In-situ U-Pb dating of phosphates in lunar basaltic breccia

Introduction: The lunar meteorites have been valuable sources for understanding the evolution of the Moon's crust, because each meteorite could potentially provide a new insight into the thermal history of unexplored regions of the Moon. In spite of their scientific interests, chronological studies of Very-Low-Ti (VLT) basaltic meteorites have not be well understood, since the most VLT basalt meteorites are brecciated and consist of mixtures of materials with different origins. In this paper, we report ion microprobe U-Pb isotopic analyses of the phosphates in brecciated lunar meteorite Yamato 981031. Yamato 981031 is a regolith breccia that consist of roughly 3:1 mix of VLT mare basalt and highland materials and is considered to be paired with Yamato 793274 [1-4]. Moreover, based on the similar launching ages in addition to many textural and mineralogical similarities, Arai and Warren [5] and Warren and Ulff-Møller [6] proposed that Yamato 793274, Yamato 981031, QUE 94281, EET 87521 and EET 96008, which are VLT mare basalt brecciated meteorites, were launched from the Moon by a single impact event. Recently, Korotev et al. [7] thoroughly investigated the chemical composition of these meteorites, and also suggested that there are not significant impediments to the hypothesis that they were launched by a single event. However, Terada et al. [8] pointed out that the previously reported chronological data do not necessarily match to this scenario. For example, Tatsumoto and Premo [9] carried out 0.5N HBr leaching and U-Pb measurements, and concluded that the Yamato 793274 clast formed at ~4.4 Ga and that a disturbance to the U-Pb system occurred at ~4.0 Ga assuming an appropriate initial lead composition. On the other hand, Eugster et al. [10] also reported a K-Ar age of 3300 Ma (without analytical uncertainties) for EET 87521, and in-situ U-Pb dating of phosphates in EET 96008 and EET 87521 are 3569 ± 100 Ma for EET 96008 [11] and 3521 ± 138 Ma for EET 87521 [8], respectively. This inconsistency of chronology ages raised the question that the validities of bulk age analyses for the Yamato 793274/981031 meteorites in the literature and/or the hypothesis of a single-crater origin. Thus, the chronological assessment of VLT meteorites still remain matter for