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

Abstract

Understanding the spatial variability of initial ²⁶Al/²⁷Al in the solar system, i.e., (²⁶Al/²⁷Al)₀, is of prime importance to meteorite chronology, planetary heat production, and protoplanetary disc mixing dynamics. The (²⁶Al/²⁷Al)₀ of calcium-aluminum-rich inclusions (CAIs) in primitive meteorites (~5 × 10^-5) is frequently assumed to reflect the (²⁶Al/²⁷Al)₀ of the entire protoplanetary disc, and predicts its initial ²⁶Mg/²⁴Mg to be ~35 parts per million (ppm) less radiogenic than modern Earth (i.e., Δ'²⁶Mg₀ = -35 ppm). Others argue for spatially heterogeneous (²⁶Al/²⁷Al)₀, where the source reservoirs of most primitive meteorite components have lower (²⁶Al/²⁷Al)₀ at ~2.7 × 10^-5 and Δ'²⁶Mg₀ 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'²⁶Mg₀ are within uncertainty of -35 ppm. Our data thus affirm a model of a largely homogeneous protoplanetary disc with (²⁶Al/²⁷Al)₀ of ~5 × 10^-5, supporting the accuracy of the ²⁶Al→²⁶Mg chronometer.