Primordial formation of major silicates in a protoplanetary disc with homogeneous Al/Al.

Understanding the spatial variability of initial Al/Al in the solar system, i.e., (Al/Al), is of prime importance to meteorite chronology, planetary heat production, and protoplanetary disc mixing dynamics. The (Al/Al) of calcium-aluminum-rich inclusions (CAIs) in primitive meteorites (~5 × 10) is frequently assumed to reflect the (Al/Al) of the entire protoplanetary disc, and predicts its initial Mg/Mg to be ~35 parts per million (ppm) less radiogenic than modern Earth (i.e., Δ'Mg = -35 ppm). Others argue for spatially heterogeneous (Al/Al), where the source reservoirs of most primitive meteorite components have lower (Al/Al) at ~2.7 × 10 and Δ'Mg of -16 ppm. We measured the magnesium isotope compositions of primitive meteoritic olivine, which originated outside of the CAI-forming reservoir(s), and report five grains whose Δ'Mg are within uncertainty of -35 ppm. Our data thus affirm a model of a largely homogeneous protoplanetary disc with (Al/Al) of ~5 × 10, supporting the accuracy of the Al→Mg chronometer.