The Isotopic Composition of Mg and the Implied Limits on ²⁶Al in the Early Solar System; Nucleosynthesis of ²⁶Al; and Nucleosynthetic Chronologies for the Galaxy

A particular isotope pair of astrophysical interest, 26Al-26Mg, was investigated in detail. A high precision mass spectrometric technique was developed whereby Mg isotopic compositions could be analyzed to better than five parts in 104. Feldspar (high Al, low Mg) mineral separates were carried out on several meteoritic and lunar samples. No anomalous 26Mg values were found. Thus, there is no evidence for the existence of 26 Al at the formation of the solar system. Upper limits on the possible amount of 26Al at solidification in the extraterrestrial samples were calculated. These limits were used to estimate the maximum change in central temperature of a planetary object which could be produced by the 26Al decay. A review is given of the production mechanisms for 26Al and it has been shown that a flux of at least 4 x 1018 protons/cm2 is required to produce enough 26 Al to melt the cores of planetary objects. The general problem of nucleosynthetic chronologies has been investigated. Nucleochronologies for the galaxy have been calculated using235U;238U,232Th/238U, 244Pu/238U (or preferably 244Pu/232Th),and 129I/127I. The systematics of the nucleochronologic equations are derived and it has been found that the mean age of the elements can be found from long-lived radioactive nucleii in a manner which is independent of the time dependent production rate. It has also been found that the interval between the termination of nucleosysthesis and the time of formation of solid bodies in the solar system can be determined model independently from the short~lived nucleii. In addition, some information on the time dependent shape of the production function can also be determined model independently.