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

Abstract

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 ^(26)Mg/^(24)Mg with ^(27)Al/^(24)Mg, yielding an isochron corresponding to an initial (^(26)Al/^(27)Al)_o = (5.27 ± 0.18) × 10^(−5) and an initial (^(26)Mg/^(24)Mg)_o = −0.127 ± 0.032‰ 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 ^(26)Al/^(27)Al. No inference about uniformity/heterogeneity of ^(26)Al/^(27)Al on a macro scale can be based on the initial (^(26)Mg/^(24)Mg)_o values. Different values of ^(26)Al/^(27)Al for samples representing the same point in time would prove heterogeneity of ^(26)Al/^(27)Al. The important issue is whether the bulk solar inventory of ^(26)Al/^(27)Al was approximately 5 × 10^(−5) at some point in the early solar system. We discuss ultra refractory phases of solar type oxygen isotope composition with ^(26)Al/^(27)Al from approximately 5 × 10^(−5) to below 0.2 × 10^(−5). We argue that the real issues are: intrinsic heterogeneity in the parent cloud; mechanism and timing for the later production of ^(16)O-poor material; and the relationship to earlier formed ^(16)O-rich material in the disk. ^(26)Al-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.

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