Excess ²⁶Mg in the Allende Meteorite

Abstract

VARIATIONS in the isotopic abundance of magnesium (78.6% ²⁴Mg, 10.1% ²⁵Mg, 11.3% ²⁶Mg) have been sought^1,2 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.