PRESOLAR MATERIALS AND NITROGEN ISOTOPE ANOMALIES IN THE UNIQUE CARBONACEOUS CHONDRITE MILLER RANGE 07687

Introduction: Presolar grains are particularly abundant in pristine extraterrestrial materials such as interplanetary dust particles [e.g., 1,2] and in the matrices of the most primitive chondrites [e.g., 3–7]. They are sensitive indicators of nebular and parent-body processes. With the exception of nanodiamonds, whose origins are ambiguous, presolar silicates are the most abundant type of presolar grain and have been reported in high abundance in a number of primitive chondrites, including the CO3s Allan Hills (ALHA) 77307 [3] and Dominion Range (DOM) 08006 [4]. Here we report the results of a NanoSIMS ion imaging study of the Antarctic carbonaceous chondrite Miller Range (MIL) 07687, which was originally classified as a CO3 [8] but may be ungrouped [9]. High presolar grain abundances (both Oand C-anomalous) indicate that this chondrite is of very low petrographic type, in agreement with petrographic observations [10]. Experimental: Areas of matrix in a thin section were identified in scanning electron microscope images and analyzed by isotopic imaging with a Cameca NanoSIMS 50L ion probe. A ~2 pA Cs primary ion beam was used to image 10×10 μm areas with a spatial resolution of ~150 nm. Two sets of measurements were performed: (i) to locate Oand C-anomalous presolar grains (O, C2, CC, Si, AlO and secondary electrons), and (ii) to characterize the N isotopic composition of carbonaceous material (O, C2, CC, CN, CN, Si and secondary electrons). It was possible to identify O-anomalous presolar grains in the second set of measurements – these grains were then re-analyzed for all three O-isotopes. Petrographic Description: MIL 07687 is very terrestrially weathered: it possesses an oxidation rind, exhibits a polygonally fractured interior and exterior, the matrix contains abundant evaporite minerals, and chondrules appear to be rust colored [8]. MIL 07687 contains abundant small (≤1 mm diameter) chondrules, chondrule fragments and mineral grains in an optically dark matrix [8]. Olivine is unequilibrated; compositions range from Fa1 to Fa50 [8]. MIL 07687 is unique in that it exhibits an unusual type of aqueous alteration that appears to be partial and localized, resulting in two apparent matrix lithologies (not to be confused with terrestrial alteration) [9]. The most altered matrix regions (either by parent body or terrestrial alteration) were avoided during analyses. On the basis of the Cr content of ferroan olivine [e.g., 11], MIL 07687 appears to have an estimated subtype of 3.00, similar to that of Acfer 094 (3.0), these results are presented in a companion abstract [10]. Results and Discussion: A fine-grained matrix area of ~14,500 μm was analyzed for O-isotopes in two areas separated by several mm (Areas 4 and 5). Nineteen presolar O-anomalous and 16 presolar SiC grains were identified (Table 1). A further matrix area, A4 (N), of ~6,500 μm (adjacent to Area 4) was analyzed for N isotopes (O-anomalous grains were also identified). Numerous regions with anomalous N-isotopic compositions were identified alongside a further 10 presolar O-anomalous and four SiC grains. C-anomalous grains. The diameters of the 20 presolar SiC grains range from ~200 to 460 nm. Other Canomalies (not associated with Si) were also present. O-anomalous grains. The 29 identified Oanomalous grains have O-isotopic compositions (Fig. 1) similar to those previously reported [12]. They range in diameter from 260 to 470 nm. For the 19 grains identified in the first set of measurements it was possible to determine the silicate versus oxide nature of the grains based on their AlO/O ratios. Four appear to be Group 1 oxides and 15 are silicates (from Groups 1, 2 and 4), yielding a silicate/oxide ratio of ~5.