What is the source lithology of mantle EM1 endmember?


Because of the compositional differences between mantle and crustal materials (e.g., oceanic crust, sediment), the subduction of crustal materials into the mantle will cause the chemical heterogeneity of mantle, recording by mantle-derived basalts. The enigmatic EM1 (Enriched Mantle 1) component found in the mantle source of some oceanic island basalts (e.g., Pitcairn basalts) is frequently speculated to be formed by this process. However, the origin of EM1 (Enriched Mantle 1) reservoir has been long debated, because melting of the ambient refractory peridotite along with the EM1 component will dilute the “EM1 fingerprints” recorded in these rocks.

Figure 1. (a) variation in Nd versus 206Pb/204Pb for typical EM1-type continental potassic basalts from Nuominhe, Wudalianchi, Erkeshan and Keluo, and the typical EM1-type OIBs from Pitcairn, Kerguelen-Heard and Walvis Ridge. (b) Geological and tectonic map of NE China, showing locations of major faults and distributions of Cenozoic basalts.

The Cenozoic potassic basalts from northeast China (K2O/Na2O>1) is recently regarded as the continental equivalents of those EM1-type OIBs since they show the least radiogenic Pb and Nd isotopic ratios in the world (Figure 1). These rocks are generally generated by low-degree partial melting, and have the chance to preserve more information on the EM1 origin than those plume-derived OIBs. The majority of studies have attributed the EM1 signatures of the potassic basalts to lithospheric metasomatism because the breakdown of hydrous phlogopite formed during the metasomatism has the chance to produce potassium-rich melts, although the lithospheric mantle (represented by the mantle xenoliths hosted in these potassic basalts) beneath these volcanic fields show significantly more depleted isotopic compositions than these potassic basalts. For this reason, the melting of subducted crustal materials (i.e., recycled sediment ± oceanic crust) directly is invoked to generate these EM1-type potassic basalts, and the stability of liebermannite (i.e., K-hollandite) is suggested to explain their K-rich characteristics. A significant difference for these two models is their source lithology. The former model is generally speculated to be derived from a phlogopite-bearing peridotite, while the lithology of the latter is inferred to be eclogite. Therefore, the identification of source lithology for these potassic basalts may provide us crucial clues for their genesis model and the origin of EM1.

Figure 2. Major and minor elemental compositions of the olivine phenocrysts hosted in the potassic basalts from northeast China.

Olivine phenocrysts hosted in the potassic basalts have low Mn/Fe and moderate Ni/(Mg/Fe) ratios, which is similar to those olivines hosted in Pitcairn basalts (Figure 2). These characteristics are quite similar to those olivines crystallized from the eclogite-derived melts, supporting the presence of eclogite in the source of the potassic basalts. High-precision Fe isotopes of these potassic basalts are also reported to constrain the source lithology. The δ57Fe values of these basalts from northeast China range from 0.15‰ to 0.28‰, which are higher than that of average fresh MORB (δ57Fe = 0.16‰ ± 0.04‰). Their δ57Fe are positively correlated with the K2O, SiO2, K/U and Rb/Y, and negatively correlated with the Nd and δ26Mg, forming binary mixing arrays. One endmember is the inferred EM1 reservoir, whereas the other is the local lithospheric mantle. Major elemental compositions of the melts released from the EM1 component resemble those sediment-derived experimental melts. Combining with their heavier Fe isotopes and higher Zn/Fe ratios relative to those mid-ocean ridge basalts (MORBs), an eclogitic source of these potassic basalts is therefore proposed to account for these features. Then, the eclogite-derived primary potassic basalts are changed by the melt-rock interaction in various degree during ascent through the thick lithospheric mantle (Figure 3).

Figure 3. Variations in δ57Fe versus εNd for the potassic basalts from northeast China and modeling for their genesis.

In summary, we emphasize that the EM-1 type OIBs and continental potassic basalts are generated from a similar eclogitic source composed of ancient subducted crustal materials (i.e., recycled sediment ± oceanic crust), and therefore highlights the important role of sediment recycling in formation of the mantle EM1 reservoir. These results are recently published on the Journal of Geophysical Research: Solid Earth (Nature IndexIF = 4.390).

Publication Information: Shi, J.-H., Zeng, G.*, Chen, L.-H., Wang, X.-J., Liu, J.-Q., Xie, L.-W., Yang, Y.-H., Zhang, H.-L., (2023). Lithology of EM1 reservoir revealed by Fe isotopes of continental potassic basalts. Journal of Geophysical Research: Solid Earth, 128, e2022JB025133. https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2022JB025133.