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Abstract:
Introduction: 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. Such grains, which can
be found today in primitive meteorites, interplanetary dust particles, and cometary dust, mostly escaped alteration and
homogenization processes in the interstellar medium as well as during the formation of the protosolar nebula and
protoplanetary disk [1]. The properties of these grains hold valuable information on stellar nucleosynthesis and evolu-
tion, grain formation in circumstellar environments, and the types of stars contributing material to the nascent Solar
System. Silicates are the most abundant type of presolar dust available for single grain analyses [2], followed by re-
fractory oxides, SiC, graphite, and Si3N4. Presolar SiC and graphite have been found to carry various amounts of
trace elements, depending on the parent star’s and circumstellar envelope’s chemical composition, physical properties
(e.g., pressure, mass-loss rate) and condensation behavior of the respective mineral phases [e.g., 3–9]. For O-rich
grains, however, data for elements heavier than Fe and Ni are virtually non-existent; only a single study found Sr in
one complex presolar grain [10]. Here, we report the first data for heavy trace elements in a presolar silicate grain.
Samples & Experimental: We conducted a search for trace elements in a particularly large presolar Ca-bearing
silicate grain, NWA7540_3A_3 (890 nm × 420 nm) with the NanoSIMS 50 at the Max-Planck-Institute for Chemis-
try. The grain was identified during previous O-isotope mapping in the ordinary chondrite Northwest Africa (NWA)
7540 (LL3.15). Measurements were performed with the Hyperion RF plasma source, by rastering a focused O– ion
beam (~15 pA, ~300 nm) over a 5×5 μm2-sized area around the presolar grain, with an integration time of ~44 min
per measurement. Three different mass sequences were applied: (1) 24Mg+, 28Si+, 88Sr+, 93Nb+, 138Ba+; (2) 24Mg+,
28Si+, 85Rb+, 98Mo+, 140Ce+; (3) 24Mg+, 28Si+, 86Sr+, 90Zr+, 139La+. Peak positions and relative sensitivity factors were
calibrated and confirmed using an NBS 611 glass standard.
Results & Discussion: The elements Sr, Zr, and Ce could be positively identified within the grain, with the fol-
lowing abundances given relative to Si: Sr/Si = 1.7±0.9 × 10–4, Zr/Si = 1.2±0.3 × 10–4, and Ce/Si = 1.2±0.5 × 10–4.
The Sr/Si ratio is in a similar range as the one reported for the O-rich complex grain by [10]. The respective Solar
System abundance ratios are Sr/Si = 2.5 ×10–5, Zr/Si = 1.15 × 10–5, and Ce/Si = 1.23 × 10–6 [11], significantly lower
than what is observed in the presolar silicate. From model data for low-mass asymptotic giant branch (AGB) stars
[12], we find that ratios of several 10–4 can occur during thermal pulses while the C/O ratios are below one, which
would favor the condensation of O-rich dust. However, the Zr/Si and Ce/Si ratios of NWA7540_3A_3 exceed the
respective model predictions, indicating additional effects governing the incorporation of these trace elements. During
previous Ca-Ti isotopic measurements [10], a heterogeneous 48Ti-distribution was observed for this presolar silicate,
and the 90Zr- and 140Ce-distributions observed here show some correlation with the area having the highest 48Ti-
intensity. For a gas of solar composition, it is predicted that Sr and Ce would condense into titanate at similar tem-
peratures (1464 K and 1478 K, respectively) [11], while Zr would form ZrO2 at T~1741 K. Since Zr shares some
chemical similarities with Ti (both are located in group 4 of the periodic table), ZrO2 might have been incorporated
into Ti-oxides, which could have served as condensation nuclei for the silicate grain [e.g., 14]. Comparison of the
trace element data from this study with SiC grain results is not straightforward. Formation of O-rich and C-rich cir-
cumstellar dust occurs in chemically different environments, and also at different stellar evolutionary stages, and both
factors likely have an influence on the availability and condensation behavior of a given trace element. Our results
show that presolar silicates have to be considered, besides carbonaceous stardust, as carriers of heavy trace elements.
Acknowledgements: We thank Elmar Gröner and Philipp Schuhmann for technical assistance with the Na-
noSIMS, and Knut Metzler (Institute for Planetology, Münster) for making the NWA 7540 sample available.
References: [1] Zinner E. (2014) In Meteorites and Cosmochemical Processes (ed. Davis A. M.). Elsevier, Am-
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B. Jr. (1995) Meteoritics 30:661–678. [4] Amari S. et al. (1995) Meteoritics 30:679–693. [5] Croat T. K. et al.
(2005) The Astrophysical Journal 631:976–987. [6] Henkel T. et al. (2007) Meteoritics & Planetary Science
42:1121–1134. [7] Marhas K. K. et al. (2007) Meteoritics & Planetary Science 42:1077–1101. [8] King A. J. et al.
(2012) Meteoritics & Planetary Science 47:1624–1643. [9] Jadhav M. et al. (2015) LPS XLVI, Abstract #2882.
[10] Leitner J. et al. (2018) Geochimica et Cosmochimica Acta 221:255–274. [11] Lodders K. (2003) The Astro-
physical Journal 591:1220–1247. [12] Cristallo S. et al. (2015) The Astrophysical Journal Supplemental Series
219:40–60. [13] Leitner J. and Hoppe P. (2022) LPS LIII, Abstract #1949. [14] Gail H.-P. & Sedlmayr E. (1998)
Faraday Discussions 109:303–319
