Chemistry of glass inclusions in olivines of the CR chondrites Renazzo, Acfer 182 and El Djouf 001.

MARIA EUGENIA VARELA; KURAT GERO,; HOPPE PETER,; BRANDSTAETTER FRANZ,

GEOCHIMICA ET COSMOCHIMICA ACTA

PERGAMON-ELSEVIER SCIENCE LTD

Año: 2002 vol. 66 p. 1663 - 1679

0016-7037

Abstract?Glass inclusions in olivines of the Renazzo, El Djouf 001, and Acfer 182 CR-type chondrites arechemically divers and can be classified into Al-rich, Al-poor, and Na-rich types. The chemical properties ofthe glasses are independent of the occurrence of the olivine (isolated or part of an aggregate or chondrule) andits composition. The glasses are silica-saturated (Al-rich) or oversaturated (Al-poor, 24% normative quartz).All glasses have chondritic CaO/Al2O3 ratios, unfractionated CI-normalized abundances of refractory traceelements and are depleted in moderately volatile and volatile elements. Thus the glasses are likely to be of aprimitive condensate origin whose chemical composition has been established before chondrule formation andaccretion, rather then the product of either crystal fractionation from chondrule melts or part melting ofchondrules. Rare Na-rich glasses give evidence for elemental exchange between the glass and a vapor phase.Because they have Al2O3 contents and trace element abundances very similar to those of the Al-rich glasses,they likely were derived from the latter by Ca exchange (for Na) with the nebula. Elemental exchangereactions also have affected practically all olivines (e.g., exchange of Mg of olivine for Fe2, Mn2, andCr3). Glasses formed contemporaneously with the host olivine. As the most likely process for growingnonskeletal olivines from a vapor we consider the VLS (vapor?liquid?solid) growth process, or liquid-phaseepitaxy. Glasses are the possible remnants of the liquid interface between growing crystal and the vapor. Suchliquids can form stably or metastably in regions with enhanced oxygen fugacity as compared to that of anebula of solar composition. Copyright © 2002 Elsevier Science Ltd