Oxygen and aluminum-magnesium isotopic systematics of presolar nanospinel grains from CI chondrite Orgueil

Abstract

Presolar oxide grains have been divided into several groups (Group 1 to 4) based on their O isotopic compositions, which can be tied to several stellar sources. Much of the available data was acquired on large grains, which may not be fully representative of the presolar grain population present in meteorites. We present here new O isotopic data for 74 small presolar oxide grains (∼200 nm in diameter on average) from Orgueil and Al-Mg isotopic systematics for 25 of the grains. Based on data-model comparisons, we show that (i) Group 1 and Group 2 grains more likely originated in low-mass first-ascent (red giant branch; RGB) and/or second-ascent (asymptotic giant branch; AGB) red giant stars and (ii) Group 1 grains with (²⁶Al/²⁷Al)<inf>0</inf> ⪆ 5 × 10⁻³ and Group 2 grains with (²⁶Al/²⁷Al)<inf>0</inf> ⪅ 1 × 10⁻² all likely experienced extra circulation processes in their parent low-mass stars but under different conditions, resulting in proton-capture reactions occurring at enhanced temperatures. We do not find any large ²⁵Mg excess in Group 1 oxide grains with large ¹⁷O enrichments, which provides evidence that ²⁵Mg is not abundantly produced in low-mass stars. We also find that our samples contain a larger proportion of Group 4 grains than so far suggested in the literature for larger presolar oxide grains (≥400 nm). We discuss this observation in the light of stellar dust production mechanisms.