Nucleosynthetic Isotope Anomalies in Cosmochemistry and Geochemistry
Authors: Bermingham Katherine , Meyer Brad 
Published: September 2024

Nucleosynthetic isotope anomalies provide some of the most informative sample-based constraints on the origin of the Solar System. An isotopic anomaly is a deviation in isotopic ratio relative to a standard that is made from natural terrestrial materials. Nucleosynthetic isotope anomalies are small (i.e., part-per-million scale) stable isotopic anomalies that are found in meteorites and some planetary bodies which are caused by the heterogeneous distribution of stardust in the protoplanetary disk. These subtle isotopic differences provide constraints on the combination of stellar precurors whose stardust comprises some of the matter in the protoplanetary disk. These anomalies also constrain how well stardust was mixed in the disk during accretion. Furthermore, discoveries of subtle nucleosynthetic isotope anomalies in samples from the Earth’s mantle have opened the door to the possibility of using nucleosynthetic isotope anomalies to trace Earth’s precursor material. New insights into the evolution of the nascent Solar System have come from interpreting nucleosynthetic isotope anomalies in the context of numerical stellar nucleosynthetic models and disk evolution models. This research is based on a thus far omnipresent isotopic dichotomy, termed the “NC–CC isotopic dichotomy”, that is recorded in the nucleosynthetic isotope composition of meteorites. This isotopic dichotomy has been interpreted to indicate that within the first few million years of Solar System history, the disk separated into two portions. This separation inhibited material in the presumptive inner Solar System (termed noncarbonaceous reservoir, NC group) from mixing with outer Solar System material (termed carbonaceous chondrite reservoir, CC group). This strict compositional division in the disk may have lasted until giant planet migration, which occurred at the tail end of the disk’s lifetime (<10 Ma of Solar System formation). Despite this application of the NC-CC isotopic dichotomy, fundamental questions remain about what part of Solar System history it preserves and if it can be used to reconstruct the architecture of the nascent Solar System.

The applications of nucleosynthetic isotope anomalies in cosmochemistry and the insights these data provide into the evolution of the Solar System and Earth are discussed. The nucleosynthetic isotope anomalies that are recorded in bulk cosmochemical and terrestrial materials are summarized. The likely stellar origins of the presolar grains responsible for the isotopic anomalies on the bulk sample scale are distilled, along with the constraints these data place on the distribution of presolar material in the disk. A review of stellar nucleosynthesis, the formation of the Solar System, the conceptual framework used to interpret nucleosynthetic isotope data, and reported bulk sample nucleosynthetic isotope anomalies is first provided. Following this, a discussion on how nucleosynthetic isotope anomalies are used to constrain the early architecture of the Solar System is presented. To conclude, possible future directions that the scientific community may pursue by applying nucleosynthetic isotope anomalies to questions about terrestrial accretion are presented.