Xenoliths in Meteorites

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Ureilites are meteorites from a differentiated asteroid (with a crust and mantle). Polymict ureilites represent the regolith of that asteroid. They contain many foreign clasts (xenoliths) that are fragments of impactors onto the surface of this asteroid. This image is an X-ray map of a polymict ureilite showing concentration of the element magnesium in red, calcium in green, and aluminum in blue. Foreign clasts have different chemical compositions from ureilites, therefore different colors. They can be identified as other meteorite types as labeled.

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Xenoliths (foreign clasts) in regolith breccias are surviving fragments of impactors onto the surface of the parent asteroid.

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The foreign clasts in polymict ureilites are extremely diverse. They include samples from many other groups of meteorites. Oxygen isotope compositions of these clasts confirm their identities as foreign meteorite types, rather than ureilitic.

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This diagram of relative abundances of asteroid types with distance from the Sun suggests that early in the history of the solar system, enstatite and ordinary chondrites dominated the inner asteroid belt, while carbonaceous chondrites and similar materials dominated the outer asteroid belt and beyond. The diversity of xenoliths mixed into polymict ureilites, and their scarcity in ordinary chondrites, might indicate that the ureilite parent asteroid had an unusual migration history.

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Igneous-textured clasts in ordinary chondrite breccias are potentially xenoliths from differentiated asteroids. However, studies of their oxygen isotopes and chemical composition show that they are similar to their hosts and probably locally derived. In contrast to ureilites, ordinary chondrites contain few true xeonoliths.

Dr. Cyrena Goodrich

Cyrena Goodrich working on the JEOL 8530F electron microprobe at ARES, JSC.

Shannon Boyle

Shannon Boyle working on the JEOL 8530F electron microprobe at ARES, JSC.

As a Staff Scientist at LPI, Dr. Cyrena Goodrich has been studying xenoliths in meteoritic breccias, working with Dr. Allan Treiman and Dr. David Kring, LPI Senior Staff Scientists. Shannon Boyle (now a graduate student at Rutgers University) joined the team as an LPI Intern in summer 2016.

The imaging and analyses of meteorites carried out in this work were done at ARES, JSC, using the JEOL 8530F electron microprobe, the Cameca SX-100 electron microprobe and the JEOL 7600F scanning electron microscope.

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