Remarkably high abundance of presolar grains in interplanetary dust particles collected from the comet Grigg-Skjellerup dust stream

Introduction: Interplanetary dust particles (IDPs) have been shown to preserve isotopic anomalies, particularly in nitrogen and hydrogen associated with molecular cloud organic material [1-3]. Furthermore, with the heightened capabilities of the NanoSIMS ion microprobe, submicrometer-sized presolar silicate grains have been identified in IDPs [3-6]. Recently, four IDPs collected during the 2003 Earth passage through the comet Grigg-Skjellerup dust stream were analyzed by isotopic and micro-Raman imaging [7]. All of the samples display heterogeneous distributions and anomalous “hotspots” in both D/H and N/N ratios (Fig. 1). Raman spectra for these samples indicate disordered C similar to the most primitive interstellar organic material in meteorites [8]. These data support the possibility that the IDPs studied by [7] did indeed originate from comet Grigg-Skjellerup. Of the four samples, L2054 E1, a fragment from a cluster IDP, and individual IDP L2054 G4 have the highest D-enrichments, with δD up to 16,000 ‰. We thus focus on these two samples to conduct further isotopic studies. Experimental: The IDPs L2054 E1, L2054 F1 (a fragment from the same cluster IDP as E1), L2054 G2, L2054 G3, and L2054 G4, all of which had been pressed into Au foils, were analyzed by raster ion imaging with the Carnegie NanoSIMS 50L ion microprobe. A focused (~100 nm) Cs primary ion beam was rastered over the IDPs, and negative secondary ions of the three O isotopes, C, Si, and CN were measured simultaneously along with secondary electrons. Integrated ion images were produced and from these, O isotopic ratio images were also calculated. Regions of interest were defined and their O isotopic ratios were normalized to the average ratio of the IDP. Grains having O isotopic compositions distinct from the isotopically normal sub-regions of the sample could thus be identified. The mineral species (i.e., oxide or silicate) was estimated from the Si⎯ signal. For L2054 E1 and G4, the three Si isotopes and MgO were analyzed in a second set of measurements. In addition, O and O were measured again to ensure that any grains previously identified as having anomalous O/O ratios could easily be located and their Si isotopic ratios deduced. The Si isotopic ratios and the O/O ratio were normalized to those of the average IDP. Results: L2054 E1 and F1: No anomalous grains Figure 1. Isotopic ratio images of L2054 E1. The N (NanoSIMS) and D (IMS-6f) measurements were performed previously by [7]. Grains having anomalous compositions appear distinct from the rest of the IDP. Presolar silicate grains are circled in the O ratio images.