Oxygen Isotope Study of Interplantary Dust Particles: Insight into the Oxygen Reservoirs of the Comet Forming Region

Introduction: Recent work on Wild2 cometary samples shows evidence for sampling of high-T grains that have strong similarities to components in carbonaceous chondrites. Such grains in cometary samples have been used as evidence for large-scale radial mixing in the early Solar System [1,2]. However, Wild2 samples are biased towards relatively coarse-grained material, with finer-grained components being either too small or dispersed along aerogel track walls [3] to be analysed with sufficient isotopic precision. Stratospheric IDPs preserve fine-grained and OHbearing phases largely unmodified during atmospheric entry and capture. This fine-grained material is arguably the most primitive Solar System material available for analysis, some of which most likely originates from comets. As such, IDPs offer the opportunity to analyse O-isotopes in Solar System materials that represent the outer Solar nebula. The majority of in-situ O-isotope analyses on cometary materials by SIMS technique have been performed on large ferromagnesian minerals from Stardust [2] and a few IDPs [4]. ’Bulk’ O-isotope analyses of a few fine-grained IDPs have been performed by SIMS [5] providing some initial insight into O-isotope systematics of outer Solar System reservoirs. In this study we use a NanoSIMS 50L to analyse Oisotopes in a set of IDPs. The NanoSIMS offers a small beam size thereby allowing more controlled and even sputtering of small, thin, pressed samples. These measurements are at a level of precision that allows for their comparison to analyses of samples from inner Solar System bodies. Of particular importance is understanding the relationship between the fineand coarsegrained phases that accreted in the outer Solar System and whether fine-grained material experienced recycling through the inner Solar System. If fine-grained IDPs represent unproccessed dust from the outer Solar nebula then they have the potential to test models for self-shielding and, in particular, the location and nature of the self-shielding environment. Methods: The IDPs analysed in this study were from large cluster particles. Three of the particles were from the Grigg-Skjellerup collection (L2055 Cluster 5 (Drake), L2055 Cluster 11 (Frobisher) and L2054 Cluster 4 (Hawkins)) and 3 were from non-specific collectors (L2005 Cluster 31 (Cortes and Pizarro) and L2006 Cluster 14 (Midford)). The particles were pressed into high-purity gold foil and characterised by FEG-SEM EDX prior to NanoSIMS analysis. Oisotope analyses were performed with a Cs + probe with a current of ~50pA that was rastered over the samples in spot mode (5x5 μm raster). 16 O was measured on a faraday cup and 17 O, 18 O (and 24 Mg 16 O) were measured on electron multipliers. Charge compensation was applied using the electron gun. The mass resolution was set to >10,000 (Cameca definition) for all analyses primarily to resolve the interference of 16 OH on 17 O. Isotope ratios were normalised to SMOW using San Carlos olivine that bracketed the analyses. IDP Hawkins was corrected to Al2O3 for matrix effect. The IDPs were also analysed in NanoSIMS imaging mode for H, C and N abundance and isotopic compositions [6]. Results: All particles display chondritic EDX spectra. Based on particle texture and C/H ratio (an indicator for the relative hydrous/anhydrous nature of the particles [7]), the IDPs can be broadly separated into three groups. 1) Anhydrous particles: Drake, Pizarro, Cortes and Midford. These show the finest-grained porous texture with relatively high C/H ratios (>1). Midford has a very high C/H (>2) and δD ~5000‰, similar to OM3 discussed in [7]. 2) Hydrous particle: Frobisher shows a predominantly platy texture resulting in a more compact, less porous, structure with a C/H ratio of <0.5. 3) Refractory grain (hydrous): Hawkins consists of a large (~8x4 μm) angular grain of Aloxide (corundum) attached to three small fine-grained porous particles (<2 μm diameter). These fine-grained particles are carbon poor but variable (C/H 0-0.3).