Magnesium isotopic variations in hibonite from the Murchison Meteorite: An ion microprobe study

Ion microprobe measurements show that some hibonite crystals from refractory‐rich inclusions in Murchison contain isotopically anomalous Mg. The observed effects include (1) the largest mass fractionation ever observed in nature for this element, ∼10% per mass unit; (2) small 26Mg excesses; (3) isotopic variability on a scale of microns or less in individual grains. These observations are corroborative evidence for the primitiveness of the Murchison inclusions. More importantly, they indicate that the types of Mg isotopic anomalies previously observed in Allende and Leoville inclusions are not confined to CV meteorites.