Was that the molten magnesium sample that was dropped from an UFO over Ubatuba, Brasil, in 1957? I also remember reading that it was also chemically VERY pure except for some oxidation, containing absolutely no aluminium or calcium which are frequent trace impurities (although harmless) in magnesium refined on Earth (usually by electrolysis of the molten chloride under argon).
If the isotope ratio of the Mg is different from that obtained in ANY natural ore source on Earth, it must indicate one of two things:
(a) It was subjected to a substantial fractionation process (e.g. putting a volatile compound, such as the hydride or dimethylmagnesium, through a gas-liquid chromatograph, possibly aided by centrifugation) that resulted in concentration of the heaviest isotope in the fraction selected for subsequent use; or
(b) Because the natural isotopic ratios of elements on Earth and elsewhere in the solar system, with slight variations due to natural geochemical mineralization processes, were fixed by the particular supernova remnant dust and gas from which the solar system was formed, the magnesium must have come from a different planet and solar system so far removed from this solar system as to have been formed from gas and dust produced by different supernovae explosions, in which different ratios of elements and different isotopic ratios of elements would have been formed.
The possibility (a) seems most unlikely, because magnesium destined for ordinary metallurgical or electronic or chemical use would not have been subjected to such a costly isotopic enrichment process. Only if particular pure isotopes of Mg for use in nuclear bombardment reactions were wanted would this have been done. It is possible that the final refining step to remove any traces of Al or Ca could have done this; but if so, such a process would have resulted in enrichment in the lightest isotope, not the heaviest as in this sample.
The slight differences in isotopic composition of Mg found on Earth would be due to the ways in which the different rocks and minerals containing it were formed, which resulted in some natural fractionation of the isotopes. Some Mg-containing minerals are of sedimentary origin, deposited from saline waters, e.g. magnesite (carbonate), dolomite (mixed Mg-Ca carbonate), epsomite (hydrous soluble sulfate), and carnallite (a mixed soluble K-Mg chloride-bromide found in some potash deposits); while others are of igneous origin, occurring mostly in basic and ultrabasic rocks which consist largely of ferrous and magnesium silicates.