Magnesium
Isotopes

Magnesium has three stable isotopes: Mg, Mg and Mg. All are present in significant amounts in nature (see table of isotopes above). About 79% of Mg is Mg. The isotope Mg is radioactive and in the 1950s to 1970s was produced by several nuclear power plants for use in scientific experiments. This isotope has a relatively short half-life (21 hours) and its use was limited by shipping times.

The nuclide Mg has found application in isotopic geology, similar to that of aluminium. Mg is a radiogenic daughter product of Al, which has a half-life of 717,000 years. Excessive quantities of stable Mg have been observed in the Ca-Al-rich inclusions of some carbonaceous chondrite meteorites. This anomalous abundance is attributed to the decay of its parent Al in the inclusions, and researchers conclude that such meteorites were formed in the solar nebula before the Al had decayed. These are among the oldest objects in the Solar System and contain preserved information about its early history.

It is conventional to plot Mg / Mg against an Al/Mg ratio. In an isochron dating plot, the Al/Mg ratio plotted is Al / Mg. The slope of the isochron has no age significance, but indicates the initial Al / Al ratio in the sample at the time when the systems were separated from a common reservoir.
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