ABSTRACT

An Al-Mg pseudo-isochron is expected for the type B inclusion (CAI) of carbonaceous chondrites on the basis of stoichiometry of anorthite (or hibonite). The linear correlation of excess ²⁶Mg with ²⁷Al/²⁴Mg ratio found for the Allende and Leoville CAI's is well-interpreted as a result of mixing in various proportions (<2%) of two end-members: one with no substantial excess of ²⁶Mg such as spinel, fassaite and melilite, and the other nearly pure anorthite with excess ²⁶Mg (possibly fossil ²⁶Mg) or it's precursor. I would like to call attention to the difficulties in attaining to a “true” Al-Mg internal isochron for meteorite CAI's.