Are the most C-rich antarctic micrometeorites exotic?
Abstract
A 400KV transmission electron microscope, equiped with both EDS and EELS (electron energy loss spectrometer) systems, has been first used to determine the C/O ratio and the concentrations of major elements on ~1000 Angstroms-zones of micrometer-sized crushed fragments from about 30 Antarctic micrometeorites, with initial sizes ranging from 30 micrometers up to 300 micrometers. Another fragment of each micrometeorite was polished and the mineralogy of its assemblage of >5 micrometers grains was determined with a SEM equiped with an EDS system, and with an electron microprobe. Another fragment of a few C-rich and C-poor micrometeorites were ultramicrotomed for further TEM observations. Finally, textural changes upon in situ pyrolysis (from 400 degrees up to 1000 degrees C) in a 3 MV microscope (at Toulouse) were continuously monitored with a video camera in preselected C-rich crushed grains already analyzed at ONERA ; modifications of their C/O ratios and chemical compositions were subsequently investigated at ONERA. The major result of this investigation are: (1) C-rich grains within a single micrometeorite. All the grains are amorphous, and they frequently show high contents of Ni, indicative of their "extraterrestrial" origin. About 5-10% of them correspond to a dirty "COPS" phase, which contains many elements, but which is especially rich in P and S. This phase, which occassionaly occurs on the external surface of micrometeorites, has also been observed by us in a CM meteorite (Murchison), just below the top layer of its fusion crust. It might just be related to the accretion of some components of the upper layers of the atmosphere along the trajectory of "shooting stars" and/or to a complex reprocessing of some primordial meteoritic components during the "hot" segment of these trajectories. There is no clear cut correlation between the C/O values and the contents of other elements (Ni, Fe, Si, Ca). Most of the carbonaceous grains start degrading around 800 degrees C; (2) C-rich micrometeorites. With the exception of the crystalline and scoriaceous particles, the C-poor fine-grained micrometeorites contain already as much carbon as Murchison. But ~20% of them have much higher contents of carbon than Orgueil. These C-rich micrometeorites could originate from comets, being different from the other micrometeorites. However, they are in fact very similar to the other micrometeorites, with some of the scoriaceous particle (that have been partially melted), containing more carbonaceous material than a truly unmelted fine grained micrometeorite. The fine-grained micrometeorites, whether C-rich or not, also show a similar differences between their assemblage of "large" (>5 micrometers) and small (<1 micrometers) minerals, with the largest minerals being related to CM and CR meteorites, and the finest sized grains bearing similarities to CV and CO meteorites. These results complicate the classification of micrometeorites. They also add to the mounting evidence suggesting that cometary material, necessarily present in the collections of stratospheric and polar micrometeorites, does not look very "exotic" with regard to the most common material observed in these collections.
- Publication:
-
Meteoritics
- Pub Date:
- July 1994
- Bibcode:
- 1994Metic..29Q.464E
- Keywords:
-
- Antarctic Regions;
- Carbon;
- Chemical Composition;
- Meteoritic Composition;
- Micrometeoroids;
- Mineralogy;
- Electron Spectroscopy;
- Scanning Electron Microscopy;
- Transmission Electron Microscopy;
- Lunar and Planetary Exploration;
- Lunar and Planetary Exploration;
- ALLAN HILLS 84001; COSMOGENIC NUCLEI; EXPOSURE AGE; MARS METEORITE; NOBLE GASES