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The Petrography and Chemistry of Cosmic Spherules from Lewis Cliff, Antarctica

Simeon S. M. Hui1, Marc D. Norman1 and Ralph P. Harvey2

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2 Department of Geological Sciences, Case Western Reserve University, Cleveland, Ohio, USA

Micrometeorites are meteoritic particles less than 1mm in size found in the deep sea, sediments, swamps and the ices of Antarctica. These particles come in a variety of sizes, shapes and textures. The micrometeorite flux to Earth has been estimated at about 30,000 tons a year and make up the majority of material accreted to the present day Earth (Love and Brownlee, 1993). More importantly, these micrometeorites can provide insights into what kinds of materials that have been accreting to Earth as early as the Achaean (Deutsch et al., 1998). Micrometeorites can originate from a variety of sources such as asteroids, comets, planets, and interstellar dust clouds (Bradley et al., 2003).

We have classified 120 spherules form the Lewis Cliff Ice Tongue Moraine deposit based upon their petrography, major element compositions determined by electron microprobe, and we have measure bulk trace element abundances on 71 of these by laser ablation ICPMS. This increases the number of previously studied spherules from this locality by about a factor of four.

Similarities in petrography and major element chemistry among cosmic spherule collections from diverse localities around the Earth suggest a consistent source supplying the spherules, and analogous processes acting on the spherules during their entry through the atmosphere. The trace element data suggests that the majority of the stony cosmic spherules derive from material similar in chemistry to chondrites, leaning towards CM (carbonaceous) or H (ordinary) type chondrites in particular. The trace element data also demonstrate significant losses of volatile lithosphile and chalcophile elements such as Rb and Cu probably due to atmospheric heating, and fractionation of siderophile elements such as Pt and W. There is also a clear depletion of siderophile elements in the silicate spherules while iron-rich cosmic spherules exhibit complementary enrichment. We interpret this as evidence for the formation, migration and possibly separation of an immiscible iron-rich core during melting.

Results of this study were presented at the 7th Australian Space Science Conference, Sydney, Australia, 24-27th September.


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