Presolar grains in the CM2 chondrite Sutter's Mill

–The Sutter’s Mill (SM) carbonaceous chondrite is a regolith breccia, composedpredominantly of CM2 clasts with varying degrees of aqueous alteration and thermalmetamorphism. An investigation of presolar grains in four Sutter’s Mill sections, SM43,SM51, SM2-4, and SM18, was carried out using NanoSIMS ion mapping technique. A totalof 37 C-anomalous grains and one O-anomalous grain have been identified, indicating anabundance of 63 ppm for presolar C-anomalous grains and 2 ppm for presolar oxides.Thirty-one silicon carbide (SiC), five carbonaceous grains, and one Al-oxide (Al 2 O 3 ) wereconfirmed based on their elemental compositions determined by C-N-Si and O-Si-Mg-Alisotopic measurements. The overall abundance of SiC grains in Sutter’s Mill (55 ppm) isconsistent with those in other CM chondrites. The absence of presolar silicates in Sutter’sMill suggests that they were destroyed by aqueous alteration on the parent asteroid.Furthermore, SM2-4 shows heterogeneous distributions of presolar SiC grains (12–54 ppm)in different matrix areas, indicating that the fine-grained matrix clasts come from differentsources, with various thermal histories, in the solar nebula.INTRODUCTIONThe Sutter’s Mill (SM) carbonaceous chondrite(CM2) fell in California on April 22, 2012. WeatherDoppler radars detected the falling meteorites, resultingin a fast recovery of some stones (Fries and Fries 2010;Jenniskens et al. 2012). The first stones found (SM 1-3)were recovered before rain and have minimal terrestrialweathering. Petrographic studies indicate that Sutter’sMill is a regolith breccia, composed of CM2 lithologieswith various degrees of aqueous alteration and thermalmetamorphism (Jenniskens et al. 2012; Zolensky et al.2012). Previous studies of acid residues show that someCM2 chondrites (e.g., Murchison, Murray) containabundant presolar material, mainly presolar SiC grains(e.g., Zinner 2013). Sutter’s Mill contains abundant fine-grained matrix areas, which are suitable for presolargrain searches. Previous studies have suggested thatdifferent secondary processes (aqueous alteration,thermal metamorphism) may preferentially destroypresolar mineral phases (presolar silicate/oxide, SiC)(e.g., Huss et al. 2003; Floss and Stadermann 2012).Here, we used NanoSIMS ion imaging to investigate thepresolar inventories of different SM fragments, forcomparison with the presolar grain abundances inMurchison and other primitive chondrites, and to gaina better understanding of the effects of secondaryprocesses on the preservation of presolar grains.SAMPLES AND EXPERIMENTALThe fragments analyzed in this study are SM43,SM51, SM2-4 (a subsample of SM2), and SM18 (forthe find locations in the strewn field, see Jenniskens