MINERALOGICAL AND CHEMICAL MODIFICATION OF COMPONENTS IN CV3 CHONDRITES - NEBULAR OR ASTEROIDAL PROCESSING

Calcium- and aluminum-rich inclusions (CAIs), chondrules, dark inclusi ons and matrices in certain CV3 carbonaceous chondrites appear to have been modified by different degrees of late-stage alteration processes that caused significant variations in mineralogy and chemistry. Some chondrules and CAIs are rimmed with fayalitic olivine. Metal in all co mponents may be oxidized and sulphidized to magnetite, Ni-rich metal a nd sulfides. Silicates in all components are aqueously altered to diff erent degrees to phyllosilicates. Primary minerals in some CAIs experi enced Fe-alkali-halogen metasomatism forming nepheline, sodalite, woll astonite, hedenbergite and other secondary minerals. In CV3 chondrites with metasomatized CAIs, nepheline, sodalite, etc. are also present i n chondrule mesostases and in matrices. McSween's (1977b) reduced subg roup of CV3 chondrites generally shows minimal alteration of all compo nents and may represent the unaltered precursors for the oxidized CV3 chondrites, which generally show major alteration. Most studies have b een focused on specific components in CV3 chondrites and have not cons idered possible relationships between alteration processes. We infer f rom the correlated occurrences of the alteration features that they we re closely related in time and space and review nebular and asteroidal models for their origins. We prefer an asteroidal model.