Submicron spatial resolution Pb-Pb dating for the formation age of Chang'e-5 basalt

Abstract

Chang'e-5 (CE-5) basalt, the youngest lunar basalt sample collected so far, is crucial for unraveling the mechanism of the prolonged volcanic activity on the Moon. To better understand the petrogenesis of this basalt, it is necessary to determine its eruption duration accurately. Previous Pb-Pb age measurements have yielded a wide range from 2040 Ma to 1963 Ma. This large range may be attributed to 80 million-year multi-eruption, analytical or interpretational biases from different laboratories, or disturbances in the U-Pb system. Here, we conducted sub-micron Pb-Pb dating on 25 Zr-rich mineral grains with various sizes (∼2 μm to 20 μm) from six CE-5 basalt fragments by a Cameca NanoSIMS 50L. After excluding anomalies caused by Pb enrichment, we obtained a weighted mean Pb-Pb age of 2059 ± 21 Ma and an isochron age of 2046 ± 29 Ma. By integrating all reported age data of the CE-5 basalt, we found inconsistencies in previous studies, probably due to analytical biases of insufficient spatial resolution, leading to the undetected presence of terrestrial common Pb. Isochron regression with a filtering process yields an age of 2031.1 ± 3.8 Ma, which we recommend as the best estimate of the formation age of the CE-5 basalt. This result suggests that all analyzed CE-5 basalt fragments have nearly the same crystallization age, indicating that the lava erupted in a very short period. Combined with previous studies on the mineralogy and geochemistry of the CE-5 basalt, this result contributes new insights into the volcanic history of the Moon.