Complementary Carbon, Nitrogen and Oxygen Isotopic Imaging of Interplanetary Dust Particles: Presolar Grains and an Indication of a Carbon

Introduction: Interplanetary dust particles (IDPs) are composed of a variety of primitive materials and are the repositories of various isotopic anomalies. They are well known to exhibit enrichments of deuterium (D) and N [e.g., 1-3], although the two types of isotopic anomalies do not appear to be directly correlated. The enrichments are generally thought to be carried by organic compounds [4-6], but C isotopic measurements of IDPs have thus far shown no clear evidence for anomalous C compositions [3,7,8]. Recently, silicate grains with extremely anomalous O isotopic compositions, indicating a presolar origin, have also been reported from IDPs [9,10]. We are carrying out complementary isotopic imaging measurements (c.f., H, C, N, O) on a suite of IDPs using the NanoSIMS at Washington University. This work is part of a larger effort to survey the isotopic microdistributions of IDPs and Renazzo matrix, in order to compare and clarify the origins of such anomalies in both types of primitive materials [e.g., 7,11,12]. Here we focus on the C, N and O isotopic systematics of two unusual IDPs. Experimental Procedures: We have so far measured the C and N isotopes in 14 cluster and 11 non-cluster IDPs from collectors L2009, L2011 and L2036, and have measured the O isotopes in a subset of eight of these. The measurements are made in raster imaging mode, in which a Cs beam is scanned over the sample surface and secondary ions are collected simultaneously at high mass resolution. Details of the measurement procedure for C and N imaging are given by [2,7]. The procedure for oxygen isotopic measurements is similar, with the measured species being O, O, O, Si and MgO. Particular care is taken to ensure complete separation of the O peak from the isobaric OH peak at high mass resolution. Results are calibrated to isotopic standards measured along with the samples. Each analysis consists of 15 to 40 scans, which, added together, constitute a single image measurement. Thus, observed isotopic anomalies can be verified through the different layers of the image. Results: Nitrogen isotopic variations are generally similar to those previously observed [2,3,7]. Most IDPs have normal bulk N compositions but may contain discrete ‘hotspots’ that are enriched to varying degrees in N. Carbon isotopes are normal in all of the IDPs, with one exception discussed below. Oxygen isotopes are also normal in the bulk IDPs, but we report below the discovery of two sub-grains with significant O enrichments. IDP ‘Kipling’ (L2011-R12): Last year we reported on the C and N isotopic compositions of this non-cluster IDP [7]. Carbon isotopic compositions are normal, but the bulk of the IDP is enriched in N, with an average dN value of +510‰. We also observed two hotspots in the center of the IDP with enriched N (dN = +1090‰ and +1250‰). We have now measured the O isotopes in this IDP and find a discrete sub-grain that is strongly enriched in O (Fig. 1). The grain, which is not associated with the N-rich hotspots, is about 350 by 500 nm in size and has a O/O ratio of 0.00081 (solar value 0.00038 [13]) and normal O/O. Figure 1 compares the isotopic composition of this grain with those of similarly sized sub-regions from the rest of the IDP, clearly demonstrating its anomalous isotopic composition. IDP ‘Benavente’ (L2036-G16): Like Kipling, the bulk N isotopic composition of this non-cluster IDP is enriched in N, although the degree of enrichment