UBIQUITOUS LOW-FEO RELICT GRAINS IN TYPE II CHONDRULES AND LIMITED OVERGROWTHS ON PHENOCRYSTS FOLLOWING THE FINAL MELTING EVENT
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dc.contributor.author | Wasson J.T. | |
dc.contributor.author | Rubin A.E. | |
dc.date.accessioned | 2021-12-26T02:54:47Z | |
dc.date.available | 2021-12-26T02:54:47Z | |
dc.date.issued | 2003 | |
dc.identifier | https://www.elibrary.ru/item.asp?id=1607973 | |
dc.identifier.citation | Geochimica et Cosmochimica Acta, 2003, 67, 12, 2239-2250 | |
dc.identifier.issn | 0016-7037 | |
dc.identifier.uri | https://repository.geologyscience.ru/handle/123456789/33838 | |
dc.description.abstract | Type II porphyritic chondrules commonly contain several large (>40 μm) olivine phenocrysts; furnace-based cooling rates based on the assumption that these phenocrysts grew in a single-stage melting-cooling event yield chondrule cooling-rate estimates of 0.01-1 K s-1. Because other evidence indicates much higher cooling rates, we examined type II chondrules in the CO3.0 chondrites that have experienced only minimal parent-body alteration. We discovered three kinds of evidence indicating that only minor (4-10 μm) olivine growth occurred after the final melting event: (1) Nearly all (>90%) type II chondrules in CO3.0 chondrites contain low-FeO relict grains; overgrowths on these relicts are narrow, in the range of 2-12 μm. (2) Most type II chondrules contain some FeO-rich olivine grains with decurved surfaces and acute angles between faces indicating that the grains are fragments from an earlier generation of chondrules; the limited overgrowth thicknesses following the last melting event are too thin to disguise the shard-like nature of these grains. (3) Most type II chondrules contain many small (<20 μm) euhedral or subhedral phenocrysts with central compositions that are much more ferroan than the centers of large phenocrysts; their small sizes document amount growth occurred after final melting event. if overgrowth thicknesses were (4-10 event, it follows fractions coarse (>40 μm) high-FeO phenocrysts are relicts from earlier generations of chondrules, and that cooling rates after the last melting event were much more rapid than indicated by models based on a single melting event. These observations are thus inconsistent with the ''classic'' igneous model of formation of type II porphyritic chondrules by near-total melting of a precursor mix followed by olivine nucleation on a very limited number of nuclei (say, =<10) and by growth to produce the large phenocrysts during a period of monotonic (and roughly linear) cooling. Our observations that recycled chondrule materials constitute a large component of the phenocrysts of type II chondrules also imply that this kind of chondrule formed relatively late during the chondrule-forming period. | |
dc.title | UBIQUITOUS LOW-FEO RELICT GRAINS IN TYPE II CHONDRULES AND LIMITED OVERGROWTHS ON PHENOCRYSTS FOLLOWING THE FINAL MELTING EVENT | |
dc.type | Статья |
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