The first billion years of the Earth's formation were one of the most puzzling periods in geological history. The high heat flow environment of Early Earth, intense meteorite bombardment, and the superposition of complex metamorphic deformations have left many key geological records incomplete. Related issues such as the nature of the early crust-mantle and the tectonic system of the early Earth are still fundamental problems in the field of earth science today. From a geochemical point of view, the silicate earth is composed of three reservoirs: the original mantle, the loss mantle and the land crust, of which the loss mantle is the remnant after the original mantle is partially melted and the melt is extracted to form the crust. If there had indeed been a large number of land crusts in the early days of the Earth, it would have caused a strong loss of the mantle. In the past decade, different scholars have speculated based on different geochemical indicators that the size of 40%-100% of the current continental crust was formed 3.8 billion years ago, but strangely, the zircon records of 3.8 billion years ago have not found credible evidence of Lu-Hf isotope loss, that is, from the perspective of zircon Lu-Hf isotopes, the early mantle of the earth is still characterized by the primitive mantle, which also means that there can be no large number of land crusts in the early days of the earth. In order to solve the above contradictions, Liu Peng, associate researcher of the State Key Laboratory of Lithosphere Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, and his collaborators Guo Jinghui, Ross Mitchell, Li Xianhua, Zhai Mingguo, etc. studied the U-Pb age and Hf-O isotopes of zircon in the Caozhuang chrome mica quartzite in the eastern part of Craton, North China.
Figure 1 It is still controversial when global mantle losses began. In recent years, Christopher Fisher, Jeffrey Vervoort and others have argued that the starting point for the evolution of the loss-making mantle was at 3.8Ga
The researchers found that about 37 percent of the rigorously screened zircons had an Hf isotopic composition higher than that of chondrites, but were still broadly distributed on both sides of the chondrite evolution line overall. This is similar to the Hf isotopic composition of ancient zircons around the world, which seems to indicate that the Hf isotope of zircon does not support the existence of loss-making mantles in the early Days of Earth. It should be noted, however, that although zircon has long been regarded as the most reliable mineral for tracing the evolution of the early crust, even the most intact zircon has been studied in past studies, and the biases caused by zircon itself and the impact of the early tectonic environment have been inevitably ignored.
The vast majority of Archaeopteric-Pluto zircons are derived from TTG (Anglo-Yun Diorite-Ocho granite-granite-granite diorite). Geochemical and experimental petrology evidence suggests that TTGs are usually distinguished by enriched magnesium sources, so the composition of the mantle source region obtained from these zircons should naturally be characteristic of the mantle-enriched mantle. In fact, statistics on global trace element and isotope data on TTG also confirm this: that is, the source area of the mantle corresponding to the original rock of TTG should also be undifferentiated. The source area of loss is not easy to form TTG, and it is naturally not easy to be recorded by zircon in TTG.
In addition, based on the isotopic characteristics of the mantle source region inverted by the Hf isotopes of zircon, an important hypothesis is that the crust given by these zircons has a very short age of residence, that is, after the magma is separated from the mantle, its Lu-Hf isotope is quickly recorded by the zircon without long-term evolution. However, it differs from today's subduction structure of strong interaction between shell mantles.
Fig. 2 Unlike today's subduction tectonic system, in the Pluto-Archaeopene, magnesium iron rocks can often be preserved for hundreds of millions of years, or even more than 1 billion years, so zircon εHf significantly underestimates the true isotopic composition of the mantle
Figure 3 The summary of global zircon data shows that the zircon Hf isotope does not show loss characteristics before 3.8Ga, but considering the bias caused by zircon itself and the impact of the earth's early tectonic environment, this means that the loss mantle is widespread at this time
In the Pluto-Archaeopene, dominated by the hysteretic cover system, magnesium ferrous rocks can often be preserved under closed systems for hundreds of millions of years, or even more than 1 billion years, if the average magnesium iron crust composition (176Lu/177Hf = 0.022) is used to calculate, its εHf value will be underestimated by an average of 0.8ε per 100 million years. Therefore, the characteristics of chondrites reflected in most Archaeopheean-Pluto zircons should be regarded as the minimum value of the mantle source area, that is, the true value of the mantle should be generally higher than that of chondrite meteorites.
More direct evidence may come from ferrous magnesium-ultramafic rocks. Ancient bodies such as West Greenland and the Isua Greenbelt Belt have developed loss-super-loss peridotite, basalt and other rock records. Studies such as the 142Nd isotope and the Mo isotope have revealed the existence of earlier mantle inhomogeneities. Combining this petrology and geochemical evidence, as well as the study in this paper, the researchers concluded that the zircon Hf isotopic composition of the early Earth 's (4.4-3.8Ga) chondrite meteorites meant that global mantle losses were already widespread by this time.
The research results were published in the international academic journal Geochimica et Cosmochimica Acta in the field of geochemistry (Liu Peng, Guo Jinghui, Ross Mitchell, Christopher Spencer, Li Xianhua, Zhai Mingguo, Noreen Evans, Li Yanguang, Bradley McDonald, Jin Mengqi. Zircons underestimate mantle depletion of early Earth [J] Geochimica et Cosmochimica Acta, 2021. DOI: 10.1016/j.gca.2021.11.015）。
Editor: Chen Feifei
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