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The chemical and isotopic characteristics of terrestrial basalts are constrained within the concept of mantle chemical geodynamics that explains the existing variety of basaltic rocks within a framework of several end-member reservoirs in Earth's mantle. In contrast, there is no comparable fully developed model explaining the isotopic composition of lunar basaltic rocks, in part owing to the lack of well-constrained age–isotope relationships in different groups of basalts identified on the Moon. Notably, the absence of agreement upon ages includes basalts from a unique group of meteorites collectively known as 'YAMM' (basalts Yamato-793169: Y-793169, Asuka-881757: A-881757, Miller Range 05035: MIL 05035 and regolith breccia Meteorite Hill 01210: MET 01210), which appear to show chemical signatures different from all other known lunar basaltic rocks. We present high-precision Pb–Pb ages and initial Pb isotopic ratios for two samples from this group, MIL 05035 and A-881757. These meteorites have Pb isotope ratios different from those of the other lunar basalts, suggesting they are derived from a distinct and depleted mantle source, with a 238U/204Pb ratio (μ value) lower than any other mantle source. Their depletion in rare earth elements, in conjunction with recalculated initial Nd and Sr isotopic ratios from published data and using our new age, appear to support this conclusion. The chemical and Sr-Nd-Pb isotopic characteristics of this low-μ source appear to be the opposite of those of the KREEP reservoir and many, if not all, features described in other lunar basalts (such as low- and high-Ti mare basalts) can be explained by a binary mixing of material derived from low-μ and KREEP-like reservoirs. This mixing might be the result of a slow, convection-like mantle overturn. [ABSTRACT FROM AUTHOR]

The Macquarie Ridge Complex (MRC) on the Australia‐Pacific plate boundary south of New Zealand is an extinct mid‐ocean ridge that has experienced a complex tectonic history and produced highly heterogeneous mid‐ocean ridge basalts (MORBs). When and how seafloor spreading ceased along the proto‐Macquarie mid‐ocean ridge remain elusive, and it is unclear how the mantle source of MORBs is affected by the gradual cessation of seafloor spreading at mid‐ocean ridges. To constrain the tectonic evolution of the MRC, the mantle source variations for MORBs at dying mid‐ocean ridges, and the mechanisms of mantle enrichment and asthenospheric heterogeneities, we report 11 pyroxene, plagioclase, basaltic glass, groundmass, and sericite 40Ar/39Ar and one zircon U‐Pb ages for the MRC MORBs. Our data reveal that basalts from the MRC seamounts were erupted between 25.9 and 1.6 Ma and Macquarie Island at ∼10 Ma. Combined age and plate reconstruction results reveal that the cessation of seafloor spreading at the MRC generally propagated from south to north along the ridge. Basalts produced by the then dying Macquarie mid‐ocean ridge at different times on different seamounts/island show a large variation in isotopic compositions and there is no clear correlation between ages and isotopic ratios. The heterogeneity of mantle source for MORBs from the proto‐Macquarie mid‐ocean ridge suggests that the upper asthenospheric mantle is heterogeneous, and such heterogeneity becomes most obvious at dying mid‐ocean ridges where the degrees of partial melting are low and a large range of melt compositions are produced. Key Points: New ages of basalts from the Macquarie Ridge Complex (MRC) at the Australia‐Pacific plate boundary are reportedThe cessation of seafloor spreading was progressed from south to north along the Macquarie RidgeUpper mantle heterogeneity becomes obvious at dying mid‐ocean ridges where the melting degrees are low and various melt compositions are produced [ABSTRACT FROM AUTHOR]

In this paper, we compare the U-Pb zircon age distribution pattern of sample 14311 from the Apollo 14 landing site with those from other breccias collected at the same landing site. Zircons in breccia 14311 show major age peaks at 4340 and 4240 Ma and small peaks at 4110, 4030, and 3960 Ma. The zircon age patterns of breccia 14311 and other Apollo 14 breccias are statistically different suggesting a separate provenance and transportation history for these breccias. This interpretation is supported by different U-Pb Ca-phosphate and exposure ages for breccia 14311 (Ca-phosphate age: 3938 ± 4 Ma, exposure age: ~550-660 Ma) from the other Apollo 14 breccias (Ca-phosphate age: 3927 ± 2 Ma, compatible with the Imbrium impact, exposure age: ~25-30 Ma). Based on these observations, we consider two hypotheses for the origin and transportation history of sample 14311. (1) Breccia 14311 was formed in the Procellarum KREEP terrane by a 3938 Ma-old impact and deposited near the future site of the Imbrium basin. The breccia was integrated into the Fra Mauro Formation during the deposition of the Imbrium impact ejecta at 3927 Ma. The zircons were annealed by mare basalt flooding at 3400 Ma at Apollo 14 landing site. Eventually, at approximately 660 Ma, a small and local impact event excavated this sample and it has been at the surface of the Moon since this time. (2) Breccia 14311 was formed by a 3938 Ma-old impact. The location of the sample is not known at that time but at 3400 Ma, it was located nearby or buried by hot basaltic flows. It was transported from where it was deposited to the Apollo 14 landing site by an impact at approximately 660 Ma, possibly related to the formation of the Copernicus crater and has remained at the surface of the Moon since this event. This latter hypothesis is the simplest scenario for the formation and transportation history of the 14311 breccia. [ABSTRACT FROM AUTHOR]