Impact melting of lunar meteorite Dhofar 458: Evidence from polycrystalline texture and decomposition of zircon

Dhofar 458 is a lunar meteorite consisting mainly of olivine-plagioclase intergrowths, pyroxene-plagioclase intergrowths, and plagioclase fragments. Pyroxene-plagioclase globules are also common. In this study, we report the discovery of a polycrystalline zircon in this lunar meteorite. The polycrystalline zircon contains small vesicles and rounded baddeleyite grains at its margin. The polycrystalline and porous texture of the zircon indicates high-pressure shock-induced melting and degassing. Baddeleyite grains are derived from decomposition of zircon under high postshock temperature. The shock features in zircon indicates that the shock pressure in Dhofar 458 was greater than approximately 60 GPa and the postshock temperature greater than approximately 1700 degrees C. The polycrystalline and degassing texture and decomposition zircon also strongly indicates that Dhofar 458 is a clast-rich impact melt rock. During this shock event, most components were melted and grains of mafic minerals are interstitial to lath-like plagioclase grains. Large fragments of olivine and chromite also formed polycrystalline texture at margins and chemically reequilibrated with surrounding melts. We suggest that pyroxene-plagioclase globules could be remains of melted target clasts, whereas vesicles may form during shock-induced degassing of the rock. The U-Pb isotopic data plot on a well-defined discordant line, yielding the age of the zircon of 3434 +/- 15 Ma (2 sigma). This age is interpreted as the time of the impact event that melted Dhofar 458 and caused decomposition and recrystallization of this zircon in Dhofar 458, which reset this zircon's U-Pb age.