Recently, Research Fellow Yang Yang from the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, in collaboration with researchers from Harvard University and the University of Tulsa in the United States, confirmed the widespread presence of recycled oceanic lithosphere mantle in the mantle of the Arctic Ocean. This research was supported by the National Natural Science Foundation of China and the Youth Innovation Promotion Association of the Chinese Academy of Sciences, with the findings published in the journal "Earth and Planetary Science Letters."
The recycling of subducted oceanic crust has long been considered a major cause of mantle compositional heterogeneity. However, the fate of the oceanic lithosphere mantle, which accounts for over 90% of the volume of subducted oceanic plates, has remained unclear.
Through systematic analysis of 267 samples of basalt from the western volcanic region and central weak magmatic activity region of the Gakkel Ridge in the Arctic Ocean, researchers found that in areas where normal thickness oceanic crust develops in the western part of Gakkel, mainly depleted N-MORB is produced, with systematic geochemical variations in magma composition from west to east revealing a gradual depletion of the mantle and a gradual decrease in partial melting. In contrast, in areas where the Gakkel midsection lacks developed oceanic crust and is dominated by exposures of olivine-rich rocks, enriched E-MORB is produced, yet major elements show a depletion of CaO and a low CaO/Al2O3 ratio.
The study suggests that the mantle beneath the western segment of Gakkel is "full" but "depleted," while the mid-segment mantle is "barren" but "enriched." Fullness and barrenness represent their ability to generate magma through melting, while depletion and enrichment indicate the degree of enrichment in trace elements. Additionally, the potential temperature of the midsection mantle is extremely low, suggesting that the combination of extremely barren mantle source regions and exceptionally low mantle potential temperatures has led to the formation of a region with minimal magmatism in the midsection of Gakkel.
Combining the geochemical composition and distribution of rocks from the Arctic Ocean ridges, the study proposes that newborn oceanic lithosphere retains a "memory" of its former self, indicating the presence of refractory mantle end-members derived from recycled oceanic lithosphere mantle beneath it, with the proportion gradually increasing from west to east until it is unable to form sufficient oceanic crust in weak magmatic activity areas, thereby exposing recycled oceanic lithosphere mantle directly on the seafloor. Quantitative calculations reveal that since plate tectonics began, the volume of subducted oceanic lithosphere mantle is nearly equivalent to the total volume of the lower mantle. Therefore, it is proposed that subducted oceanic lithosphere mantle recycles into the convecting mantle, forming a part of the global newborn oceanic lithosphere.
For more information on the related paper, visit: https://doi.org/10.1016/j.epsl.2023.118553
