Uranium‐lead ages for lunar zircons: Evidence for a prolonged period of granophyre formation from 4.32 to 3.88 Ga

Abstract

Abstract 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.