The Mg isotope composition of presolar silicate grains from red giant stars

We report O and Mg isotope compositions of presolar silicate grains which likely formed around asymptotic giant branch stars. Our grains represent the most abundant Mg-rich presolar grain group and their Mg isotope composition provides thus far missing information about the contribution of isotopically anomalous presolar dust to the Mg isotope inventory of the early Solar System. Presolar silicate grains were identified in situ, using the NanoSIMS, in the matrix of the ungrouped carbonaceous chondrite Acfer 094. O isotope compositions suggest that the presolar grains of the present study formed in the stellar winds of low mass (M <= similar to 2.2 x M-solar) red giant or asymptotic giant branch stars of close-to-solar metallicity and thus belong to the most abundant presolar silicate grain group. In order to minimise matrix contributions during spatially poorly resolved Mg isotope analyses (spatial resolution comparable to average grain size), meteorite matrix in the presolar grains' vicinity was removed using a focussed Ga ion beam. To monitor accuracy, we prepared and analysed O-isotopically regular (Solar System) matrix grains the same way as the presolar grains. The Mg-25/Mg-24 ratios of all seven successfully analysed presolar silicate grains are identical to that of the Solar System at the precision of our measurements. The Mg-26/Mg-24 ratios of five grains are also solar but two grains have significant positive anomalies in Mg-26/Mg-24. On average, however, Mg-25/Mg-24 and Mg-26/Mg-24 ratios are higher than solar by a few %. All grain compositions are consistent with Galactic chemical evolution and, possibly, isotope fractionation caused by interstellar or Solar System processing (sputtering and/or recondensation). The grain with the strongest enrichment in Mg-26 relative to Mg-25 (delta Mg-25 = 34 +/- 25 parts per thousand, delta Mg-26 = 127 +/- 25 parts per thousand; where delta Mg-x = 1000 x [(Mg-x/Mg-24)(grain)/(Mg-x/Mg-24)(meteorite matrix)) - 1] with x = 25 or 26; the reported uncertainty corresponds to 1 sigma), probably incorporated Al-26 during grain condensation. Our and previously reported Mg isotope data on presolar oxide and silicate grains indicate that the isotopically anomalous O-rich dust component of the Solar System's parent molecular cloud was heterogeneous with respect to Mg isotope compositions and probably had a higher Mg-26/Mg-24 ratio on average than that of the present-day Solar System. (C) 2014 Elsevier Ltd. All rights reserved.