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A chemical and petrographic study of refractory inclusions from Kaba (CV3) chondrite



Graduation date: 1990Refractory inclusions from Kaba (CV3) chondrite were studied by\ud instrumental neutron activation analysis (INAA) and electron probe\ud microanalysis (EPMA). The inclusions observed cover a wide range of\ud chemical patterns, mineral assemblages and textures, indicating the\ud complexity of early solar nebular conditions. The elemental\ud abundances normalized by Cl chondritic abundances vary smoothly as a\ud function of their volatilities, suggesting these inclusions were\ud formed by condensation/evaporization processes in the primeval solar\ud nebula.\ud The average REE pattern of Group III inclusions has an\ud enrichment of 3lxCl with approximately equal depletions of Eu and Yb\ud (Eu/Eu*=0.33, Yb/Yb*=0.29). Group III inclusions are irregularly-shaped\ud aggregates of rounded or sub-rounded melilite-rich nodules\ud rimmed with mono-mineralic layers of high-Al phase, Ti-Al pyroxene\ud and hedenbergite.\ud KI53 is an irregular aggregate. It shows a strongly fractionated\ud REE pattern, but differs from typical the Allende Group II pattern by\ud exhibiting normal Eu and Yb abundances. KI77 and KI88 inclusions consist of anorthite, diopside and\ud enstatite, with Si-rich phase in fine-grained areas. Their element\ud patterns suggest they were condensed from a gaseous environment rich\ud in moderately volatile elements.\ud The lower enrichment of Al, Ti and Ca is interpreted by the\ud partial vaporization of the inclusion by a transient event in an\ud oxidizing environment near the nebular midplane. This process caused\ud the partial loss of Al, Ti, Ca and more volatile elements. The rim is\ud the resultant vaporization residue. This model combines the\ud modification of refractory elemental pattern and the rim formation in\ud one event. The partial evaporization of a precursor inclusion with an\ud unfractionated REE pattern would form a rim where Yb and Eu are\ud strongly depleted in different degrees, and the interior is not\ud effected. A Group III pattern would be observed to be the summation\ud of two components, rim and interior.\ud The similarities between Kaba and Allende in whole-rock\ud composition and their inclusion mineralogies and textures suggest\ud they were formed in a generally similar nebular region. Kaba\ud inclusions are less altered by secondary reactions with a gas phase\ud at lower temperature, therefore informations of early solar nebular\ud conditions are better preserved

Year: 1989
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