IN SITU TEM HEATING EXPERIMENTS OF METEORITE MATRICES: CONSTRAINTS ON THERMAL PROCESSING OF ORGANICS AND SILICATE GRAINS IN CARBONACEOUS ASTEROIDS

The response of primary fine-grained materials in carbonaceous chondrites to secondary alteration is important for understanding active processes on the surfaces and within the chondrite-parent asteroids. Thermal metamorphism, in particular, could have played an important role in processes such as melting, volatile loss, elemental diffusion between grains, and driving hydrothermal processing.

To better understand the effect(s) of heating on the composition and microstructure of volatile-rich material in asteroids, we carried out in-situ flash- and step-heating experiments of fine-grained matrix material of the Murchison (CM2), Tagish Lake (C2-ung.) and Acfer094 (C2-ung.) carbonaceous chondrites using the Hitachi Blaze heating holder in a Hitachi SU9000 scanning transmission electron microscope (STEM/SEM) in vacuum (< 10^-5Pa) at temperatures up to 1075˚C. The SU9000 isequipped with an Oxford X-Max 100TLE energy-dispersive X-ray spectroscopy (EDS)system and Hitachi electron energy-loss spectroscopy (EELS) system.

In-situ heating of fine-grained matrix material from these three meteorites shows significant changes to their microstructure and elemental compositions, such as melting and formation of Fe-Ni nanoparticles, occurs only after heating above 600°C. The nanoparticles do not appear to be pure Fe-Ni metal but contain significant Si (with low Ni content). The microstructure and composition of these particles are similar to the Fe-Ni metal inclusions identified inside chondrule ‘dusty’ olivine grains. Heating above 800°C caused a significant loss of volatiles (e.g., S) and the graphitization of the carbonaceous matter.