Multiple generations of fractionated hibonite-rich CAIs sampled the solar nebula at different degrees of isotopic heterogeneity

Abstract

Calcium-aluminum-rich inclusions (CAIs) are the oldest dated objects that formed in the solar system [1]. A few of these show large massdependent enrichments in the heavy isotopes of some elements, attesting to a formation by melt distillation of low-temperature precursors, and are often classified as fractionated (F) CAIs ([2] and references therein). Some fractionated CAIs also have large mass-independent anomalies of nucleosynthetic origin and are classified as FUN (Fractionated and Unidentified Nuclear effects) CAIs. Other CAIs with large unidentified nuclear (UN) anomalies lack fractionation effects. While F and FUN CAIs are often not petrologically distinct from regular CAIs, UN effects are most commonly found in hiboniterich CAIs like platy hibonite crystals (PLACs) [3]. We have studied 80 hibonite-rich CAIs separated from the Murchison meteorite for their UN characteristics [4, 5] and incorporation of 26Al at their time of formation. Here, we report the mass-dependent and massindependent isotope effects in O, Mg, Ca and Ti in 7 of these hibonite-rich CAIs that have F(UN) characteristics. We also infer their initial 26Al/27Al ratios [(26Al/27Al)0] from model and internal isochrons.