A basaltic meteorite, but from where?

The vast majority of meteorites represent bodies in the Solar System that never became parts of planets; they are fragments of planetesimals.  Of the 20,000 collected meteorites, only about 50 have been suggested from their geochemistry to hail from existing planetary bodies.  They travelled to Earth as fragments that violent impacts on these bodies ejected from their surfaces.  Since most meteorites have been recovered either from glacial ice or the surface of deserts, such suspected planetary fragments arrived recently in geological time, but had probably been travelling for immense periods of time since an impact dislodged them.  Oddly, there are few if any meteorites with Earthly compositions, and only the Moon and Mars seem to be represented in collections.  Suspected planetary meteorites have basaltic compositions, but so too do some likely to have originated from planetesimals.  One of the keys to sorting them is analysis of their oxygen isotopes, as well as conventional element analyses and noble-gas composition.  It was the resemblance of noble gases in the notorious Antarctic meteorite ALH84001, and others like it, to the very imprecise measurements made by the Viking lander in the 1970s that encouraged the view that it was from Mars.  Their odd oxygen-isotope composition has also been said to indicate a Martian origin, mainly because they don’t fit with other specimens most likely to have originated from planetesimals.

In these uncertain times for manned and unmanned space missions, basaltic meteorites are probably as close as planetary scientists will ever get to the objects of their longing, perhaps for several generations. It is hardly surprising that collectors seize on petrogenetically evolved meteorites with glee.  Such a desirable chunk from a desert surface in NW Africa has been analysed comprehensively by scientists from Japan and the USA (Yamaguchi, A. et al. 2002. A new source of basaltic meteorites inferred from Northwest Africa 011.  Science, v.  296, p. 334-336).  Its chemistry fits with no planetesimal or suspected planetary meteorite class, although for the most part it does resemble the eucrites, considered to originate from the large asteroid Vesta.  Rare-earth elements, siderophile metals and oxygen isotopes put it in a class of its own.  Although the authors are content to conclude that it probably evidences a range of planetesimals that underwent differentiation to produce basaltic magmas, some have been tempted to speculate on a planetary origin, perhaps on Mercury (Palme, H. 2002.  A new Solar System basalt.  Science, v, 296, p. 271-273).  I am left wondering why the supposed Martian meteorite class, with all the kudos that such a suggested origin brings, has not been tempered by the likelihood of origin in a large planetesimal; but I am no specialist.


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