerals were obtained on the carbon-coated, polished thin sections by using a JEOL JSM-6400 scanning electron microscope at the Museum of Natural History from Vienna. The instrument was operated at an accelerated voltage of 15 kV, a 38.5 nA beam current and 39 mm working distance. About 30 points of both olivine and orthopyroxene were measured on each thin section.
The microscopic study in thin sections revealed the presence of the following types of chondrules in the Ohaba chondrite: readily distinguished PO â€" porphyritic olivine, RP â€" radial pyroxene, BO â€" barred olivine (sometimes polysomatic) and GOP â€" granular olivine-pyroxene chondrules, ranging in size from 300 μm to up 1200 μm. The matrix is recrystallized, the feldspar (An15Or3) occurs in grains smaller than 50 m and the igneous glass is absent. Pyroxenes are mainly orthopyroxenes but less than 10% of the grains are clinopyroxenes with Wo37 (mole percentage), which is similar to diopside composition. Other minerals identified in polished thin sections are kamacite, troilite, taenite, plessite, cromite and magnetite.
Based on petrographic data, the Ohaba chondrite is classified as petrologic type 5, consistent with the data printed in Graham et al. (1985).
Analysis of 15 olivine grains from two thin sections shows a variation in composition from Fa14 to Fa15 mole% fayalite (avg. Fa15; PMD 2.2%). Twelve orthopyroxene grains from thin sections show a range in composition from Fs13 to Fs16 mole % ferrosilite (avg. Fs14; PMD 6.06%).
Taking into account the iron-content of olivine plotted against iron-content of orthopyroxene (Keil and Fredriksson, 1964), Ohaba meteorite may be considered as an ordinary chondrite â€" H5, belonging to the primitive meteorites class (Fig. 1), consistent as well with the data published by Graham et al. (1985).
In order to determine the mineralogical effects of shock metamorphism in the Ohaba meteorite, ten to twenty of the largest, randomly distributed olivine single crystals were examined in every thin section by optical polarizing microscope with 20x- or 40x- objectives (cf. StÃ¶ffler et al., 1991) and with a JEOL JSM-5400 scanning electron microscope for higher magnifications. Because olivine shows planar fractures (Fig 2), undulatory extinction and irregular fractures and plagioclase has also undulatory extinction, the shock degree of this chondrite may be estimated as S-3, weakly shocked.
This work was supported in part by a Grant from the Romanian National University Research Council.

References: Graham, A.L., Bevan, A.W.R., Hutchison R. (1985) Catalogue of meteorites; with special reference to those represented in the collection of the British Museum (Natural History), 4th ed., British Museum (Natural History), London, 460. Mason B. (1963) Olivine composition in chondrites. Geochimica et Cosmochimica Acta, 27, 1011-1023. StÃ¶ffler D., Keil K., Scott E. R. D. (1991) Shock metamorphism of ordinary chondrites, Geochim. Cosmochim. Acta, 55, Pergamon Press. Keil K., Fredriksson K. (1964) The iron, magnesium and calcium distribution in coexisting olivines and rhombic pyroxenes in chondrites. J. Geophys. Res., 64, 3487-3515.

Poster presented at:
First International Conference on Impact Cratering in the Solar System - 2006

Website link:http://mail.uaic.ro/~ogiancu/