|Title:||On an Irradiation Origin for Magnesium Isotope Anomalies in Meteoritic Hibonite|
|Authors:||M.‐C. Liu, and K.D. McKeegan|
|Publication:||ApJ, v. 697, p. L145‐L148.|
We investigate spallogenic effects on magnesium isotopic compositions in solids with hibonite‐like (CaAl12O19) chemistry under an irradiation setting in the early solar system. We consider a series of nuclear reactions triggered by gradual flare irradiation, with an energy spectrum dN/dE α E−2.7 and a proton flux Fp (E ≫ 10 MeV) = 1.9 × 1010 cm−2 s−1, on isotopically normal hibonite‐like solids. The proton fluence is constrained by matching the 10Be/9Be measured in meteoritic hibonite platy crystals. The net result, accounting for both production and destruction reactions of Mg isotopes, shows small deviations from the terrestrial isotopic composition with the sign and magnitude of Δ26Mg effects dependent on both low energy cutoff and total fluence. The total span of predicted spallogenic deviations can explain small apparent 26Mg excesses observed in some hibonite grains, but does not account for the magnitude of apparent 26Mg deficits found in other grains. As previously indicated, irradiation by gradual flares decouples Δ 26Mg variations and 26Al/27Al from 10Be/9Be.