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Chronology of chrondrule and CAI formation: Mg-Al isotopic evidenceDetails of the chondrule and Ca-Al-rich inclusion (CAI) formation during the earliest history of the solar system are imperfectly known. Because CAI's are more 'refractory' than ferromagnesian chondrules and have the lowest recorded initial Sr-87/Sr-86 ratios of any solar system materials, the expectation is that CAI's formed earlier than chondrules. But it is not known, for example, if CAI formation had stopped by the time chondrule formation began. Conventional (absolute) age-dating techniques cannot adequately resolve small age differences (less than 10(exp 6) years) between objects of such antiquity. One approach has been to look at systematic differences in the daughter products of short-lived radionuclides such as Al-26 and I-129. Unfortunately, neither system appears to be 'well-behaved.' One possible reason for this circumstance is that later secondary events have partially reset the isotopic systems, but a viable alternative continues to be large-scale (nebular) heterogeneity in initial isotopic abundances, which would of course render the systems nearly useless as chronometers. In the past two years the nature of this problem has been redefined somewhat. Examination of the Al-Mg isotopic database for all CAI's suggests that the vast majority of inclusions originally had the same initial Al-26/Al-27 abundance ratio, and that the ill-behaved isotopic systematics now observed are the results of later partial reequilibration due to thermal processing. Isotopic heterogeneities did exist in the nebula, as demonstrated by the existence of so-called FUN inclusions in CV3 chondrites and isotopically anomalous hibonite grains in CM2 chondrites, which had little or no live Al-26 at the time of their formation. But, among the population of CV3 inclusions at least, FUN inclusions appear to have been a relatively minor nebular component.
Document ID
19950012899
Acquisition Source
Legacy CDMS
Document Type
Conference Paper
Authors
Macpherson, G. J.
(National Museum of Natural History Washington, DC, United States)
Davis, A. M.
(Chicago Univ. Chicago, IL., United States)
Date Acquired
September 6, 2013
Publication Date
January 1, 1994
Publication Information
Publication: Lunar and Planetary Inst., Papers Presented to the Conference on Chondrules and the Protoplanetary Disk
Subject Category
Astrophysics
Accession Number
95N19314
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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