Shock melting of ordinary chondrite powders and implications for asteroidal regoliths

Abstract

A series of 59 impacts in the laboratory reduced a coherent 460 g piece of the L6 ordinary chondrite ALH 85017 to a coarse-grained regolith. We then subjected the 125-250 micrometer fines from this sample to reverberation shock stresses of 14.567 GPa in order to delineate the melting behavior of porous, unconsolidated, chondritic asteroid surfaces during meteorite impact. The initial pore space (40-50%) was completely closed at 14.5 GPa and a dense aggregate of interlocking grains resulted. Grain-boundary melting commenced at <27 GPa and ~50% of the total charge was molten at 67 GPa; this stress corresponds to typical asteroid impacts at ~5 km/sec. Melting of the entire sample most likely mandates >80 GPa, which is associated with impact velocities >8 km/sec. The Fe-Ni and troilite clasts of the original meteorite melted with particular ease, forming immiscible melts that are finely disseminated throughout the silicate glass. These metal droplets are highly variable in size, extending to <100 nm and most likely to superparamagnetic domains; such opaques are also observed in the natural melt veins of ordinary chondrites. It follows that melting and dissemination of pre-existing, Fe-rich phases may substantially affect the optical properties of asteroidal surfaces. It seems unnecessary to invoke reduction of Fe2+ (or Fe3+) by sputtering or impact-processes--in analogy to the lunar surface--to produce space weathering effects on S-type asteroids. We note that HED meteorites contain ample FeO (comparable to that in lunar basalts) for reduction processes to take place, yet their probable parent object(s), Vesta and its collisional fragments, display substantially unweathered surfaces. Howardites, eucrites, and diogenites (HEDs), however, contain little native metal (typically <0.5%), in contrast to ordinary chondrites (commonly 10-15%) and their S-type parent objects. These considerations suggest that the modal content of native metal and sulfides is more important for space weathering on asteroids than total FeO.