Stable Isotope Measurements in Presolar Grains

The most abundant presolar grains in primitive meteorites are nanodiamonds. These grains survived the formation of the solar system and kept their own individuality. However, existing measurements of their isotopic pattern do not allow for a consistent understanding of their origin. The reasons are mainly due to their small size (nm) and the low abundance of trace elements. Most likely, such nanodiamonds were formed in a supernova environment. Positive measurements of trace-element isotopic signatures will help understanding heavy element nucleosynthesis in massive stars and dust formation from their ejecta. They will also enable testing predictions of r-process nucleosynthesis and understanding the uniformity of the “main r-process”.
A first approach applying AMS to measure Pt in nanodiamonds was performed at TU Munich [1]. We have continued our search for stable isotope anomalies in nanodiamonds at VERA. Recent experience showed that AMS provides the required measurement precision together with a low Pt machine background. Moreover, we observed for the first time enhancements of ¹⁹⁸Pt/¹⁹⁵Pt isotope ratios in two diamond residues from the Allende meteorite, which were prepared by different separation techniques. Variations in other isotopic ratios were within analytical uncertainty, and no anomaly was identified in a third diamond fraction. An enhanced ¹⁹⁸Pt/¹⁹⁵Pt ratio is predicted by models that either include or exclude a simultaneous negative anomaly in ¹⁹⁴Pt/¹⁹⁵Pt. This negative anomaly, however, was not observed via AMS and is in contrast to data obtained for tellurium.
The robustness of these first results needs to be verified by detailed and systematic studies of possible mass-fractionation effects and potential scatter due to inhomogeneities within samples. We will present our measurement approach and plans for other isotopes. The latter is part of the Eurogenesis programme organized by the European Science foundation.

[1] S. Merchel et al., Geochim et Cosmochim Acta 67 (2003) 4949.