The content and stable isotopic composition of carbon in spherical micrometeorites

Yates, Paul David (1993). The content and stable isotopic composition of carbon in spherical micrometeorites. PhD thesis The Open University.



Micrometeorites are extraterrestrial bodies that have survived atmospheric entry to be found on Earth as grains <1mm in diameter. Of the annual flux of extraterrestrial material to the Earth ca.99% is supplied by the micrometeorites. The lifetime of the micrometeroids in the interplanetary medium in extremely ephemeral, but their population is maintained largely by ejection of particles from cometary nuclei and from collisional debris from the asteroid belt. As such, the micrometeorites are of fundemental importance to cosmogony, because they may provide the only means, at present, to directly study cometary material, and sample a far less biased cross section of the asteroid belt than conventional meteorites.

A static gas source mass spectrometer for carbon isotope measurements built in the Planetary Sciences Unit has been tested and shown capable of routinely analysing ca.1 nanogram of carbon (as carbon dioxide) to precisions of ca.±1‰. To enable this gain in sensitivity to be exploited, a sample preparation and carbon dioxide extraction system has been devised that typically imparts a total of less than 20 nanograms of extraneous carbon contamination to the samples. An advantage of the mass spectrometer is its ability to make accurate isotopic abundance measurements of the blank contribution for the purpose of making corrections to the analytical data. The instrument and inlet system has been applied to studies of micrometeorites >200∝m in diameter.

A procedure has been developed involving three analytical techniques (chemical/petrographic, X-ray diffraction and stable carbon isotope). Carbon isotopic analyses of multiple-sample aliquots of deep-sea, and individual Greenland and Antarctic micrometeorites has enabled the identification of components that are similar to those within the stony chondritic meteorites. More specifically, components have been revealed that may indicate a generic connection to primitive chondrites. Carbon isotopic characterisation of acid-resistant residues prepared from micrometeorite-rich Greenland cryoconite suggested that the micrometeorites contain pre-solar dust grains of silicon carbide, graphite and diamond. which provides an additional indication of the primitive nature of some micrometeorites. Experiments to produce micrometeorite analogues by pulse-heating microgram-sized samples of whole-rock meteorites showed that indigenous carbonaceous species can survive pulse-heating above their normal combustion temperatures. The overall conclusion of the investigation is that the flux of >200∝m micrometeorites is dominated by a steroidal material and that the atmospheric entry heating process is poorly understood.

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