Abstract
The published high abundances and unusual ratios of siderophiles in the chassignite Brachina (refs 1, 2 and R. A. Schmitt, personal communication in ref. 1) in comparison with its apparent relatives Chassigny, shergottites, and nakhlites2–11, suggest that Brachina might be an impact melt. If so, the projectile was chemically similar to either some iron meteorites12 or the Eagle Station trio13 of pallasites. As contaminants, the latter meteorites would also explain the deviant oxygen isotope ratios14 of Brachina without recourse to a separate parent planet. We show here that all these meteorites could have formed on Mars, with Brachina ejected as a large tektite-like melt blob, along with shocked and unshocked target rocks. The hypothesis creates predictions which can be tested with data from radiometric ages and dynamic crystallization experiments.
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