Equilibrated Aggregates in Cometary Idps: Insights into the Crystallization Process in Protoplanetary Disks.

Introduction. Processes occurring within proto-planetary disks partly convert amorphous interstellar silicate grains into crystalline grains [e.g. 1,2]. The details of the transformation mechanism(s) and the subsequent transport of the crystalline grains from the hot inner part of the disk to cooler regions are poorly constrained [3]. Having formed in the coldest, most distant region of the Solar System, comets harbor well preserved remnants of these processes, and provide an important counterpoint to inner solar system materials that comprise meteorites. Initial studies of cometary materials returned by the Stardust spacecraft reveal abundant refractory minerals whose origins are traced to the inner solar system [e.g. 4], indicating efficient transport of processed materials to the outer reaches of the solar nebula. The best preserved samples of original nebular materials may be found in anhydrous chondritic porous (CP) interplanetary dust particles (IDPs) whose origins are linked to short-period comets [e.g. 5]. We are analyzing the crystalline grains within cometary IDPs to gain insights into the crystallization processes in the early solar nebula. Many of the crystalline sili-cates in IDPs occur as single crystals with compositions and microstructures that are most consistent with high T condensation processes [6]. In previous studies we have shown the great majority of crystalline sili-cates have solar O isotopic compositions, and thus likely formed in the inner regions of the solar system [7]. Crystalline silicates in IDPs also occur in poly-crystalline grains that are referred to as equilibrated aggregates (EAs) [8]. We are exploring the chemistry, mineralogy and isotopic characteristics through coordinated analyses of EAs in IDPs to determine if they represent some of the earliest examples of grain growth and coagulation in the early nebula. Methods and Samples. Nanometer-scale quantitative compositional maps of equilibrated aggregates in microtome thin sections of chondritic porous IDPs (L2005AL5, L2011B10, W7029C28, L2005AA7) were obtained using a JEOL 2500SE 200 keV field-emission scanning-transmission electron microscope (STEM) equipped with a Noran thin window energy-dispersive X-ray (EDX) spectrometer. Following EDX mapping, the sections were subjected to O isotopic imaging with the JSC NanoSIMS 50L. Results. Equilibrated aggregates are 0.1-2.0 μm sized irregularly-shaped, polycrystalline objects that are a component of cometary IDPs (Fig.1a). Their abundance in individual IDPs is highly variable from a few vol.% to nearly the entire volume of an IDP. The