U-pb Ages of Lunar Apatites

Introduction: Apatite is one of the minerals that is rarely utilized in U-Pb geochronology, compared to some other U-rich accessory phases. Relatively low U concentration, commonly high proportion of common Pb and low closure temperature of U-Pb system of apatite inhibit its application as geochronological tool when other minerals such as zircon are widely available. However, zircon appear to be restricted to certain type of lunar rocks, carrying so called KREEP signature , whereas apatite (and whitlockite) is a common accessory mineral in the lunar samples. Therefore, utilizing apatite for lunar chronology may increase the pool of rocks that are available for U-Pb dating. The low stability of U-Pb systematics of apatite may also result in the resetting of the system during meteoritic bombardment, in which case apatite may provide an additional tool for the study of the impact history of the Moon. In order to investigate these possibilities, we have analysed apatites and zircons from two brec-cia samples collected during the Apollo 14 mission. Both samples were collected within the Fra Mauro formation, which is interpreted as a material ejected during the impact that formed the Imbrium Basin. Samples: Sample 14306 is classified as a crystalline matrix breccia (Simonds et al., [1]), consisting of three fragment generations. The oldest fragments are coarsely crystalline and partly glassy basaltic (noritic) microbreccias, the second generation are dark grey micro-to cryptocrystalline polymict microbreccias and the third generation is the matrix, similar in composition to the second generation fragments (Anderson et al., [2]). Polished thin section 14306-60 contains fragments and matrix material that are consistent with the above descriptions. All of the apatite and zircon grains dated in this sample are contained within one large fragment (5x4 mm) of noritic gabbro microbreccia. This large fragment contains coarse-grained aggregates and broken crystal fragments (up to 1 mm) of plagio-clase and pyroxene set in a dark glassy matrix. The large fragment also contains numerous apatite, zircon and ilmenite grains ranging in size up to 0.7 mm for the largest apatite and 0.4 mm for the largest zircon (both of which are part of coarse-grained gabbroic aggregates). Apatite and zircon grains vary from eu-hedral crystals in aggregates to broken fragments in the matrix. Sample 14066 is classified as a crystalline matrix breccia (Simonds et al., [1]) with medium grey lithic fragments in a pale grey matrix. Polished thin section