The deep carbon cycle of the earth has become one of the hot topics in earth science research at home and abroad. Past research has confirmed that the mantle is a huge carbon reservoir, and its carbon storage exceeds the total carbon content of all other reservoirs on the earth, which indicates that most of the carbon on the surface has returned to the mantle through the subduction zone. The large amount of carbonate rocks carried by the altered oceanic crust will not be released during the subduction and dehydration process, but will be carried to the deep part of the mantle, thereby significantly changing the physical and chemical properties of the mantle (such as solid line, seismic wave velocity, density, etc.) . The C isotope has been widely used for tracing recycled carbonate rocks, and it is very effective for tracing organic carbon. However, 95% of the carbonate rocks recycled into the mantle through the subduction zone are inorganic carbon. Therefore, this requires a new isotope system to more effectively trace the deep carbon cycle.
Yang Wei, associate researcher at the Nano Ion Probe Laboratory, Institute of Geology and Geophysics, Chinese Academy of Sciences and his collaborators conducted related research on this. They found that the huge Mg isotope difference between carbonate and silicate rocks makes the Mg isotope system likely to be An effective means of tracing and recycling carbonate rocks, and based on this, a systematic Mg isotope study of Mesozoic and Cenozoic basalts in the North China Craton was carried out.
The results show that the> 120 Ma Yixian Formation basalt has an Mg isotopic composition similar to that of the mantle (δ26Mg = -0.31 ~ -0.25), with an average value of -0.27 ± 0.05. On the contrary, both <110 Ma Fuxin and Taihang Mountain basalts have a light Mg isotopic composition (δ26Mg = -0.60 ~ -0.42), with an average value of -0.46 ± 0.10 (Figure 1).
Simulation calculations show that this light Mg isotope composition is likely to be derived from the replacement of basalt source areas by recycled carbonate rocks (Figure 2), and other geochemical characteristics of basalt (high U / Pb, Th / Pb ratio and The deficient Sr-Nd isotope characteristics) also support this conclusion. Since the Tethys and Mongolian oceans were closed in the Triassic and no light Mg isotope was found in the> 120 Ma basalt, the carbonate melt that affected the Cenozoic mantle of the North China Craton may have originated from the subducted Pacific Plate.
This study not only provided an example of using Mg isotope tracing to recycle carbonate rocks, but also provided important constraints for the impact of the Western Pacific subduction on the evolution of the North China Craton.
The result was published in the internationally renowned geochemical journal Chemical Geology (Yang et al. Magnesium isotopic systematics of continental basalts from the North China craton: Implications for tracing subducted carbonate in the mantle. Chemical Geology. 2012. 328: 185-194) .
Figure 1: Meso-Cenozoic basalt δ26Mg vs MgO in the North China Craton. The gray area indicates the Mg isotopic composition of the average mantle.> 120 Ma Yixian basalt has a Mg isotopic composition similar to that of the mantle, while <110 Ma Fuxin and Taihangshan basalt have a δ26Mg significantly lower than the mantle value.
Figure 2: δ26Mg vs MgO map of peridotite and basalt in the North China Craton. Simulation calculations show that it is difficult to form a low δ26Mg mantle source area by pure silicate melt metasomatism (very high melt / peridotite ratio 5.2 is required), and a small amount of carbonate melt must be added.
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