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.

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The ages of a number of small fragments of lunar granophyre have been determined by the in situ U‐Th‐Pb isotopic analysis of zircon using a sensitive high mass‐resolution ion microprobe (SHRIMP I). The zircon from lunar granophyre is characterized by consistently high U and Th contents (most 200–500 ppm and 100–300 ppm, respectively) compared to zircon from mafic lunar rocks. Some fragments of lunar granophyre are found to be as old as 4.32 Ga, supporting other evidence that the original lunar magma ocean crystallized completely within ∼200 Ma of the formation of the Moon itself. Other fragments are as young as 3.88 Ga, which is much later than the time of formation of most of the lunar crust. The older lunar granophyres have rare‐earth‐element (REE) patterns that are similar to lunar KREEP, whilst the younger granophyres have bow‐shaped REE patterns that feature a greater relative enrichment in the heavy REE. The wide range of ages of numerous lunar zircons, lunar granophyres and other rocks indicates that zircon‐forming magmatism in the lunar highlands was most active prior to 4.3 Ga but continuous until at least 3.88 Ga. The U‐Pb isotopic composition of much lunar zircon is near concordant, but the effects of isotopic disturbance as late as ∼1.0 Ga are observed in some zircon, both within granophyre fragments recrystallized by reheating and within fragments in which the original delicate silica‐K‐feldspar granophyric intergrowth is well preserved. It is therefore essential to make multiple analyses of individual zircon grains, and preferably analyses of suites of zircons from lunar igneous rocks if they are to be dated reliably by the U‐Pb method. It is possible that some of the younger lunar granophyres are the product of large‐scale silicate‐liquid immiscibility within late‐stage differentiates, but this remains unproven until remnants of demonstrably cogenetic, Fe‐rich, immiscible liquid are positively identified.

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Results are reported of an investigation of the age and origin of the exceptional zircon aggregate in an anorthositic clast from lunar breccia 73235. Cathodoluminescence and birefringence images show that the aggregate consists of numerous angular fragments of sector zoned primary zircon in a matrix of secondary zircon with an overall texture that resembles a pseudotachylite. SIMS U-Pb analyses of the primary fragments and the matrix yielded two clearly defined ages, an age of 4.315 ± 0.015 Ga and initial Th/U ratio of 0.21-0.35 for the primary zircon and an age of 4.187 ± 0.011 Ga and Th/U of 0.04-0.17, for the secondary zircon matrix. A Raman spectroscopic study the secondary matrix zircon was undertaken to investigate its structure. Results showed that the matrix has a zircon structure but there is also evidence for the presence of an amorphous component. Implications of the structural and U-Pb age data are discussed in terms of the origin and evolution of the aggregate and the history of lunar events. It is proposed that an original single, millimetresized, sector zoned zircon, formed at 4.31 Ga, was subjected to a severe shock event at 4.18 Ga. This event resulted in the fracturing of the zircon, the displacement and rotation of fragments, the compression of the aggregate to a lensoid shape, and the shock reduction of zircon to sub-micron-sized and amorphous granules in crush zones in the mosaic of fractures. Volatilisation loss of Pb and the addition of U to the secondary zircon is attributed to processes activated by the extreme thermal pulse which accompanied the 4.18 Ga shock event. Shock effects are seen in some of the primary fragments but Raman spectra of the primary and secondary zircon show no evidence for pressure-induced transformation of zircon to a scheelite structure. The zircon U-Pb system has not been affected by the ca. 3.95 Ga thermal pulse that accompanied formation of the host breccia although this event has largely reset the K-Ar systems.