U‐Pb geochronology of zircons from lunar breccia 73217 using a sensitive high mass‐resolution ion microprobe

Abstract

U‐Pb ages of four zircons in thin‐sections of lunar breccia 73217 have been determined using the sensitive high‐resolution ion microprobe, SHRIMP. All crystals show remarkably little loss of radiogenic Pb and give U‐Pb ages that are within 10% of concordance at 4356−14+23 m.y. Two of the crystals show evidence of radiogenic initial Pb that evolved in a source with μ ∼2000. One zircon is an inclusion in an ilmenite of the clast assemblage, which shows that the zircons belong to that assemblage and strongly suggests that they date formation of the (igneous) rock from which the clasts were derived. The principal loss of radiogenic Pb from the zircons occurred significantly later than 3900 m.y. but probably earlier than 1000 m.y. Two of the zircons are optically and chemically zoned and only partly metamict. U, Th, Yb, Y, and P are relatively enriched in the metamict region of one.