Automated thin-film analyses of anhydrous interplanetary dust particles in the analytical electron microscope

Abstract

An analytical electron microscope (AEM) equipped with digital beam control was used to obtain quantitative point count analyses from thin-sections of eight anhydrous chondritic interplanetary dust particles (IDPs). The electron beam was rastered on a two-dimensional grid over each thin-section. A preset video threshold was used to locate chondritic material. Between 200 and 500 X-ray analyses were collected from each thin-section with a total of 2804 spectra from the eight IDPs. Each spectrum was analyzed for Mg, Al, Si, S, Ca, Cr, Mn, Fe, and Ni using a Cliff-Lorimer thin-film correction procedure. Cluster analysis was used to identify major and minor mineral phases and their relative abundances. Petrographic features of each thin-section were determined from brightfield and darkfield images. Two types of anhydrous chondritic aggregates were observed in the eight IDPs. One was highly porous with an average grain size < 10 2 nm and the other was less porous with grains between 10 2 and 10 3 nm. Most IDPs were dominated either by the fine- or coarse-grained aggregates and there was a tendency towards enstatite enrichment in the fine, and olivine enrichment in the coarse components. One IDP contained both fine- and coarse-grained aggregates. Pyroxene and olivine in the fine-grained aggregates usually contain < 5 mole % Fe while those in the coarse aggregates contain up to 30 mole % Fe. The eight anhydrous IDPs are mixtures of fine- and coarse-grained aggregates, large ( > 10 3 nm) mineral grains, glass, and carbonaceous material. On a scale of less than a micrometer, elemental concentrations follow those of solar abundances, suggesting that they are chemically pristine and unperturbed by aqueous alteration. Using an automated AEM and cluster analysis we are able to rapidly characterise IDP thin-sections. It now appears possible to develop an IDP classification scheme based on AEM data.