Pyroxene-selective impact smelting in ureilites

Abstract

A characteristic feature of ureilite meteorites is reduction of FeO. But the reduction is usually confined to the rims of olivine. 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 compromised to allow smelting, and yet a minor proportion of the pyroxene escaped smelting and survived with its original composition (En 74.1Wo 10.2). 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 SiO 2, 23 Al 2O 3, 11 CaO, 3.7 Na 2O) 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 probably 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 CO x gas). The impact-smelted pyroxene of LAP 03587 is enigmatic because this ureilite also features little-shocked euhedral graphite laths and no olivine mosaicism. The fine-grained ureilitic component of Almahata Sitta appears to have likewise formed by impact smelting, but with more extensive melting of pyroxene (especially a Ca-rich pyroxene component), more pulverization and melting of olivine, and more displacement of both. However, in places the original coarse-equant ureilite texture is still discernible in relict form. Ordinarily, an impact shock melts olivine before, or at least no later than, pyroxene. But in the case of LAR 04315 and LAP 03587, the great shock event evidently occurred when the material was already anatectic or very nearly so; and thus the difference in melting temperature between pyroxene and olivine, ∼300 degrees lower for pyroxene, was decisive. If literature inferences of extremely fast cooling rates, implying shallow burial depths, are accurate, the proportion of CO x gas generated by ureilite smelting exceeded by a very large factor (of order 10 3 but possibly much greater) the volume represented as porosity in the final ureilites. The outflow of so much gas may have, by near-surface explosive expansion and jetting, enhanced the thoroughness of the impact-triggered catastrophic impact disruption of the parent asteroid.