ORIGIN OF FAYALITIC OLIVINE RIMS AND LATH-SHAPED MATRIX OLIVINE IN THE CV3 CHONDRITE ALLENDE AND ITS DARK INCLUSIONS

We have studied an Allende dark inclusion by optical microscopy, scann ing electron microscopy, electron microprobe analysis and transmission electron microscopy. The inclusion consists of chondrules, isolated o livines and matrix, which, as in the Allende host, is mainly composed of 5-20 mu m long lath-shaped fayalitic grains with a narrow compositi onal range (Fa(42 +/- 2)) and nepheline. Olivine phenocrysts in chondr ules and isolated olivine grains show various degrees of replacement b y 5-10 mu m wide fayalitic rims (Fa(39 +/- 2)) and 100-1000 mu m wide translucent zones, which consist of 5-20 mu m long lath-shaped fayalit ic grains (Fa(41 +/- 1)) intergrown with nepheline. These fayalitic ol ivines, like those in the matrix of the dark inclusion, contain 10-20 nm sized inclusions of chromite, hercynite, and Fe-Ni sulfides. The fa yalitic rims around remnant olivines are texturally and compositionall y identical to those in Allende host, suggesting that they have simila r origins. Chondrules are surrounded by opaque rims consisting of tiny lath-shaped fayalitic olivines (<1-3 mu m long) intergrown with nephe line. As in the Allende host, fayalitic olivine veins may crosscut alt ered chondrules, fine-grained chondrule rims and extend into the matri x, indicating that alteration occurred after accretion. We infer that fayalitic olivine rims and lath-shaped fayalites in Allende and its da rk inclusions formed from phyllosilicate intermediate phases. This exp lanation accounts for (1) the similarity of the replacement textures o bserved in the dark inclusion and Allende host to aqueous alteration t extures in CM chondrites; (2) the anomalously high abundances of Al an d Cr and the presence of tiny inclusions of spinels and sulfides in fa yalitic olivines in Allende and Allende dark inclusions; (3) abundant voids and defects in lath-shaped fayalites in the Allende dark inclusi on, which may be analogous to those in partly dehydrated phyllosilicat es in metamorphosed CM/CI chondrites. We conclude that the matrix and chondrule rims in Allende were largely converted to phyllosilicates an d then completely dehydrated. The Allende dark inclusions experienced diverse degrees of aqueous/hydrothermal alteration prior to complete d ehydration. The absence of low-Ca pyroxene in the dark inclusion and i ts significant replacement by fayalitic olivine in Allende is consiste nt with the lower resistance of low-Ca pyroxene to aqueous alteration relative to forsteritic olivine. Hydrothermal processing of Allende pr obably also accounts for the low abundance of planetary noble gases an d interstellar grains, and the formation of nepheline, sodalite, salit e-hedenbergite pyroxenes, wollastonite, kirschsteinite and andradite i n chondrules and Ca,Al-rich inclusions.