Abstract
VARIATIONS in the isotopic abundance of magnesium (78.6% 24Mg, 10.1% 25Mg, 11.3% 26Mg) have been sought1,2 in meteorites because of the possible addition of radiogenic 26Mg by the decay of 26Al. This short-lived nuclide (half-life 7.4×105 yr) is formed by proton bombardment of light elements3 and is of particular interest in theories of nucleosynthesis and heat generation during the early history of the Solar System. Analysed material must have a high Al/Mg ratio to avoid the dilution of any radiogenic 26Mg by common Mg. Consequently, previous workers have chosen mainly feldspars from basaltic achondrites and metamorphosed chondrites. The apparent absence of excess 26Mg in such meteorites is, however, not suprising, because the condensation of chondritic material from the solar nebula now seems4 to predate its later metamorphism by some 80 Myr, and the magmatic activity that produced the basaltic achondrites is younger still. Thus, the feldspars that formed as a result of these processes would incorporate aluminium that was separated from a nucleo-synthetic event by at least 80 Myr and because of its short halflife in-sufficient 26Al would remain to generate a detectable 28Mg anomaly.
Published Version
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