Chang'e-5 samples reveal how young volcanic activity occurs on the moon

The Moon formed about 4.
5 billion years ago and has a mass of ~1% that of Earth, and for such a small body, it should theoretically cool quickly and stop volcanic activity
early.
However, Chinese scientists' research on the Chang'e-5 basalt has confirmed that lunar volcanic activity can continue until 2 billion years ago, which not only refreshes human understanding of the history of lunar magma activity and thermal evolution, but also raises new scientific questions: why lunar volcanic activity lasts so long
.
 

Lunar basalt is a rock
formed by the melting of part of the lunar mantle magma formed by volcanic eruption to the lunar surface to cool and crystallize.
For the continuously cooling moon, there are two possible ways for the lunar mantle to partially melt: (1) heating (such as heating of radioactive elements); (2) Reduce the melting point (such as the lunar mantle is rich in water or other volatile components).

Scholars have used impact crater dating to predict that some areas of the radioactive element-rich storm Ocean Cripp body still have young volcanic activity, so the mainstream hypothesis is that the heating of radioactive elements is the main mechanism
that maintains young volcanic activity on the moon.
However, recent studies of the Chang'e-5 basalt, also from the Storm Ocean Krip massif, have revealed that its lunar mantle source region is not rich in radioactive thermogenic elements, directly rejecting this mainstream hypothesis; At the same time, the study also found that the source area of Chang'e-5 basalt is very "dry", further ruling out the conjecture
that the lunar mantle is rich in water and lowers the melting point.
Therefore, why the lunar volcanic activity continues for so long has become an unsolved mystery
in a new round of lunar research.
 

The essence of partial melting of the lunar mantle is that the temperature of the lunar mantle exceeds its solidus line
.
Therefore, lunar volcanic activity can not only cause the lunar mantle to warm up above the solid phase line and then melt through other heating effects (such as tidal action and impact effect), but also induce the lunar mantle melt by changing the material composition of the lunar mantle source area to make the solid phase move in the direction of low temperature
.
Therefore, the breakthrough to crack the cause of the young volcano of the moon lies in accurately defining the temperature and pressure of the origin of magma, restoring its source area composition, and comparing it with the ancient Apollo basalt on this basis to reconstruct the lunar thermal-magma evolution model
.
Defining the origin temperature and pressure of Chang'e-5 basalt and restoring its initial magmatic composition are key
.
 

Based on this research idea, Associate Researcher Su Bin, Researcher Chen Yi and Former Jiangyan of the Institute of Geology and Geophysics, Chinese Academy of Sciences, together with Researcher Yang Wei, Researcher R.
N.
Mitchell, Associate Researcher Wang Hao, Associate Researcher Tian Hengci, Academician Li Xianhua, Academician Wu Fuyuan and Professor Hui Hejiu of Nanjing University, selected 27 The representative Chang'e-5 basalt cuttings (large particle size, many internal mineral particles and uniform distribution) were analyzed, and the main composition of the whole rock of the cuttings was analyzed by the newly developed scanning electron microscopy energy spectroscopy, combined with a series of magma separation crystallization simulation and thermodynamic simulation calculations, the initial magmatic composition of Chang'e-5 basalt and Apollo low titanium basalt was restored (Fig.
1, Fig.
2), and the mantle source area composition of lunar volcanic rocks in different periods was defined (Fig 3), the temperature and pressure conditions under which they melt (Figure 4) are obtained, and the understanding obtained is as follows: 

(1) Compared with the ancient Apollo low-titanium basalt, the initial magma of the young Chang'e-5 basalt contains higher CaO and _TiO2, as well as lower MgO (Figure 2);  

(2) Compared with the Apollo low-titanium basalt, the source area of Chang'e-5 basalt contains higher (~20%) monoplioxene-ilmenite pile crystals formed in the late magma ocean, and their addition will significantly reduce the melting point of the lunar mantle and induce the formation of young volcanoes (Figure 3);  

(3) The origin depth of Chang'e-5 basalt and Apollo basalt is roughly the same, but the formation temperature of Chang'e-5 basalt is lower, indicating that the internal temperature of the moon has only decreased by ~80 °C from 3.
8-3.
1 billion years ago to 2 billion years ago (Figure 4).

 

Fig.
1 Simulation results of initial magmatic separation and crystallization of Apollo low-titanium basalt

Fig.
2 Simulation results of reverse separation and crystallization of initial magma of Chang'e-5 basalt

Fig.
3 Simulation results of crystal mixing in lunar magma piles

Fig.
4 Calculation results of initial magmatic multiphase saturation temperature and pressure of Apollo and Chang'e-5 basalts

This work shows that although the interior of the moon continues to cool slowly, the mixing of lunar mantle materials caused by the overturning of crystals in the late magma ocean pile may gradually strengthen in the long-term evolutionary history of the moon, which can not only change the material composition of the lunar mantle source area, but also effectively reduce the melting point of the rocks in the source area, offset the general trend of the lunar mantle gradually and slowly cooling, and trigger long-term continuous lunar volcanism
.
This work quantifies the thermal evolution of the slow cooling of the moon's interior, providing a new mechanism for the important scientific question of "the origin of the moon's young volcanoes", and deepening our understanding
of the moon's origin and thermal evolutionary history.
 

The research results have been published in the international academic journal Science Advances (Su Bin #*, Yuan Jiang Yan #, Chen Yi^*, Yang Wei, Ross N.
Mitchell, Hui Hejiu, Wang Hao, Tian Hengci, Li Xianhua, Wu Fuyuan .
Fusible mantle cumulates trigger young mare volcanism on the cooling Moon [J].
Science Advances, 2022, 8: eabn2103.
DOI: 10.
1126/sciadv.
abn2103
。 The achievement was jointly funded
by the Key Deployment Project of the Chinese Academy of Sciences (ZDBS-SSW-JSC007-15) and the Key Deployment Project of the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS-202101).