Allende Dark Inclusions Affected by Different Degrees of Aqueous Alteration

Details Allende Dark Inclusions Affected by Different Degrees of Aqueous Alteration Allende Dark Inclusions Affected by Different Degrees of Aqueous Alteration

Sep 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.529k&link_type=abstract

Meteoritics, vol. 30, no. 5, page 529

Mathematics

Logic

5

Alteration, Aqueous, Inclusions, Dark, Metamorphism, Meteorites, Allende, Phyllosilicates, Veins

Scientific paper

A variety of dark inclusions (DIs) have been reported from Allende and other CV3 chondrites [e. g., 1-3]. It has been controversial whether they were formed in the solar nebula [3] or on the meteorite parent body [2]. Recent studies of some DIs [4, 5] revealed evidence suggesting that they experienced intense aqueous alteration and subsequent thermal metamorphism on the meteorite parent body. We here present the results of petrographic and scanning electron microscope studies of two DIs in Allende that are very different in mineralogy and texture from each other. One of the two DIs (16-S-1), which was previously studied by Fruland et al. [1] and Zolensky and Buchanan [6], contains abundant chondrules that are composed mainly of Mg-rich olivine and pyroxene, embedded in a fine-grained matrix. Although the general petrographic feature appears to be common to the CV3 chondrites, 16-S-1 shows several features indicative of secondary alteration. Mesostasis in most chondrules is filled with fine-grained aggregates of Si-, Al-, Ca-rich phases, which were probably formed by replacing glass. Mg-rich olivine and pyroxene, especially those adjacent to mesostasis and matrix, have Fe-rich rims. Fracture-filling veins mainly of Fe-rich olivine occur both in chondrules and matrix. As reported by Zolensky and Buchanan [6], Fe-rich olivine grains in some portions of the matrix and chondrule mesostasis show fibrous to acicular morphology, suggesting that they were produced by dehydration and thermal transformation of phyllosilicate. The textures suggest that 16-S-1 was affected by relatively minor aqueous alteration, so only the chondrule mesostasis and the edges of Mg-rich olivine and pyroxene were affected. The other DI (DN1) lacks chondrules, but instead contains rounded to oval-shaped inclusions (< 600 micrometers, mostly about 150 micrometers in diameter) that are composed mostly of fine grains (<1 to about 20 micrometers in diameter) of homogeneous Fe-rich olivine (Fo(sub)65 to about 68). Olivine grains in the inclusions commonly show fibrous morphology, suggesting pseudomorphs after phyllosilicate. Matrix is also composed mostly of Fe-rich olivine which is similar in composition to that in the inclusions. Fracture-filling veins abundantly occur around relatively large inclusions. The mineralogical and textural features suggest that DN1 was once involved in intense aqueous alteration, so that original chondrules were almost entirely replaced by phyllosilicate. Subsequently, it was dehydrated by mild heating and transformed to a nearly homogeneous aggregate of Fe-rich olivine grains. The present study reveals that both 16-S-1 and DN1 show evidence that they experienced aqueous alteration and subsequent thermal metamorphism on the meteorite parent body, although they are very different in texture and mineralogy. This suggests that such a sequence of secondary process is not unique to the DIs previously studied [4, 5] but is common to other DIs. The differences in mineralogy and texture of DIs are probably explained by different degrees of aqueous alteration in which each DI was involved before thermal metamorphism. The sequence of alteration was probably a common event that occurred near the surfaces of meteorite parent bodies. We believe that systematic studies of DIs would provide a more precise view regarding the evolution of carbonaceous chondrite parent bodies. References: [1] Fruland R. M. et al. (1978) Proc. LPSC 9th, 1305-1329. [2] Bunch T. E. and Chang S. (1983) LPS XIV, 75-76. [3] Bischoff A. et al. (1988) LPS XIX, 88-89. [4] Kojima T. et al. (1993) Meteoritics, 28, 649-658. [5] Kojima T. and Tomeoka K. (1994) Meteoritics, 29, 484. [6] Zolensky M. E. and Buchanan P. C. (1995) LPS XXVI, 1565-1566.

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