~(16)O-rich melilite in CO3.0 chondrites: possible formation of common, ~(16)O-poor melilite by aqueous alteration

摘要

A problem of long-standing is the origin of the high Δ~(17)O (= δ~(17)O - 0.52·δ~(18)O) values of melilite in the refractory inclusions of carbonaceous chondrites. The spinel and, usually, also the diopside in the inclusions have "primitive" Δ~(17)O ≤ -20‰, but in the widely studied CV3 chondrite Allende, the melilite commonly has a Δ~(17)O of about -4 ± 2‰. Recent studies of oxygen diffusion coefficients in melilite and spinel have shown that 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 might occur during aqueous alteration by studying melilite in CO chondrites showing variable degrees of alteration. Our results show that almost all melilite in CO3.2 Kainsaz and CO3.3 Ornans has Δ~(17)O > -15‰, whereas in CO3.0 colony (and inclusions in CO3.0 Yamato 81020 studied by Itoh et al. [2000]) Δ~(17)O in the melilite is <-20‰ (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 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 reprecipitate with an O-isotopic composition intermediate between the initial composition and that of the H_2O. A possible implication of our study is that slope ≈0.9 arrays observed in the minerals of refractory inclusions of many carbonaceous chondrites could be mainly the result of aqueous alteration.