Author's personal copy Pyroxene-selective impact smelting in ureilites

A characteristic feature of ureilite meteorites is reduction of FeO. But the reduction is usually confined to the rims of oliv-ine. In the LAR 04315, LAP 03587 and Almahata Sitta ureilites, pyroxene was extensively reduced by impact smelting. In LAR 04315, the impact caused nearly all of the original pigeonite to melt or otherwise become sufficiently structurally com-promised to allow smelting, and yet a minor proportion of the pyroxene escaped smelting and survived with its original com-position (En74.1Wo10.2). Olivine mosaicism confirms that LAR 04315 experienced a major shock event. The smelted pyroxenes also show a distinctive patchiness in their interference colors (although each grain’s basic optical continuity, often including twinning, is still discernible). They also have reduced compositions, are ubiquitously porous (15%), and contain sprinklings of Fe-metal and felsic glass. For the most part the olivine underwent only very slight reduction. Much of the (small) pyroxene component of LAP 03587 shows the same oddly porous texture. LAR 04315 also contains large traces of silica and felsic glass (with a typical composition of, in wt%, 61 SiO2, 23 Al2O3, 11 CaO, 3.7 Na2O) glass; these two phases together form selvages that line the walls of many of the largest voids in the rock. Silica is a by-product of pyroxene smelting. The felsic glass prob-ably derives largely from interstitial basaltic melt that predated the impact. However, the comparatively stiff surrounding/ included silica may have promoted unusually high melt retention within LAR 04315 through the smelting episode (one aspect of which was a major stream-out, through the same large voids, of COx gas). The impact-smelted pyroxene of LAP 03587 i