Progress on the formation of volatile lunar nuclei by using Cu and Zn isotopes

[ Instrument Network Instrument R & D ] The Big Collision Hypothesis holds that the moon formed in a large collision between a Mars-sized planet and the primitive earth 4.5 billion years ago. After the collision, the material splashed into space, accumulating and accumulating in the lunar orbit, forming a hot molten lunar magma ocean, and then differentiating the structure of the moon's core, mantle, and moon crust. The large collision event profoundly changed the planetary environment and chemical composition of the Earth-Moon system, and is the most important planetary event that the Earth-Moon system experienced. It is generally believed that the volatilization during large collisions will change the composition of the moon's elements and isotopes, but little is known about the differentiation of the lunar sphere, especially the formation of the moon's nucleus, mainly due to the lack of understanding of the geochemical behavior of the elements and isotopes .

Copper (Cu) and zinc (Zn) are both volatile elements (which can restrict the volatilization effect during collision) and iron-sulfur elements (which can restrict the nuclear-mantle differentiation process), so they can be used to explore the lunar nuclear separation The effect of difference on the chemical composition of the moon. After years of hard work, the team of Huang Fang, a professor at the School of Earth and Space Sciences, University of Science and Technology of China, established a high-precision Cu-Zn isotope analysis method. High-pressure experimental petrology accurately measured the equilibrium fractionation coefficients of Cu and Zn isotopes between silicate melt and metal melt, which restricted the composition and formation process of lunar nuclei.

The study found that the sulfur-containing metal melt was significantly richer in light Cu and Zn isotopes than the silicate melt, while the sulfur-free metal melt and silicate melt had a smaller fractionation. This result explains the difference in the composition of stable isotopes of metals between the Earth and the Moon. The larger Zn isotope fractionation between the moon and the earth clearly reflects the influence of the volatilization process; although the volatility of Cu is weaker than that of K and Ga, the difference in the composition of Cu isotopes between the moon and the earth is greater than that of the K and Ga isotopes . This may be because the Cu isotopic composition of the moon is not only controlled by volatilization during large collisions, but also by the separation of sulfur-containing metal melts from the lunar magma ocean during the formation of the lunar nucleus; but K and Ga do not enter the lunar nucleus, so Affected by the formation of lunar nuclei. This work suggests that there may be a certain amount of S in the lunar nucleus, which is of great significance for understanding the volatile composition of the moon, the condensation of the lunar nucleus, the maintenance of the lunar magnetic field, and the late accumulation between the earth and the moon.

The paper, titled The effect of core segregation on the Cu and Zn isotope composition of the silicate Moon, was recently published in the International Geochemical Journal Geochemical Perspective Letters (Geochem. Persp. Let. 12, 12–17). Xia Ying, a PhD graduate of the School of Earth and Space Sciences, is the first author, Kate Kiseeva is the second author, and Huang Fang and Jon Wade are co-corresponding authors. This work was supported by the Chinese Academy of Sciences Pilot Project B and the National Natural Science Foundation of China.