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it home reported on september 6 that the duration of lunar volcanic activity is of great significance to the construction of the lunar thermal evolution model and has always attracted much attention.

the nine lunar samples returned by the us apollo program and the former soviet union's luna, as well as all the collected meteorite samples, are basically older than 3 billion years, suggesting that the moon's volcanic activity ended long ago.

the precise dating result of the chang'e-5 basalt sample is 2.03 ± 0.04 billion years, proving that there was still large-scale magma activity on the moon 2 billion years ago. is this age the end of the lunar volcanic activity?

▲ diagram of the formation pattern of volcanic glass beads and impact glass beads

based on the lunar soil samples from chang'e-5, researchers from the institute of geology and geophysics of the chinese academy of sciences found that the lunar magma activity can be traced back to 120 million years ago. this research result once again refreshed people's understanding of the time limit of lunar volcanic activity.

the relevant research results have been published today in the journal science (doi: 10.1126/science.adk6635).

▲ sulfur isotope fractionation characteristics in lunar samples

in addition to the rocks formed by large-scale magmatic activities, the products of lunar volcanic activities also include volcanic debris brought by smaller-scale volcanic fountains, including volcanic glass beads. the dating of such glass beads can also trace the history of lunar magmatic activities. however, impact glass beads are common on the moon, which are formed by the melting of lunar surface materials, the sputtering and separation of melts, and the rapid cooling caused by meteorite impacts. it is a huge challenge to identify volcanic glass beads that only exist as a possibility from a large number of glass beads.

the youngest chang'e-5 lunar soil sample is an ideal object for finding younger volcanic glass beads. researcher li qiuli from the institute of geology and geophysics, chinese academy of sciences, took this as his research goal and applied for and obtained approval for about 3 grams of chang'e-5 lunar soil samples. doctoral student wang biwen, postdoctoral fellow zhang qian, and associate researcher he yuyang and other collaborators conducted a detailed study of approximately 3,000 glass beads selected from them.

▲ s content and isotopic characteristics of ce-5 glass beads and apollo volcanic glass beads

the research team conducted s isotope analysis on the six potential volcanic glass beads selected and found that only three of them had δ³⁴s is a negative value, ranging from -12.3‰ to -9.5‰. combining the structure-composition-major-trace-isotope criteria, it can be determined that these three are volcanic glasses.

the u-pb isotope dating results of these three volcanic glasses are 123 ± 15 ma. combined with the following characteristics:

(1) the three glass beads are relatively intact, with only the third one experiencing brittle fracture, and there is no sign of significant impact modification;

(2) the rb content of the three glass beads is significantly higher than that of other impact glass beads. no significant loss of rb occurs, indicating that pb with similar volatility is not significantly affected either.

(3) the three glass beads have the characteristic of significantly negative sulfur isotopes, because the loss process caused by the impact will cause the sulfur isotopes to fractionate in the heavy direction, eliminating the influence of impact modification.

therefore, the measured age is the formation time of the three volcanic glasses, representing the existence of this period of volcanic activity on the moon.

▲ bse images and u-pb dating results of three ce-5 volcanic glass beads

the confirmation of this young volcanism extends the lunar volcanic activity time by nearly 1.9 billion years based on the age of the chang'e 5 basalt, far exceeding previous empirical inferences, which poses a greater challenge to previous lunar thermal evolution models. the research team believes that although no large-scale late magmatic activity has been observed on the moon, there are still small-scale volcanic eruptions caused by local warming.