Isotopische Untersuchungen zur Identifikation von präsolaren Phasen in den ältesten Festkörpern des Sonnensystems
Zusammenfassung der Projektergebnisse
• Allgemeinverständliche Darstellung der wichtigsten wissenschaftlichen Fortschritte und ggf. ihrer Anwendungsaspekte. Calcium-aluminum-rich inclusions (CAIs) are the first solids that condensed in the cooling protoplanetary disk (e.g., Amelin et al., 2010; Connelly et al., 2012). Therefore, CAIs are used to mark the start of the Solar System and they provide insightful information about the origin and processing of the earliest reservoir(s) in the Solar System. Furthermore, CAIs are small objects, ranging from the micrometer to centimeter scale, and it is believed that CAIs formed near the young Sun before they were transported outwards to the chondrite accretion regions (e.g., Desch et al., 2018). Investigations of the isotope compositions of CAIs can help in identifying a sample’s provenance and can be used to calculate formation ages of these objects. Specifically, isotope compositions can be broken down into two separate categories: massindependent and mass-dependent effects. Mass-dependent isotope signatures reflect processing events of CAIs such as evaporation or condensation. However, massindependent isotope signatures of CAIs reflect processes which include 1) radiogenic ingrowth (used to determine the age of these objects), 2) effects caused by irradiation from the young Sun, or 3) nucleosynthetic anomalies arising from the incomplete homogenization of presolar material. Importantly, CAIs of all varieties (i.e., platy hibonite crystals (PLACs), spinel-hibonite inclusions (SHIBs), fractionation and unknown nuclear effects (FUN inclusions), normal CAIs) are known to exhibit both nucleosynthetic isotope anomalies and mass-dependent isotope fractionation relative to later-formed solids such as chondrules, chondrites, and terrestrial planets (e.g., Dauphas and Schauble, 2016). However, while significant research efforts have been dedicated to understanding the formation and evolution of CAIs, several questions remained unanswered with regards to CAI formation and also in linking CAIs to other early Solar System solids. In this study we investigated nucleosynthetic anomalies of the elements Ti, Fe, and Zr as well as mass-dependent isotopic compositions of Ti, Fe, and Mg in a unique CAI sample set. We processed four CAIs through a sequential acid leaching procedure and then made the aforementioned isotopic measurements. We observed little, if any, massindependent Ti isotopic variability between the CAI leachates. However, bulk CAIs along with Ti isotopic laser ablation data from various CAIs indicate significant interand intra-CAI isotopic heterogeneity in 50Ti. We found that the range of Ti isotope compositions recorded by CAIs can be accounted for by the averaging of hibonite grains and that the variable Ti isotope compositions of hibonite grains can be explained by the averaging of isotopically diverse presolar grains present in the Sun’s parental molecular cloud. We further investigated this effect of averaging by applying the central limit theorem to the Ti isotopic data (Shollenberger et al., in revision). This averaging effect was an important finding from this work and may be applicable to other isotopic systems in CAIs. Additionally, we were able to put forth an idea of how early formed solids in the protoplanetary disk are related. Our Fe isotopic results indicate that bulk CAIs have Fe isotope anomalies that are indistinguishable at the current level of precision from bulk meteorites. This likely indicates that most of the Fe in the CAI forming region/process was present in the gas phase and had an Fe isotopic composition similar to bulk meteorites because Fe is well mixed in the gas phase. However, small Fe isotopic variations in our later leachate samples likely derive from refractory dust incorporated into CAIs. The isotopically distinct Fe from the dust is such a small fraction of the total CAI Fe budget that it is only revealed with an acid leaching procedure that essentially removes all the CAI Fe derived from the gas phase. The mineralogical host of the refractory Fe component is yet to be determined. The Zr isotopic results from our CAI leachates indicate a heterogeneity between different CAI leachate steps. However, further work is required to fully understand the Zr isotopic anomalies in CAI leachates as our chemical blanks were a concern. The Mg isotope results obtained thus far indicate that these CAIs have experienced condensation and evaporation events as indicated by light and heavy Mg isotopic compositions and the final samples are still being processed. We are still evaluating our data in terms of the nucleosynthetic anomalies and mass-dependent isotopic compositions. Altogether, this study demonstrates a link between early formed solids in the Solar System and provides evidence that the solar nebula was well mixed prior to CAI formation.
Projektbezogene Publikationen (Auswahl)
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(2021) 54Fe Heterogeneity in sequential acid leachates of refractory inclusions. Lunar and Planetary Science Conference, #1733
Shollenberger Q.R., Tang H., and Young E.D.
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(2021) Titanium isotope systematics of refractory inclusions: Echoes of molecular cloud heterogeneity. Lunar and Planetary Science Conference, #2316
Shollenberger Q.R., Render J., Jordan M.K., McCain K.A., Ebert S., Bischoff A., Kleine T., and Young E.D.
