Microcharacterization of Interplanetary Dust Collected in the Earth's Stratosphere.

This thesis involved an examination of the internal structure of thirteen 10 (mu)m aggregates using selected techniques from the field now known as analytical electron microscopy. The aggregates were collected in the earth's stratosphere at 20 km altitude by impactors mounted on NASA U-2 aircraft. Eleven of them exhibited relative major element abundances similar to those found in chondritic meteorities. For this and other reasons, these eleven particles are believed to represent relatively-unaltered interplanetary dust. Interplanetary dust is thought to be of cometary origin, and comets in turn provide the most promising reservoir for unaltered samples of materials present during the collapse of the solar nebula. This thesis shows that the "chondritic" aggregates probably contain important information on a wide range of processes in the early solar system. In the course of this study, significant developments were necessary in the techniques of analysis for: (i) selected area electron diffraction (SAED) data; (ii) energy dispersive X-ray spectra; and (iii) spatial heterogeneity in geological materials. These developments include a method for analysing single crystal SAED patterns using spherical geometry. The method makes possible much more efficient use of diffraction data taken with a goniometer specimen stage. It allows major portions of the analysis to be done by a microprocessor, and it has potential for a wide range of on-line applications. Also, a comprehensive approach to the study of point-to-point heterogeneity in geological materials was developed. Some statistical, comparative, petrographic, and physical applications are described in the thesis. Emphasis in past literature has been on similarities between individual aggregates. In light of results here, the similarities are: (i) They tend to have relative major element abundances for elements heavier than sodium within a factor of 2 of the "chondritic" average; (ii) in the scanning electron microscope they show reentrant structures constructed from rounded nodules in the 0.1 to 1 micron size range; (iii) in the transmission electron microscope they frequently show submicron pyroxene, olivine, and pyrrhotite crystals coated by or embedded in a low-Z amorphous matrix; and (iv) X-ray and electron powder diffraction often indicates the presence of magnetite. The examination of 11 chondritic aggregates has shown that in spite of these similarities, significant differences between particle compositions, internal morphologies, and mineralogies do exist. For example, one aggregate contains silicate and carbide-coated metal droplet crystals similar to those predicted for condensation in a circumstellar nebula with an oxygen to carbon ratio less than 1. This particle is probably the first laboratory sample of primitive unmetamorphosed material which was formed in a highly reducing environment. The existence of such environments in our solar nebula has long been predicted by the existence of the more reduced chondritic meteorites. A second aggregate contains silicates decorated with magnetite, similar to ones which have been invoked to explain absorption and polarization features due to interstellar dust. For other aggregates, there is presently no clear indication for the possible origin. If the particles were 10 (mu)m aggregates while in the interplanetary dust cloud, it is likely that silicate crystals in the aggregates accumulated a detectable density of solar flare iron-ion tracks, Unannealed solar flare tracks are not present in the silicates examined. The absence of tracks is not easily explained unless many of the particles represent fragments of still larger aggregates which broke up on encounter with the earth's atmosphere. The observations are consistent with the hypotheses that: (i) the particles represent fragments of interplanetary dust; (ii) some of them have not been significantly altered by thermal or radiation processes since their assembly; (iii) interplanetary dust is of cometary origin; and (iv) the dust parent materials consist of a wide range of relatively unaltered "leftovers" from the collapse of the solar nebula.