16O-rich melilite in CO3.0 chondrites:: Possible formation of common, 16O-poor melilite by aqueous alteration

problem of long-standing is the origin of the high Delta O-17 (= delta O-17 - 0.52.delta O-18) value, of melilite in the refractory inclusions of carbonaceous chondrites. The spinel and, usually, also the diopside in the inclusions have "primitive" Delta O-17 less than or equal to -20 parts per thousand, but in the widely Studied CV3 chondrite Allende, the melilite commonly has a Delta O-17 of about -4 +/- 2 parts per thousand. Recent studies of oxygen diffusion coefficients in melilite and spine] have shown Ilia[ the traditional model for altering the melilite-diffusive exchange with the gas in the solar nebula-cannot occur under plausible nebular scenarios. Because of increasing evidence for aqueous alteration of Allende and other carbonaceous chondrites, we tested the hypothesis that melilite alteration [night occur during aqueous alteration by studying melilite in CO chondrites showing variable degrees of alteration. Our results show (hat almost all melilite in CO3.2 Kainsaz and CO3.3 Ornans has Delta O-17 > -15 parts per thousand, whereas in CO3.0 Colony (and inclusions in CO3.0 Yarnato 81020 studied by Ron et al, [2000]) Delta O-17 in the melilite is < -20 parts per thousand (with the exception of one unusual inclusion that has undergone two melting episodes). We suggest that the O-isotopic composition of the melilite is reset by dissolution of the primitive melilite and reprecipitation in (lie presence of H2O having a high Delta O-17 value similar to that inferred from studies of CV magnetite (Choi et al,, 1997, 2000. Although alteration of melilite can produce phases such as nepheline, andradite, and hedenbergite, it appears that, depending on the composition of the microenvironment, melilite present in a high-energy state (because of fine grain size, dislocations, other defects) can dissolve and reprecipitate with an O-isotopic composition intermediate between the initial composition and that of (lie H2O. A possible implication of our study is that the slope approximate to0.9 arrays observed in the minerals of refractory inclusions of many carbonaceous chondrites could be mainly the result of aqueous alteration, Copyright (C) 2001 Elsevier Science Ltd.