Mg isotopic heterogeneity, Al-Mg isochrons, and canonical 26Al/27Al in the early solar system

There is variability in the Mg isotopic composition that is a reflection of the widespread heterogeneity in the isotopic composition of the elements in the solar system at approximately 100 ppm. Measurements on a single calcium-aluminum-rich inclusion (CAI) gave a good correlation of 26Mg/24Mg with 27Al/24Mg, yielding an isochron corresponding to an initial (26Al/27Al)o = (5.27 +/- 0.18) x 10-5 and an initial (26Mg/24Mg)o = -0.127 +/- 0.032 parts per thousand relative to the standard. This isochron is parallel to that obtained by Jacobsen et al. (2008), but is distinctively offset. This demonstrates that there are different initial Mg isotopic compositions in different samples with the same 26Al/27Al. No inference about uniformity/heterogeneity of 26Al/27Al on a macro scale can be based on the initial (26Mg/24Mg)o values. Different values of 26Al/27Al for samples representing the same point in time would prove heterogeneity of 26Al/27Al. The important issue is whether the bulk solar inventory of 26Al/27Al was approximately 5 x 10-5 at some point in the early solar system. We discuss ultra refractory phases of solar type oxygen isotope composition with 26Al/27Al from approximately 5 x 10-5 to below 0.2 x 10-5. We argue that the real issues are: intrinsic heterogeneity in the parent cloud; mechanism and timing for the later production of 16O-poor material; and the relationship to earlier formed 16O-rich material in the disk. 26Al-free refractories can be produced at a later time by late infall, if there is an adequate heat source, or from original heterogeneities in the placental molecular cloud from which the solar system formed.