Unusual chondrules in the Mbale ordinary chondrite

Details Unusual chondrules in the Mbale ordinary chondrite Unusual chondrules in the Mbale ordinary chondrite

Jul 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29q.496m&link_type=abstract

Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 496

Mathematics

Logic

Chondrites, Chondrule, Metamorphism (Geology), Meteoritic Composition, Chromium, Mineralogy, Petrology, Spinel, Volatility

Scientific paper

Four chondrules with properties that distinguish them from the usual ferromagnesian chondrules have been found in the Mbale, Uganda, chondrite; three are dominated by chromite-rich and chromian spinel-rich phases and a fourth by an SiO2 phase. These chondrules are characterized by clearly defined visual chondrule boundaries, which is unexpected given the chondrite petrologic type (L6). Sharp chondrule boundaries appear to have remained due to the unique mineralogy of these chondrules, which enabled them to resist the effects of metamorphism and maintain their shape. Chondrule A is spherical in shape, about 850 microns in apparent diameter and black in color. Chondrule B is black in color, spherical, and about 1775 microns in apparent diameter. Chondrule C is spherical and about 3.0 mm in apparent diameter. Chondrule matrix is black in color and surrounds a conspicuous 800 x 900-microns, white, subhedral Ca-phosphate crystal. Chondrule D is ovate in shape with dimensions of approximately 4.5 x 3.5 mm. A 0.6-mm-thick, pale-green, fibrous, orthopyroxene rim is the most prominent characteristic of this chondrule. The current consensus regarding chondrule formation involves melting of preexisting dust by a transient heat source. However, phases such as chromite, chromian spinel, and SiO2 are not predicted to form via equilibrium condensation in the solar nebula. Volatile fractionation, which could produce a refractory-rich precursor and a volatile-rich precursor, or condensation under highly oxidizing, nonequilibrium conditions may be responsible for producing the chromite-rich chondrules. The small chromite grains may, however, result from the breakdown of a Cr-rich silicate to plagioclase and chromite during parent-body metamorphism. Chondrules A and B occur on the same thin section. Chondrules C and D were found in the same sample about 1 cm apart and represent two very different chondrule types.

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