Aluminum 26‐magnesium 26 dating of feldspar in meteorites

A search for ‘fossil’ Mg26 from the decay of 0.72 m.y. Al26 in an aluminum‐rich (plagioclase feldspar) phase of several stone meteorites has been carried out. An electron impact ion source in a tandem mass spectrometer was used for a series of measurements of the relative abundances of table magnesium isotopes in meteoritic and terrestrial feldspar, as well as in unseparated (‘common’) magnesium from meteorites, from meteoritic chondrules, and from laboratory reagents. Relative to terrestrial magnesium, the meteorites Bruderheim and Khor Temiki show a definite Mg26 excess (4–6‰). Haraiya and Mocs also show this effect although the experimental evidence is not nearly as secure as in the first two examples; Moore County is a borderline case. Magnesium contents of the separated feldspar phases were measured by isotope dilution, allowing a calculation of Mg26 (radiogenic), relative to Si = 106 atoms for the meteorites. The following conclusions can be made: (1) the feldspar of some meteorites cooled to below the temperature of appreciable Mg diffusion (∼650°C) within a time spread of about 3 m.y.; (2) only a short time (≤5 m.y.) could have elapsed between the end of Al26 production and formation of the feldspar phase in some meteorites; (3) the process that produced Al26 early in the history of the solar system (most probably solar proton irradiation of surrounding material) was of short duration (<5 m.y.); (4) if the levels of Al26 deduced from measurements of excess Mg26 were present at the time of formation of objects a few tens of km in radius, sufficient energy would have been produced by radioactive decay of Al26 to heat the centers of these objects to near their melting points.