What mysteries of the moon can be revealed by the first lunar samples collected by Chang'e 5? On December 13, the Zijinshan Observatory of the Chinese Academy of Sciences announced that researcher Xu Weibiao and his planetary chemistry research team, together with Nanjing Institute of Geology and paleontology, studied the lunar samples and found that the samples contained extremely high ilmenite content, which was determined as high titanium basalt. Previously, other domestic research teams found low titanium and medium titanium basalt types in Chang'e 5 lunar samples. Combined with this study, scholars speculate that there may have been many volcanic eruptions in the lunar landing area of Chang'e 5. The results were recently published online in the English version of China's comprehensive academic journal Science Bulletin. Extremely high ilmenite content was found in lunar samples On December 17, 2020, the chang'e-5 spacecraft successfully returned to the ground with 1731 grams of lunar samples, realizing the sampling and return of extraterrestrial objects in China for the first time. Zijinshan Observatory of the Chinese Academy of Sciences is one of the first 13 scientific research institutions to obtain lunar scientific research samples in China. Researcher Xu Weibiao, as the person in charge of use, was approved two lunar basalt samples with a total of 15 mg. "The lunar samples collected this time are lunar sea basalts, which are mainly distributed in the basin on the front of the moon and may be formed by partial melting of the lunar mantle from 100 km to 400 km deep." Xu Weibiao is also the corresponding author of this paper. He introduced that the research team carried out detailed mineralogical and three-dimensional tomography research on one of the samples by using high-resolution micro CT, scanning electron microscope and electron probe. The results show that the sample belongs to lunar volcanic basalt with fine-grained medium grained sub diabase structure. A small amount of olivine phenocrysts are distributed in the matrix composed of pyroxene, plagioclase, ilmenite and other accessory minerals. The chemical composition and evolution trend of the main constituent mineral pyroxene are highly consistent with the types of high titanium basalt retrieved from the Apollo program of the United States and the moon program of the Soviet Union. Xu Weibiao said that the sample contains extremely high ilmenite content, with a volume percentage of 17.8vol%, and the abundance is close to the highest value of 18vol% of Yuehai basalt retrieved by the Apollo program of the United States. It provides clues for studying the energy source of lunar volcanic magma and the composition of lunar mantle "High titanium basalt is rare. At present, high titanium basalt is basically not found in all collected lunar meteorites. Because ilmenite is located in the shallow layer of the moon, it is generally distributed in the areas below the lunar crust and above the lunar mantle. Basalt is a kind of rock formed by the eruption of magma generated by high-temperature melting of lunar mantle material in the depths of the moon to the lunar surface and solidification after cooling, so it is in basalt The titanium content should be very low. " Xu Weibiao explained that the reason for the emergence of high titanium basalt is that there is a hypothesis in the academic community that ilmenite has a gravity unstable structure of heavy on the upper mantle and light on the lower mantle because of its heavy proportion. Ilmenite has overturned and sunk to the deep lunar mantle. After melting, it is sprayed out together with magma, and is "blocked" in basalt after cooling. "This situation does not often occur, which shows that in the area where Chang'e 5 landed and sampled, the lunar mantle may have overturned, which will produce high titanium basalt." Xu Weibiao thinks. In this sample, researchers also found phosphate minerals with a content of 0.5%. "Phosphate minerals are generally rich in rare earth elements and radioactive heat generating elements, such as thorium and uranium, which are the energy source for producing basaltic magma." Xu Weibiao said. It is noteworthy that earlier, domestic scientific research teams released that low titanium and medium titanium lunar sea basalt types were found in the lunar samples collected by Chang'e 5, "At the same place, the magma erupted each time comes from the same area of the lunar mantle. Low titanium and medium titanium basalts have been found in this area before. This time, a relatively rare high titanium lunar sea basalt enriched in rare earth elements has been found, which indicates that there have been at least three and most likely many volcanic eruptions in the history of the Chang'e 5 landing area, which is also the source area of the bright lunar mantle The chemical composition is uneven. " Xu Weibiao said that this provides an important clue for the study of the evolution of the moon. "The findings of this study make us more curious about why there are so many magma and where the heat source inside the moon comes from in the late known evolution of the moon." Xu Weibiao believes that the discovery of high titanium basalt is expected to interpret the different material components in the lunar mantle source area, the energy source of volcanic magma formation and the fine temporal and spatial distribution law of late lunar volcanic activity. The follow-up deepening research work is in progress. (Xinhua News Agency)
Edit：Li Ling Responsible editor：Chen Jie
Source：Science and Technology Daily
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