SPACE/COSMOS
Chang’e-6 lunar samples reveal 2.83-billion-year-old basalt with depleted mantle source
Chinese Academy of Sciences Headquarters
image:
Fig. 1 Landing site of the Chang’e-6 mission on the Moon’s far side
view moreCredit: Image by GIGCAS
The Moon has a global dichotomy, with its near and far sides having different geomorphology, topography, chemical composition, crustal thickness, and evidence of volcanism.
To better understand this dichotomy, Professor XU Yigang’s team from the Guangzhou Institute of Geochemistry of the Chinese Academy of Sciences investigated lunar soil samples from the far side South Pole-Aitken (SPA) Basin of the Moon returned by the Chang’e-6 mission.
Their work was published in Science on Nov. 15.
“The samples returned by Chang’e-6 provide a best opportunity to investigate the lunar global dichotomy,” said Professor XU.
Volcanic eruptions flooded parts of the surface with lava, producing rocks known as mare basalts, which are more common on the near side, where they cover ~30% of the surface compared to 2% of the far side. It is obvious that to investigate the lunar global dichotomy, samples from both the near and far sides are needed.
The Chang’e-6 lunar soils contain two types of mare basalts: low-Ti and very low-Ti (VLT). The predominant low-Ti basalt represents the local basalt unit around the landing site, whereas the VLT basalt possibly came from the unit to the east of the landing site (Figure 1B).
The high-precision Pb-Pb dating of Zr-bearing minerals and Rb-Sr dating of plagioclase and late-stage mesostasis of the low-Ti basalt yield consistent isochron ages of 2.83 Ga (Figure 2), indicating that “young magmatism also exits on the lunar far side,” according to the study.
Compared to the near side samples returned by the Apollo and Chang’e-5 missions, the Chang’e-6 low-Ti basalt has a low μ value and 87Sr/86Sr and a very high εNd value (Figure 3), suggesting a very depleted mantle source.
Crustal thickness has been suggested as a key factor in accounting for asymmetry in the abundance of volcanism between the lunar near side and far side. However, this model has been questioned since the SPA basin on the far side, which has an anomalously thin crust, appears deep and significantly underfilled by volcanism.
Based on the investigation of Chang’e-6 low-Ti basalt, XU’s team suggested that the composition of the mantle source is another important factor controlling the generation of lunar volcanic activity.
“Although the SPA basin has a thin crust, the depleted and refractory mantle source beneath the SPA basin hinders partial melting to a large degree,” said XU.
This work also provides an additional calibration point at 2.83 Ga for the lunar crater chronology and implies a constant impact flux after 2.83 Ga. This newly calibrated chronology model improves the age estimation tool based on crater statistics for both the Moon and other terrestrial bodies, and also has additional implications for the evolution of lunar impactors, potentially related with early planet migration in the early Solar System.
This work was financially supported by the Chinese Academy of Sciences and the lunar research program of GIGCAS.
Figure 2 The two types of basalts in Chang’e-6 soils and isochrons of the Chang’e-6 low-Ti basalt.
Figure 3 Initial Pb and Sr-Nd isotopes of lunar basalts.
Figure 3 Initial Pb and Sr-Nd isotopes of lunar basalts.
Figure 4 Incorporating the Chang’e-6 landing site into a lunar crater chronology model.
Figure 4 Incorporating the Chang’e-6 landing site into a lunar crater chronology model.
Credit
Image by GIGCAS
Journal
Science
Article Title
A sample of the Moon's far side retrieved by Chang'e-6 contains 2.83-billion-year-old basalt
Article Publication Date
15-Nov-2024
Chang’e-6 basalts offer insights on lunar farside volcanism
image:
Distribution of distinct volcanic episodes on the Moon’s farside. The remote sensing image highlights the Chang’e-6 landing site along with nearby mare and cryptomare basaltic units.
view moreCredit: Image by YANG Muhan
Basalt samples returned by the Chang’e-6 mission have revealed volcanic events on the lunar farside at 2.8 billion years ago (Ga) and 4.2 Ga, according to research conducted by Prof. LI Qiuli’s lab at the Institute of Geology and Geophysics of the Chinese Academy of Sciences. This work was recently published in Nature.
“Unraveling the volcanic history of the lunar farside is crucial for understanding the hemispheric dichotomy of the Moon,” said Prof. LI.
The asymmetry between the Moon’s nearside and farside—encompassing differences in basalt distribution, topography, crustal thickness, and thorium (Th) concentration—has long been a mystery. However, China’s Chang’e-6 mission—the first to return samples from the lunar farside—has created a unique opportunity to explore volcanic activity in this hemisphere, with the 1,935.3 g lunar soil sample it retrieved.
Led by Prof. LI, postdoctoral researcher ZHANG Qian conducted systematic radioisotope dating on 108 basalt fragments from this sample. Of these, 107 fragments revealed a consistent formation age of 2807 ± 3 million years ago (Ma), indicating the eruption age of local basalts at the Chang’e-6 landing site. Notably, this 2.8 Ga volcanic episode has not been observed in nearside samples.
The remaining fragment, of high-aluminum basalt and dating to 4203 ± 4 Ma, is thought to have originated from a cryptomare region south of the landing site. It is the oldest lunar basalt sample yet returned whose age has been precisely determined.
These data indicate that volcanic activity on the lunar farside persisted for at least 1.4 billion years, from 4.2 Ga to 2.8 Ga. Initial lead isotope analysis suggests that these basalts derive from distinct mantle sources: The 4.2 Ga basalt came from a KREEP-rich source, i.e., one with abundant potassium (K), rare earth elements (REE), and phosphorous (P), while the 2.8 Ga basalt came from a KREEP-poor source.
The close alignment between the 2.8 Ga basalt age and crater-counting estimates suggests that the cratering chronology model, established based on nearside observations, is also applicable to the lunar farside. The radioisotope ages of the Chang’e-6 basalts provide an essential calibration point for refining lunar crater-counting chronology, thus enhancing its accuracy, according to Prof. LI.
This research was conducted in collaboration with the National Astronomical Observatories, with samples provided by the China National Space Administration. Funding was provided by the National Natural Science Foundation of China, the Strategic Priority Research Program (Category B) of the Chinese Academy of Sciences, and the Key Research Program of the Institute of Geology and Geophysics, Chinese Academy of Sciences.
Journal
Nature
Article Title
Lunar farside volcanism 2.8 billion years ago from Chang’e-6 basalts
Article Publication Date
15-Nov-2024
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