Mixing of oxidised and reduced components in the early Solar System by LondonGlobal


									Mixing of oxidised and reduced components in the Solar System: evidence from
clasts in regolith breccia meteorites

Principal Supervisor: Prof Hilary Downes (BBK/UCL); Other supervisors: Dr David
Mittlefehldt (NASA Johnson Space Centre, Houston), Dr Caroline Smith (Natural
History Museum).

The photo (above right) shows a planetary disc forming within the Orion nebula. Our
Solar system (above left) probably formed from material like this. The proposed project
will shed light on mixing of material formed in the primitive solar nebula during the early
history of the Solar System. The project involves the study of fragments of one meteorite
type that are embedded within meteorites that are derived from the rubbly surface of a
separate and apparently unrelated asteroid [see reference 1]. The fragments are pieces of
an oxidized and undifferentiated type of meteorite (R-chondrites), situated within
brecciated meteorites derived from a highly reduced and strongly differentiated body
(ureilites). To what extent does this represent an unusual situation in the early Solar
System or was it common to have exchange and mixing between such different
planetesimals? Could this R-chondrite material actually have contributed to the
composition of our own planet Earth?

There are approximately 20 meteorites identified as breccias formed on the surface of the
ureilite parent body [see ref 2]. During our recent study, we identified R-chondrite clasts
in four ureilite breccias, and similar material has been reported (although not always
correctly identified!) in several other ureilite breccias. Thus we conclude that, far from
being an oddity, R-chondrite debris is common on the surface of the ureilite asteroid.
This is surprising, as R-chondrite meteorites are extremely rare and represent a very
unusual, highly oxidized, component within the material left over from Solar System
formation, whereas ureilites are relatively common, strongly reduced, and have
experienced extensive igneous processing. We want to investigate whether debris from
the oxidized R-chondrite parent body was common in the early Solar System. Such
material could have also impacted on the early Earth, perhaps contributing to the “late
veneer” of oxidized material required by some models of Bulk Earth composition [see ref
The project involves petrographic analysis (mostly using the scanning electron
microprobe), mineral analysis by wavelength-dispersive electron microprobe and trace
element analyses by Laser Ablation ICPMS. The material that will be studied comprises
the R-chondrite clasts in ureilitic breccia meteorites and a range of R-chondrite
meteorites. Meteorites will be obtained either from the Antarctic meteorite collection
housed at Johnson Space Centre, Houston, or from North Africa. The possibility of R-
chondritic material contributing to the overall composition of the Earth will also be
investigated by determining the bulk-chemical compositions of the clasts and modeling
the effect of late-stage mixing of this component on bulk-Earth compositions.


 1. Downes H and Mittlefehldt D W 2006. Identification of a common R-chondrite
    impactor on the ureilite parent body. 69th Annual meeting of the Meteoritical
    Society. Abstract # 5147.
 2. Goodrich C A, Scott E R D and Fioretti A M 2004. Ureilitic breccias: clues to the
    petrological structure and impact disruption of the ureilite parent asteroid. Chemie
    der Erde 64, 283-327
 3. Drake M J and Righter K 2002. Determining the composition of the Earth. Nature
    416, 39-44

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