Portales Valley: Petrology of a metallic-melt meteorite breccia

Abstract

Portales Valley (PV) is an unusual metal-veined meteorite that has been classified as an H6 chondrite. It has been regarded either as an annealed impact melt breccia, as a primitive achondrite, or as a meteorite with affinities to silicated iron meteorites. We studied the petrology of PV using a variety of geochemical-mineralogical techniques. Our results suggest that PV is the first welldocumented metallic-melt meteorite breccia. Mineral-chemical and other data suggest that the protolith to PV was an H chondrite. The composition of FeNi metal in PV is somewhat fractionated compared to H chondrites and varies between coarse vein and silicate-rich portions. It is best modeled as having formed by partial melting at temperatures of ~940-1150 °C, with incomplete separation of solid from liquid metal. Solid metal concentrated in the coarse vein areas and S-bearing liquid metal concentrated in the silicate-rich areas, possibly as a result of a surface energy effect. Both carbon and phosphorus must have been scavenged from large volumes and concentrated in metallic liquid. Graphite nodules formed by crystallization from this liquid, whereas phosphate formed by reaction between P-bearing metal and clinopyroxene components, depleting clinopyroxene throughout much of the meteorite and growing coarse phosphate at metal-silicate interfaces. Some phosphate probably crystallized from P-bearing liquids, but most probably formed by solid-state reaction at ~975-725 °C. Phosphate-forming and FeO-reduction reactions were widespread in PV and entailed a change in the mineralogy of the stony portion on a large scale. Portales Valley experienced protracted annealing from supersolidus to subsolidus temperatures, probably by cooling at depth within its parent body, but the main differences between PV and H chondrites arose because maximum temperatures were higher in PV. A combination of a relatively weak shock event and elevated pre-shock temperatures probably produced the vein-and-breccia texture, with endogenic heating being the main heat source for melting, and with stress waves from an impact event being an essential trigger for mobilizing metal. Portales Valley is best classified as an H7 metallic-melt breccia of shock stage S1. The meteorite is transitional between more primitive (chondritic) and evolved (achondrite, iron) meteorite types and offers clues as to how differentiation could have occurred in some asteroidal bodies.