ELEMENTAL REDISTRIBUTION IN TIESCHITZ AND THE ORIGIN OF WHITE MATRIX

Two types of mesostasis coexist within some porphyritic chondrules in Tieschitz. One type is smooth. The other, confined to chondrule margin s, is blocky on a 5-10 mu m scale. Mesostases in one porphyritic olivi ne-pyroxene (POP) chondrule and one porphyritic olivine (PO) chondrule were analysed by scanning electron microscopy (SEM) and energy-disper sive x-ray spectrometry (EDS), as was white matrix nearby. Mesostases in the PO chondrule and in four others were analysed by ion probe. Pyr oxene phenocrysts or dendrites extend across contacts between smooth a nd blocky mesostasis with no compositional change. Relative to smooth mesostasis, blocky mesostasis is enriched in Al, alkalis, Ba, F, and C l but depleted in Si, Fe, and Ca, White matrix fills channels between the chondrules. It is physically and chemically similar to blocky meso stasis, but three ion probe analyses indicate that, unlike the mesosta ses, it is poor in Sc and has variable and fractionated rare earth ele ments (REEs). Smooth mesostasis is interpreted as solidified primary c hondrule liquid; whereas blocky mesostasis is its alteration product o r, less likely, a precipitate replacing smooth mesostasis leached out by aqueous fluid. White matrix may have formed by secondary alteration or replacement of mesostases that had been expelled from chondrules d uring accretion, or as a precipitate filling interchondrule voids. Iro n may have been lost from the bulk meteorite, but most other elements merely underwent internal redistribution. Disturbed isotopic systems i ndicate that aqueous fluid may have been active on the Tieschitz paren t body only 2 Ga ago. If correct, this would be the first evidence tha t an ordinary chondrite parent body underwent internal reprocessing si gnificantly later than 4.5 Ga ago.