Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.253m&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 253
Mathematics
Logic
5
Scientific paper
The discovery of excess ^26Mg correlated with ^27Al/^24Mg in refractory inclusions in chondritic meteorites (Lee et al., 1976), interpreted as being due to in situ decay of the extinct radionuclide ^26Al, was followed shortly by the realization that ^26Al was probably heterogeneous in the early solar nebula and therefore of questionable value either as a heat source for early planetary melting or as a fine-scale chronometer of early solar nebula events (e.g. Hutcheon, 1982). Recent correlated petrologic/isotopic studies (e.g. Podosek et al., 1991) have indicated that the "disturbed" Al-Mg isotopic systematics within many individual inclusions may be due to complex evolutionary histories rather than initial isotopic heterogeneities. For this reason a reevaluation of the nebular distribution of ^26Al seems in order. A compilation of nearly 640 published and unpublished Al-Mg isotopic analyses (mostly by ion microprobe) of Al-rich objects in 10 meteorites (mostly Ca-Al-rich inclusions and Ca-Al-rich separated mineral grains) shows a remarkably consistent upper limit for initial ^26Al/^27Al of ~5 x 10^-5. This upper limit apparently transcends meteorite classification within the carbonaceous chondrites; at a minimum, enough data exist to indicate that there is no significant difference between inclusions from any of the CV3 or CM2 meteorites. Calculation of initial ^26Al/^27Al for all analyses of non-FUN samples (assuming uniform initial ^26Mg/^24Mg = 0.13932) shows a bimodal distribution (Fig. 1) of isotopic compositions, with peaks near ~4.5 x 10^-5 and ~0. Of the analyses showing near-zero initial ^26Al/^27Al, most come either from CM2 hibonite grains or from CV3 inclusions for which good evidence exists for later reprocessing. Collectively, the data suggest that ^26Al was in fact rather uniformly distributed, i.e. throughout the regions of the early solar nebula where parent bodies for several chondrite subtypes formed. A widespread upper limit for initial ^26Al/^27Al implies that the nebular isotopic heterogeneity required by FUN inclusions and many CM2 hibonite grains was a second order phenomenon. The marked bimodality of initial ^26Al/^27Al is the result of two processes, nebular heterogeneity and secondary reprocessing. However, the two Al-isotopic reservoirs must have remained remarkably separate or else always mixed in nearly the same proportions in order for the ^26Al/^27Al of = ~5 x 10^-5 end member to be so well represented. Much reprocessing of inclusions apparently took place at least several half-lives of ^26Al after initial CAI formation. Reprocessing cannot, however, easily explain the preservation of nuclear isotope anomalies in e.g. FUN inclusions and CM2 hibonites that show little or no excess ^26Mg. References: Hutcheon I.D. (1982) Amer. Chem. Soc. Symposium Ser., 176, 95-128. Lee T., Papanastassiou D.A., and Wasserburg G.J. (1976) Geophys. Res. Lett. 3, 109-112. Podosek F. A., Zinner E. K., MacPherson G. J., Lundberg L. L., Brannon J. C., and Fahey A. J. (1991) Geochim. Cosmochim. Acta 55, 1083-1110. Figure 1, which in the hard copy appears here, shows all non-FUN CAI and refractory grains with Al/Mg > 100.
Davis Aileen M.
MacPherson Glenn J.
Zinner Ernst K.
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