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

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 rep-resentative of the presolar grain population present in meteorites. We present here new O isotopic data for 74 small presolar oxide grains (~& nbsp;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 (Al-26/Al-27)(0) ' 5 x 10(-3 )and Group 2 grains with (Al-26/Al-27)(0) / 1 x 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 tem-peratures. We do not find any large( 25)Mg excess in Group 1 oxide grains with large( 17)O enrichments, which provides evidence that Mg-25 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. (c) 2021 Elsevier Ltd. All rights reserved.