In the 1980s, mantle geochemistry pioneers Alan Zindler and Stan Hart defined four mantle terminal components based on the radioisotopic composition of oceanic basalt: DMM (loss-bearing mantle), HIMU (high 238U/204Pb mantle), EM1 (type 1 enriched mantle), and EM2 (type 2 enriched mantle). Of the four mantle components, em1 has the most controversial source. Typical EM1 features include very low 206Pb/204Pb, 143Nd/144Nd, 176Hf/177Hf, and medium 87Sr/86Sr. Potential sources of EM1 include: (1) recycling of the Ancient Continent Lithosphere Mantle, (2) recycling of the Lower Ancient Continent crust, and (3) recycling of the Oceanic Crust + Ancient Sediment. The root cause of the long-standing debate is that all three of these can exhibit the characteristics of EM1 over a long period of time (>70 million years) of evolution. Ma Liang, Associate Researcher of Mantle Geochemistry Discipline Group, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, and Academician Xu Yigang, together with Researcher Li Jie, Professor Chen Lihui of Northwest University, and Dr. Liu Jianqiang of Hohai University, used the Mo stable isotope system to define the origin of the mainland EM1 component from the recycled oceanic crust + ancient sediments.
The Mo stable isotope system can effectively identify continental crusts, continental lithosphere mantles, recycled oceanic crusts and pelagic sediments. Chondrites have a homogeneous Mo isotopic composition (δ98/95Mo=-0.15‰; δ98/95Mo is the 98Mo/95Mo ratio relative to the Reference Standard NIST SRM 3134); the Mo isotopic composition of the mantle (-0.20‰) is slightly lighter than that of chondrites; and the δ98/95Mo (-0.21±0.18) of the global mantle peridot inclusion; 2sd, n=15) is close to the mantle value; continental crusts have heavy Mo isotopic characteristics (0.00‰ to 0.35‰), while recycled oceanic crusts (-0.68‰ to -0.13‰) and pelagic sediments (-1.87‰ to 0.11‰) have light Mo isotopic characteristics. Significant differences in Mo isotopic composition between continents, oceans and mantles provide excellent conditions for tracing the origin of em1 components.
Fig. 1 Radioisotopic composition characteristics of the Normin River basalt.
The researchers conducted a detailed Mo isotope study of a typical continental EM1 basalt, the Cenozoic Nomin River potassium basalt (260Pb/204Pb<17.5) in northeast China (Figure 1). The analysis results showed that the Normin River basalt had Mo isotopic composition characteristics lighter than chondrite meteorites, and its δ98/95Mo changed from -0.49 to -0.15‰, and Mo/Ce changed from 0.009 to 0.037. δ98/95Mo is positively correlated with Mo/Ce, Lu/Hf, 176Hf/177Hf, and negatively correlated with La/Yb, Zr/Yb, Hf/Yb, and some incompatible elements (La, Nb, Ta, Pb, Zr, Hf).
The Mo isotopes of the Nomin River basalt are not affected by hydrothermal alteration, crustal blending, Mo-rich mineral separation crystallization and melt residues. The relationship between δ98/95Mo, Mo/Ce and radioisotopes suggests that EM1 terminals have very light Mo isotope composition and very low Mo/Ce (Figure 2). Since neither continental crust nor lithospheric mantle can contribute light Mo, the EM1 component is most likely derived from light Mo isotopes, low Mo/Ce recycled ocean crust + ancient pelagic sediments.
Fig. 2 Mo/Ce, Mo isotope and Hf isotope relationship diagram of Nomin River basalt.
The covariance relationship between Mo isotopes and radioactive isotopes and trace elements can be explained by the inconsistent melting process of peridot + garnet (including sediments). Information on enriched, fusible garnet (containing sediment) melts is mainly recorded during low partial melting, i.e., low δ98/95Mo, Mo/Ce, Lu/Hf, high La/Yb, Zr/Yb, Hf/Yb and incompatible elements, as well as radioisotopic characteristics of EM1 type, while geochemical information of loss-prone mantle peridots is recorded at high degree partial melting (Figure 3).
The study not only supports the idea that the EM1 component in the mantle originates from the recycled oceanic crust + ancient sediments, but also demonstrates that light Mo in residual plates can be preserved deep in the mantle (at least 410-660 km deep in the mantle transition zone) and circulated to the surface by volcanic action. The recycled oceanic crust material forms a reservoir of light Mo in the deep mantle, which compensates for the continental crust characterized by heavy Mo isotopes. The Mo isotope is expected to be a powerful tracer for recycling oceanic crust material in deep mantles.
Fig. 3 Diagram of the Mo isotope and partial melting degree of the Nomin River basalt.
The research results were published in the international journal Geochimica et Cosmochimica Acta. The research was co-funded by the Pilot B Project of the Chinese Academy of Sciences, the National Natural Science Foundation of China, the Natural Science Foundation of Guangdong Province, and the Guangdong Provincial Laboratory of Marine Science and Engineering (Guangzhou).
论文信息：Ma Liang*, Xu Yi-Gang, Li Jie, Chen Li-Hui, Liu Jian-Qiang, Li Hong-Yan, Huang Xiao-Long. Ma Qiang, Hong Lu-Bing, Wang Yu. 2022. Molybdenum isotopic constraints on the origin of EM1-type continental intraplate basalts. Geochimica et Cosmochimica Acta. 317, 255-268.