Primordial formation of major silicates in a protoplanetary disc with homogeneous (26)Al/(27)Al

Understanding the spatial variability of initial (26)Al/(27)Al in the solar system, i.e., ((26)Al/(27)Al)(0), is of prime importance to meteorite chronology, planetary heat production, and protoplanetary disc mixing dynamics. The ((26)Al/(27)Al)(0) of calcium-aluminum–rich inclusions (CAIs) in primitive meteorites (~5 × 10(−5)) is frequently assumed to reflect the ((26)Al/(27)Al)(0) of the entire protoplanetary disc, and predicts its initial (26)Mg/(24)Mg to be ~35 parts per million (ppm) less radiogenic than modern Earth (i.e., Δ′(26)Mg(0) = −35 ppm). Others argue for spatially heterogeneous ((26)Al/(27)Al)(0), where the source reservoirs of most primitive meteorite components have lower ((26)Al/(27)Al)(0) at ~2.7 × 10(−5) and Δ′(26)Mg(0) 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′(26)Mg(0) are within uncertainty of −35 ppm. Our data thus affirm a model of a largely homogeneous protoplanetary disc with ((26)Al/(27)Al)(0) of ~5 × 10(−5), supporting the accuracy of the (26)Al→(26)Mg chronometer.