The search for trace elements in stardust silicate grains

Isotopically anomalous dust grains that formed in the outflows of evolved stars and in the ejecta of stellar explosions are a minor, but important component of primitive Solar System materials. These grains carry records of stellar nucleosynthesis and evolution, circumstellar grain formation, and stars contributing material to the nascent Solar System. Silicates are the most abundant type of presolar dust available for single grain analyses [1]. Presolar SiC and graphite contain various amounts of trace elements [e.g., 2], while for O-rich grains, only one study reported Sr in an O-rich grain [3]. Here, we report on a search for trace elements in a large presolar silicate by NanoSIMS. Measurements were performed with the Hyperion RF plasma O-source using three different mass sequences. Strontium, Zirconium, and Cerium were identified, with Sr/Si in compliance with astrophysical models. Zr/Si and Ce/Si ratios exceed model predictions, indicating additional effects governing their incorporation. The 90Zr- and 140Ce-distributions are correlated with 48Ti. For a gas of solar composition, Sr and Ce would condense into titanate, while Zr forms ZrO2 at higher temperatures. Due to chemical similarities between Zr and Ti, ZrO2 might have been incorporated into Sr- and Ce-bearing Ti-oxides, serving as condensation nuclei for the silicate [4].

References: [1] Floss C. & Haenecour P. (2016) Geochemical Journal 50: 3–25. [2] Amari S. et al. (1995) Meteoritics 30:679–693. [3] Leitner J. et al. (2018) Geochimica et Cosmochimica Acta 221:255–274. [4] Gail H.-P. & Sedlmayr E. (1998) Faraday Discussions 109:303–319.