Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.588t&link_type=abstract
Meteoritics, vol. 30, no. 5, page 588
Other
4
Aqueous Alteration, Chondrites, Carbonaceous, Cv, Meteorites, Allende, Phyllosilicates
Scientific paper
Aqueous alteration is an important process that prevailed in the early solar system. In order to better understand the aqueous alteration processes and conditions, we embarked in hydrothermal alteration experiments of the Allende CV3 chondrite. We here present the first results of our experiments. Samples of Allende were sealed in gold tubes with 1-N HCl and heated in a reactor vessel at 450 degrees C and 800 bars for 4 to 6 weeks. Thin sections were made from the run products and were observed and analyzed by a scanning electron microscope equipped with an EDS spectrometer. Our experiments produced remarkable alteration textures in the Allende chondrite. Parts of internal areas of most of the chondrules are replaced by brownish-to-greenish phyllosilicate, exhibiting an appearance closely similar to the altered ("spinach"-bearing) chondrules in CM chondrites. Fractures and interstices in chondrules and matrix, up to 20 micrometers in width and 1 mm in length, are filled with phyllosilicate, producing remarkable veins similar to those observed in CI chondrites. Mainly two kinds of phyllosilicates were produced. One contains major Mg and Fe and has compositions similar to Fe-rich saponite. This is the most common phyllosilicate that occurs in both chondrules and matrix; it is primarily formed by replacing olivine and low-Ca pyroxene. The other kind of phyllosilicate contains major Mg and Fe and variable Al; it is probably a mixture of two phases, so we tentatively call it high-Al phyllosilicates (HAP). HAP occurs mainly in chondrule mesostasis, where it is formed by replacing mesostasis glass. Chondrules and aggregates are altered from their edges to inward. Olivine in outer areas of chondrules and aggregates are enriched in Fe, and most of individual olivine grains show strong Fe-Mg zoning, indicating substantial Fe was added from matrix to chondrules. Mesostasis in central areas is preferentially replaced by HAP, while that in outer areas is replaced by saponite. There are remarkable differences in degree of alteration by phyllosilicates among the precursor minerals in chondrules. The resistance to alteration increases in the order, glass, low-Ca pyroxene, olivine. Aggregates of Fe-rich saponite ranging in diameter from 10 to 50 micrometers are produced in places in matrix. However, most of small original olivine grains (1 to 10 micrometers in diameter) still remain unaltered. This is surprising to us, taking into account the fact that chondrule internals were altered to the extent up to 200x200 micrometers^2 in area. This indicates that the alteration proceeds much faster in chondrule mesostasis than in the fine-grained matrix. Drastic chemical exchanges took place between chondrules and matrix. Among major elements, Fe was added from matrix to chondrules mainly as an olivine component, whereas Mg, Al, Ca, Na and S were lost from chondrules. In chondrule mesostasis, Ca is almost completely lost, confirming that the mobility of Ca is particularly high. As a result of these elemental exchanges, the meteorite became homogenized as a whole. The texture and the phyllosilicate mineralogy produced in the altered Allende samples are closely similar to those observed in the Mokoia and Kaba CV3 chondrites [1,2]. These similarities suggest that the alteration in Mokoia and Kaba occurred in situ on the CV parent body under conditions somewhat related to the present experimental condition, although the pressure and temperature should have been much lower than those used in our experiments. References: [1] Tomeoka K. and Buseck P. R. (1990) GCA, 54, 1745-1754. [2] Keller L. P. and Buseck P. R. (1990) GCA, 54, 2113-2120.
Kojima Takeo
Tomeoka Kazushige
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