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

摘要

Understanding the spatial variability of initial sup26/supAl/sup27/supAl in the solar system, i.e., (sup26/supAl/sup27/supAl)sub0/sub, is of prime importance to meteorite chronology, planetary heat production, and protoplanetary disc mixing dynamics. The (sup26/supAl/sup27/supAl)sub0/sub of calcium-aluminum–rich inclusions (CAIs) in primitive meteorites (~5 × 10sup?5/sup) is frequently assumed to reflect the (sup26/supAl/sup27/supAl)sub0/sub of the entire protoplanetary disc, and predicts its initial sup26/supMg/sup24/supMg to be ~35 parts per million (ppm) less radiogenic than modern Earth (i.e., Δ′sup26/supMgsub0/sub = ?35 ppm). Others argue for spatially heterogeneous (sup26/supAl/sup27/supAl)sub0/sub, where the source reservoirs of most primitive meteorite components have lower (sup26/supAl/sup27/supAl)sub0/sub at ~2.7 × 10sup?5/sup and Δ′sup26/supMgsub0/sub 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′sup26/supMgsub0/sub are within uncertainty of ?35 ppm. Our data thus affirm a model of a largely homogeneous protoplanetary disc with (sup26/supAl/sup27/supAl)sub0/sub of ~5 × 10sup?5/sup, supporting the accuracy of the sup26/supAl→sup26/supMg chronometer.