Continued Characterization of Presolar Silicate Grains from the Acfer 094 Carbonaceous Chondrite

Introduction: Among circumstellar grains produced around O-rich evolved stars, silicate grains are the most abundant [1]. Until recently, the identification of presolar silicate grains in extraterrestrial materials proved difficult due in part to their destruction by parent body processes, requiring analysis of very primitive samples, and to the experimental challenge presented by their submicron diameters and the predominance of grains of solar-system origin. Presolar silicate grains were first identified in anhydrous interplanetary dust particles (IDPs) [2, 3], and subsequently in meteorites. By far the largest number of presolar silicates in meteorites has been found in the very primitive carbonaceous chondrite Acfer 094 [4-7]. Six additional grains have been found in Semarkona and Bishunpur [8, 9], and NWA 530 [5]. We previously reported the discovery of 9 presolar silicate grains among Acfer 094 matrix grains 0.1-0.5 μm in diameter [4]. This was achieved by isotopic raster ion imaging of dense grain areas in the NanoSIMS ion microprobe. The chemical composition of six grains was determined by X-ray analysis. In addition, the Mg isotopic composition of one grain revealed a high (Al/Al)o ratio of 0.12, explained by cool bottom processing (CBP) in a low-mass thermally pulsing asymptotic giant branch (AGB) star. Here we report additional analyses made on a coarser grain size separate and on a thin section of Acfer 094. Experimental: A grain size separate of Acfer 094 containing matrix grains 0.5-1 μm in diameter was produced in the same manner described previously [4]. Dense grain areas on a gold substrate were chosen for isotopic analysis in the NanoSIMS. While grain size separates allow us to focus on silicate grains of a specific size range, in situ measurements on a polished thin section afford analysis of all matrix grains. For both samples, a ~100 nm Cs primary ion beam was rastered over 30x30 μm (grain separate) or 20x20 μm (thin section) areas. In addition to the three O isotopes, MgO and Si were measured simultaneously as negative secondary ions to produce integrated 256x256 pixel ion images. Oxygen isotopic ratio images were calculated and used to identify anomalous grains. A grain had to have an O isotopic composition distinct from that of the surrounding matrix material, most of which is isotopically normal, to be considered presolar. The MgO and Si signals could then be correlated with a grain of interest to deduce its silicate nature. We also measured the three Si isotopes as negative secondary ions for 10 anomalous silicate grains. For these measurements, a Cs primary beam was rastered over 10x10 μm areas around an anomalous grain and 128x128 pixel images were acquired. In addition, we succeeded in producing a focused ion beam (FIB) liftout section of one presolar silicate identified from a grain dispersion mount. This resulted in the first transmission electron microscopy (TEM) study of a presolar silicate grain from a meteorite. Details of this technique are given in [10]. Results and Discussion: We have identified 11 new presolar silicate grains and 8 presolar oxide grains with diameters between 100 and 600 nm. According to the MgO/O ratio of the presolar oxides, 3 appear to be spinel and 5 appear to be corundum. The O isotopic ratios of these grains along with our previously discovered grains from Acfer 094, and those from [6] are shown in Fig. 1. All four of the previously defined presolar oxide groups [11] are represented among these grains. Most are group 1 grains, believed to have formed in the atmospheres of low-mass red giant branch (RGB) and AGB stars.