Petrographic, isotopic, and chemical studies of cristobalite- and tridymite-bearing chondrules and clasts in ordinary chondrites

Details Petrographic, isotopic, and chemical studies of cristobalite- and tridymite-bearing chondrules and clasts in ordinary chondrites Petrographic, isotopic, and chemical studies of cristobalite- and tridymite-bearing chondrules and clasts in ordinary chondrites

Jul 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29r.448b&link_type=abstract

Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 448-449

Computer Science

Chondrites, Chondrule, Meteoritic Composition, Mineralogy, Oxygen Isotopes, Chemical Analysis, Chemical Composition, Rare Earth Elements, Trace Elements

Scientific paper

The cristobalite in chondrules and clasts is uniquely O-16 depleted for chondritic material. Six alpha-cristobalite-bearing chondrules from Parnallee (LL3.6), one alpha-cristobalite xenolith from Farmington (L5), and one tridymite-bearing clast from Parnallee have been studied. Silica polymorphs were identified by x ray diffraction (XRD). Some of these chondrules have been described briefly. The tridymite-bearing clast is larger, 1.6 cm diameter, and contains clusters of needlelike tridymite grains (40 modal%) enclosed by clinoenstatite (En(88-91.5)). The clinoenstatite shows extreme compositional zonation towards its margins, to Wo20, Fs(75.6), En(4.3). Minor plagioclase (An(71-83)) is present. Bulk compositions of the tridymite-bearing clast and two alpha-cristobalite-rich chondrules were obtained by averaging Electron Probe Microanalysis (EMPA) analyses of 280-300 points in arrays on polished sections. The assemblage protoenstatite or clinoenstatite enclosing cristobalite and tridymite crystallizes at cooling rates of 0.01-0.23 C/s in experimental charges of 65.1 wt% SiO2 (remainder MgO) from starting temperatures of around 1550 C. Typical chondrule cooling rates also lie within this range, suggesting that these samples originated through flash melting of SiO2-rich, alkali- and Rare Earth Element (REE)-depleted solids. During or shortly after the flash heating events, the cristobalite and tridymite exchanged O with an O-16-poor gas. High degrees of O diffusion from an ambient gas may be due to the open structure of tridymite and cristobalite. The Si-isotopic ratios (P7-CONCEPT ion probe), of two alpha-cristobalite-bearing chondrules lie on the terrestrial fractionation line, showing that the chondrules are not derived from an exotic Si reservoir. All the silica-bearing samples analyzed so far are plotted on an O three-isotope plot with o.c chondrules. A least-squares best-fit line of slope 0.76 is defined, showing a marked deviation from the equilibrate chondrite line (ECL) line of slope 1.0. The O-16-poor gas may be an end member at some extension of the cristobalite line (CRIL).

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