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

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 25Mg/24Mg ratios of all seven successfully analysed presolar silicate grains are identical to that of the Solar System at the precision of our measurements. The 26Mg/24Mg ratios of five grains are also solar but two grains have significant positive anomalies in 26Mg/24Mg. On average, however, 25Mg/24Mg and 26Mg/24Mg 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 26Mg relative to 25Mg (δ25Mg = 34 ± 25‰, δ26Mg = 127 ± 25‰; where δxMg = 1000 × [(xMg/24Mg)grain/(xMg/24Mg)meteorite matrix) − 1] with x = 25 or 26; the reported uncertainty corresponds to 1 σ), probably incorporated 26Al 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 26Mg/24Mg ratio on average than that of the present-day Solar System.