A protracted timeline for lunar bombardment from mineral chemistry, Ti thermometry and U-Pb geochronology of Apollo 14 melt breccia zircons

摘要

New zircon U-Pb and trace element investigations from Apollo 14 lunar impact breccia sample #14311 reveal at least three distinct (Concordia, 2 sigma) age populations at 4334 +/- 10, 4245 +/- 10 and 3953 +/- 10 Ma. Titanium-in-zircon thermometry (Ti-xln) results correlated with U-Pb ages range from similar to 800-1200 degrees C. Lattice strain models used to infer zircon versus whole-rock rare earth element contents, and partitioning calculations against lunar impact breccia component compositions, quantitatively constrain formation conditions for the different age populations. A compilation of new data with published work shows that Apollo 14 zircons older than ca. 4300 Ma formed by igneous processes associated with lunar crust formation. Compositional variability in the ca. 4240 Ma zircon age population is interpretable, however, via a mixture of inherited and melt-generated components from one or more large impacts perhaps related to a marked increase in bombardment flux. Ages from the youngest zircon group at ca. 3950 Ma coincide with the classical "late heavy bombardment" (LHB) as documented from previous lunar geochronologies. These results lend support to the idea that instead of a simple unimodal LHB scenario, or a monotonic decline in impacts, the Moon was battered by multiple cataclysms since ca. 4240 Ma. Such a "Picket fence"-like bombardment to the Moon best describes the mode and tempo of impacts that accompanied the late stages of solar system formation and giant planet migration.