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Mg isotopic heterogeneity, Al‐Mg isochrons, and canonical 26 Al/ 27 Al in the early solar system
Author(s) -
WASSERBURG G. J.,
WIMPENNY Josh,
YIN QingZhu
Publication year - 2012
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/maps.12014
Subject(s) - isochron , isochron dating , isotope , homogeneous , geology , mineralogy , chemistry , physics , geochemistry , thermodynamics , nuclear physics
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.