Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27..311z&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 311
Mathematics
Logic
32
Scientific paper
One of the important questions for the history of the early solar system is whether or not there was enough ^26Al to melt small planetary bodies through the heat released by its decay. Although there is ample evidence for the existence of live ^26Al in refractory inclusions (Wasserburg and Papanastassiou, 1982; Hutcheon, 1982; Podosek et al., 1991), CAIs are special objects with peculiar properties and their Al is not necessarily representative of that of their host meteorites nor the early solar systems. Furthermore, some inclusions do not show any evidence for ^26Al (Wasserburg and Papanastassiou, 1982; Ireland, 1990; Virag et al., 1991), raising the possibility of ^26Al heterogeneity. The only previous observation of ^26Mg excesses attributed to the decay of ^26Al outside of CAIs was in an igneous clast from Semarkona (Hutcheon and Hutchison, 1989) leading to the conclusion that ^26Al indeed could have been a heat source for planetary melting. We have measured Al-Mg in plagioclase grains from the H4 chondrite Ste. Marguerite by ion microprobe mass spectrometry. Feldspars from H4 chondrites are good samples for addressing the problem of ^26Al as heat source because most Al resides in this phase and some H4s experienced fast cooling (Pellas and Storzer, 1981); in fact, the possibility of live ^26Al in feldspars from H4 chondrites that cooled fast has been predicted by Pellas and Storzer (1981). Furthermore, extremely precise absolute Pb/Pb ages exist for these meteorites (Gopel et al., 1991). Figure 1 shows the measurements on five feldspar crystals. All show ^26Mg excesses. A fit through the data points and the normal ^26Mg/^24Mg ratio of 0.13962 obtained from Lake County plagioclase measured under the same instrumental conditions as the Ste. Marguerite samples yields a (^26Al/^27Al)(sub)0 ratio of (2.0 +- 0.6) x 10^-7. If interpreted chronologically this ratio dates the retention of radiogenic ^26Mg in Ste. Marguerite feldspar to 5.6 +- 0.4 Ma after the formation of Allende inclusions. This time interval is a little longer than but still consistent with the difference of the Pb/Pb age of 4.5627 +- 6 Ga for Ste. Marguerite (Gopel et al., 1991) and the U-Pb age of 4.566 +- 2 Ga for Allende inclusions (Manhes et al., 1988). It thus appears that there was sufficient ^26Al to melt early accreting small planetary bodies. References: Gopel C., Manhes G., and Allegre C. J. (1991) Meteoritics 26, 338. Hutcheon I. D. (1982) In Nuclear and Chemical Dating Techniques: Interpreting the Environmental Record (eds. L. A. Curie), Amer. Chem. Soc. Symposium Series No. 176, 95-128. Hutcheon I. D. and Hutchison R. (1989) Nature 337, 238. Ireland T. R. (1990) Geochim. Cosmochim. Acta 54, 3219-3237. Manhes G., Gopel C., and Allegre C. J. (1988) Comptes Rendus de l'ATP Planetologie, 323-327. Pellas P. and Storzer D. (1981) Proc. R. Soc. Lond. A374, 253- 270. 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. Virag A., Zinner E., Amari S., and Anders E. (1991) Geochim. Cosmochim. Acta 55, 2045-2062. Wasserburg G. J. and Papanastassiou D. A. (1982) In Essays in Nuclear Astrophysics (eds. C. A. Barnes et al.), Cambridge Univ. Press, 77-140.
Göpel Christa
Zinner Emst
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