THE PORTALES VALLEY METEORITE BRECCIA: EVIDENCE FOR IMPACT-INDUCED MELTING AND METAMORPHISM OF AN ORDINARY CHONDRITE

Abstract

The Portales Valley H-chondrite fall is an annealed impact-melt breccia with coarse metal interstitial to angular and subrounded silicate clasts. The large metal-rich regions exhibit a Widmanstatten structure and contain very little troilite. We were able to examine a 16.5 kg metal-rich specimen of Portales Valley. Silicates contain numerous flecks of metallic Cu and curvilinear trails of tiny metallic Fe-Ni blebs, characteristic of shocked and annealed chondrites. One silicate clast appears to have experienced little (<10%) or no melting; it is essentially identical to normal H6 chondrites. Other clasts are finer grained and have a low abundance of recognizable relict chondrules; they are significantly enriched in troilite and depleted in metallic Fe-Ni relative to typical H chondrites. Their low metal abundance indicates that they are not simply ultra-recrystallized H6 chondrites. If the silicates in these clasts started off as normal H-chondrite material and were recrystallized to the same extent as the progenitor of the H6-like clast, then their low modal abundance of chondrules indicates that they experienced significant crushing and/or impact melting. We infer that most of the metal and troilite was lost from these silicate clasts during impact melting; it appears that troilite was reintroduced into the silicates, perhaps by an S2-rich vapor (that formed FeS by reacting with Fe vapor or residual metal). Portales Valley probably formed on a low-density, porous H-chondrite asteroid by a high-energy impact event that caused crushing and melting; the target material was buried deeply enough to undergo slow cooling. Meteorites that appear to have formed, at least in part, by analogous processes include IIE-an Netschaevo and EL6 Blithfield.