Thermal Processing History of the Solar System’s First Solids Inferred from Isotope Fractionation of Refractory Elements

Abstract We present analyses of mass-dependent isotope fractionation of strontium (Sr) and neodymium (Nd) from a number of petrologically and chemically diverse calcium–aluminium-rich inclusions (CAIs) from the CV chondrite Allende. Combined with literature data, our results reveal systematic variations of elemental and isotopic fractionation signatures that depend on element volatility and CAI type. In most CAIs with little volatility-driven elemental fractionation (which we collectively refer to as “nongroup-II” CAIs), moderately refractory elements such as Nd and Ti are not isotopically fractionated relative to chondrites, while less refractory elements such as Sr and Ca show light isotope enrichments correlated with their lowered abundances. In contrast, in CAIs with the “group-II” type elemental fractionation pattern, refractory elements show more variable isotopic compositions uncorrelated to element abundances. The coupled elemental and isotopic fractionation in nongroup-II CAIs are consistent with a simple evaporation–condensation–reevaporation origin in the solar nebula, while group-II CAIs require more extensive and complex thermal processing histories. The disparate thermal evolutions of group-II and nongroup-II CAIs indicate distinct physical conditions in their formation pathways, corresponding to different localities in the protoplanetary disk. Nongroup-II CAIs may represent materials that were more effectively transported between high-temperature and low-temperature disk zones and thus experienced more intensive heating and cooling, while group-II CAIs represent particles that more frequently traversed between those zones but were neither heated nor cooled as thoroughly as the former.